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1. INTRODUCTION

The aim of this report is to understand the basic concept of crosswall construction, and a proposal for 'Armature Crosswalls' to be used as earthquake hazard mitigation for reinforced concrete and masonry infill-wall buildings vulnerable to collapse. RC frame and infill construction is common throughout the world and often has proved lethal in earthquakes. The paper traces the history of masonry infill construction from pre-modern forms that have shown earthquake resistance in the past, to the early modern steel skeleton frame buildings that survived the 1906 San Francisco earthquake. the construction flow at real time in step by step procedures and quantifying the benefits and studying their applications. This is done by making a comparison in usage of the same technique in two different countries namely India and the United Kingdom. This comparison is aimed at producing an insight into the technique and to identify the areas of improvement.

2. Crosswall construction

2.1 Definition:

Crosswall is a modern method of building construction using division walls which transfers the floor loads through the building to the foundations. The name crosswall itself signifies the masonry connection is on either sides of the main wall. It employs concrete components like lift and stair cores, precision engineered (highly skilled), and precasted in the factories.

2.2 Applications:

This type of pre-cast single-skin or flat panel concrete construction is becoming more popular in commercial applications such as hotels, motels, prisons, military barracks and student accommodations. These types of structures up to 16 storeys have been finished in U.K using the crosswall technique. The use of such panels can result in fast, simple construction process on site followed quickly by finishing trades. Crosswall is mainly used for medium and high rise buildings.

2.3 Benefits:

Cross wall construction method reduces the wet trades and creates very early dry boxes for subsequent trades. This helps in speeding up the construction process without delay in work. Crosswalls also helps to eliminate internal cladding and other items such as party walls because the concrete crosswalls and floors do more than load carrying system. Crosswall technique reduces the labour onsite. A precast panel provides concrete frames without structural down stands.

Cross wall construction which has been developed for providing very fast and high quality in

repetitive

accommodation units such as hotels and the opportunity for multi storey apartments is rising, student accommodation buildings, stadiums etc. but the crosswall technique optimisation in low rise buildings is minimal(single houses) because the dimensions of each room may not be the similar.

The Crosswall construction is incorporated with a series of both horizontal and vertical ties which are designed in such a way to prevent the failures and collapses according to building specifications. Other precast work can be started simultaneously before the precast units are erected.

The crosswall buildings have less maintenance service and have good acoustics values. The acoustic performance of crosswall is excellent because of its mass and effective damping. Crosswalls reduces the risks of failing pre completion acoustic tests. (The Concrete Centre, 2006)

Pre cast walls and floor units are more than just a structure (highly effective in costs, speed, early start for other trades, and provision of fire separation panels), wall panels are provided with good finish and air tight tolerance. Tight tolerance enables fitting of bathroom pods, carpets and built in furniture. Flat pack construction is very quick and cost effective because it reduces the material waste on the site and the party walls are largely eliminated.

The separation of acoustics and additional finishes are reduced as the wall panels provide the sufficient airborne noise separation.

Thermal mass is provided by concrete and the thermal is utilised in crosswall constructions because the concrete widely spreads on the unfinished surfaces and the thermal mass reduces the risk of over heating in summer season by keeping the surrounding cool.( Doebber, Ellis M.W; 2005)

The main features incorporated in cross wall construction are

  • Direct decorative finish to the walls with only minor pre decoration treatments.

  • Solid room sized slabs pre finished for direct ceiling decorations.

  • Reduced structural zone without downstands.

  • Cross wall system generally utilises stair cores and lift cores for overall stability.

  • Pre fitted windows eliminates internal cladding.

  • Optimal methods of floor construction, allowing flexibility for individual client requirements.

Construction of stairs and lift cores can permit early access for subsequent trades. (The Concrete Centre, 2007)

2.4 Limitations:

In spite of several benefits offered by the precast panels, it has found that there is no wide acceptance of the precast panels in construction because of

  • Highly skilled engineers or labours are required for manufacturing, placing and erecting the moulds on required positions.

  • Lack of awareness and initiatives especially in Indian construction industry because of invariable labour intensive methods which leads to delay in the construction and that result in cost over-run and poor workmanship.

  • Shortage of skilled and semi-skilled personnel availability may bring poor finishes, leakages in the buildings, corrosion of structural elements. These defects can be only minimised by the use of mass scale projects such as schools, offices, hospitals and other similar projects.(A.B.Shah,2005)

  • Precast wall panel construction may be more or equal expensive as masonry construction because it mainly depends upon the transportation of offsite fabricated materials to onsite. (Havel.G, 2006)

  • Difficult to transport heavy weight and size of the precast panels.

  • The profit margin is very low in the small scale constructions.

  • For single and two storey dwellings it would be typical to use single storey height walls panels between 90-300mm thick concrete for external walls and 70-100mm thick panels for the internal walls. The variation in designs leads to the problem and the single housing clients are not happy with the precast technique even though it provides a higher quality and good finish.

  • This crosswall technique is mostly used for large repetitive structures may not be used for small scale construction like individual housing because the crosswall is more expensive tool when compared. (Glass.J, 2000)

2.4.1 Fire Conditions:

The building with large precast wall panels often has high ceilings and heavy fire loadings, as in retail stores, factories and warehouses. The fire in one of these occupancies require large volumes of water from large lines, if the fire is not controlled master stream appliances will be needed.

The fire in large precast buildings is likely to cause an early collapse of the roofs even if they are no weak connections. In this type of precast panel construction all the parts of the building are tied together as part of a structure, the roof collapses are more likely to tip the walls inwards and outwards.

As we know, the roof supports are more securely connected to the walls, the chances of roof collapsing is high before the wall connection fails.

The failure of any part of the structure may affect the stability of the other parts of the building. (Havel.G, 2006)

3. Historical background:

The precast concrete wall panel system was devised in England by William H.Lascelles (1832-85) of Exeter. In 19th century the pre-casting concrete for the structural purposes was started. Francois Hennebique (1842-1921) first introduced precast concrete into cast in-situ flour mill in France. White and Morris had given the historical accounts of the early development on precast concrete.

(Elliot.K.S; 2000)

Later in 1930s the use of precast as been expanded by companies such as Bison, Trent concrete and girling. Whilst precast concrete usage was stand at the first place it differs from country to country. One of the reason was the structural timber became more limited in some of the countries which led to development and improvement of precast usage. During the next 25 years the development in the precast frames, precast cladding as increased the market share to around 15% in industrial, commercial sectors. (Richardson.J.G, 1973)

Since the 1990s, a significant amount of research has been conducted on the seismic behavior and the design of precast wall paneled structures that do not emulate the behavior of cast-in-place reinforced concrete construction.

4.Development Of Precast Concrete Panel Frame Systems:

Precast panel frame systems have been successfully emerged from the research use of post tensioning between the precast beam and columns to achieve the lateral load resistance needed in seismic areas. (Seismic design of friction-damped precast concrete frame structure)

For the Docklands project on constructing student accommodation blocks in London, the concrete was prefabricated in Belgium and transported because the northern Europe was only able to cope up with the demands of the project and other small local markets were tightened.

Precast concrete wall panels in buildings speeds up the building process by adopting the precast concrete moulds. Decorative and light weight blocks have a great to offer visual values and technical values. The small store buildings from the precast industries offer excellent means of construction within the budget. The farm buildings, ware houses, industrial buildings are required to be constructed on exposed sites so the materials of standard precast frame components are supplied.

Precasting of simple lintels offers opportunities of time saving on the site. The schools, universities are built by using the precast modular components.

4.1

Comparison of Conventional and Large precast panel Construction

Serial no.

Description

In-situ Conventional systems

Precast Large Panel Systems

Benefits in Precast Systems

1.

Construction time

100 percent

50 percent

50 percent

2.

Economy/Cost

100 percent

80 percent

20 percent

3.

Quality

Average

Excellent

Not Measurable

4.

Aesthetics

Thick Sections

Very Slender Sections

Aesthetically Pleasant

5.

Ht. of the structure

Tall

Short

5 Percent Reduction in Height

6.

Dead Loads

100 percent

75 percent

Overall savings in cost

7.

Earthquake Resistant

Moderate

Good

Not measurable

8.

Corrosion resistance

50 percent

100 percent

Not measurable

9.

Durability

50-75 percent

75-100 percent

25 percent more life

10.

Maintenance

100 percent

25 percent

Large savings

11.

Eco-friendly construction

20 percent

80 percent

60 percent more friendly

(Shah.A.B, 2005)

The above table clearly signifies the benefits of large precast construction over conventional construction methods in various fields.

4.2 Structural Superiority:

The precast panel structural system can be quiet efficient compared to other systems and it was tested and proven. (Fintel.M, July1991).

In cast in-situ concrete structures, the large stresses may built up in the structure due to the curing, shrinkage, creep, temperature etc. However no such stresses are built in large precast panel systems during casting. Due to these special characteristics of large panels it has proved its efficiency for more than 50 years in Europe, America and other developed countries. (Shah.A.B, 2005)

5. Precast Concrete Panel Manufacture Process

5.1 Casting:

Precast concrete panels can be manufactured by various casting methods.

5.1.1 Wet casting

: It is generally used for small number of units having similar specifications. The moulds are manufactured by heating them in the enclosed and covered zones. Skilled engineers design the type of moulds depending upon the requirement for the project and also under the guidance of trained supervisions.

As this casting is used for small units, it can be manufactured manually depending upon the number of units to be prepared. This casting can be provided for small housing. Wet casting provides the concrete in the cube strength ranging from 21 to 50N/mm, the slump varying from 0 to 175mm with a compacting factor varying from 0.8 to 0.97.(specifications are derived from CP 110 Unified code for structural concrete)

5.1.2 Gang casting:

It is usually used when the moulds are combined together into a large unit assembly. The gang casting is developed by the modification of wet castings. The gang casting could be used when the similar unit requirement is more.

For example: gang casting can be used in the production of stair case in the multi storied buildings.

The main factors for adopting gang casting process is because of the designed units, and the general components of concrete. The gang casting can be arranged horizontally and the outputs which are achieved from gang casting can be more enhance in the stack casting. The main advantage of gang casting is it allows the concrete to place faster and than the concrete is compacted with the help of immersion-type vibrators.

The greater accuracy of the component units are produced with the gang casting. The gang moulds are tied up in series so that the pressure loading on each individual unit is counteracted by the adjacent units, by this way it reduces the number of tie members. The gang moulds can be used for long horizontal spans, but the filling of gang moulds must be carefully controlled such that the intermediate moulds are not subjected to differential loading which may cause deflections and waves along the line of moulds. (Gibb.A, 1999)

5.1.2 Stack Casting:

Stack casting is the slight modification to the flat and gang casting units. In this casting process gang moulds are filled and hardened after the hardening process the divider plates are driven into the mould up to an appropriate depths so that the next layer will be ready for casting. Stack casting is used to produce 'A' frames and can be used in the repetitive structures such as prisons constructions. All the rooms are of same size and dimensions.

Precasters have found that the incorporation of through holes, barrels or by insertion of anchors allows fastening the mould sides and bring them to the subsequent positions with exact casting thickness.

5.1.3 Battery casting:

It is mostly used on wider scale, battery moulds has become more popular in large concrete wall panel constructions and the casting technique is also used for the manufacture of floor slabs and for decorative cladding components.

The battery casting can be used in the manufacture of 'L' shaped components such as balcony elements and lift enclosures. In this type, the units are generally cast in batteries of two or more. The battery mould can also have eight to twelve cells in the same mould. The changes on the casting process which provides a continuous casting, hardening, and curing schedule prior to de moulding. The battery mould which basically contains a series of plates that are spaced a part by the other mould members.

A Battery mould allows high density of casting to be carried in the available space. Care and proper supervision is required in the assembly operations so it results in securely tied moulds which will be impossible for the concrete member to get separate from the mould. (Richardson.J.G, 1973)

5.2. Direct Casting and Inverted Shell Procedures:

The precasted concrete panels can be obtained from the flexible formwork. The fabric formwork can be used to produce two basic types of concrete panels such as direct casting and inverted shells. In the direct cast panels, the concrete moulds are formed by the sandwiching fabric between the two rectangular frames. Firstly, the lower frame has the intermediate supports place inside it. The lower frame is X- shaped intermediate supports. The fabric membrane is than prestressed between the lower and intermediate supports.

Finally, the upper frame is placed over the membrane and than aligned with the lower frames. When the concrete is placed in the mould, the fabric form bends downwards and creates three dimensional tension curves between the available supports. Using the direct casting method, a single membrane can be moulded to form different varieties of designs by simply changing the design of the intermediate supports. The produced precasted frames with different designs can be used as various building components. The direct cast panel can be used as moulds to produce light weight shaft panels with compressed shells are caste from the mould. The panel has a minimum thickness of 38mm at the apex and a maximum thickness of 127mm at the perimeter and the diagonal X beams.

The two glass fiber reinforced concrete panels can also be produced by optimising the same process of tilting the direct cast panel to produce a compression shell panel. The final obtained glass fibre reinforced concrete panel cast from the mould which significantly varies in thickness from 13 to 38mm.

Unlimited desired number of different pattern or designs can be produced from these methods. Each intermediate supports produces its own set of compression shells. The moulds can be produced by providing the compression resistance to different load patterns by the changing the loading which is used in direct cast mould productions.

This method helps in developing an architectural quality concrete finishes using the industrial concrete mixing. Also expands the architectural potential of concrete constructions. But the problem was identified that there was no suitable method for predicting the magnitude of deflections in the formwork membranes under the variety of loading and also structural behaviour of some of the structures are not examined, precise engineers may solve the problem of structural behaviour in the precast concrete panels.

(West.M, April 2004)

6.Crosswall Construction Procedures

Precasting the elements such as foundations, wall panels, floors, stairs, chajjas, water tanks are manufactured at the factory and they are directly installed on the site.

6.1

Foundations:

The foundations along with the walls up to the plinth levels can be cast by using M20 grade concrete in the factories and further construction can be processed by using precast panels.

6.2 Precast

Wall Panels:

The precast wall panels can be made of concrete with the reinforcement provided as per the specifications. The wall panels can be cast in horizontal positions and than they are lifted from the casting beds after the concrete attains minimum required strengths. They are three main types of wall elements that are solid panels, panels with door openings and panels with window openings.

6.2.1 Sandwich Panels:

The sandwich panel which involves a precast concrete outerleaf, and the choice of simulated stone finishes or facings, insulating layer and a blacking leaf of plane grey concrete. The insulation which is installed under the factory conditions is well protected by the concrete. The thickness of the insulation contained in the sandwich panels can be varied to achieve the required U-values.

The precast concrete sandwich panels are often used for the building exteriors cladding and also serves as shear walls.

The two sandwich layers are generally connected by the stainless steel connectors, which may consists of wind and the shear connectors. Several insulation types such as mineral fibre insulation materials can be used. A cavity can also be introduced if necessary. Mineral fibre insulation is environmentally friendly, fire resistant when compared to the expanded polystyrene products.

The sandwich panels may support floors, slabs and beams. The main advantage of the load bearing panels is they may not require perimeter columns and instead increases the floor area and gives flush wall profiles. The applied finish panels may include terracotta, glazed bricks, tiles, granite, and limestone. A panel may incorporate more than 1000 bricks or 100 stones.

(Dawson.S, 2004)

The sandwich panel system which includes polystyrene insulation sandwiched between the two concrete walls. The interior of the sandwich panel is thicker because several studies as shown the thermal capacitance of benefits are greater when the thermal capacitance is within the insulation barrier

(Kossecka & Kossny, 2002) The two types of precast concrete panel systems are One is "waffle" precast concrete panel system which is currently used in the light commercial and residential industry and the other is "sandwich" precast concrete panel system which represents the available wall technologies which has greater thermal performance than the waffle panel system.

The inner leaf of the sandwich panel may be used as a load bearing structural element for giving support to the floor units. This provides more efficiency to the construction process and minimises the need to integrate different trades.

Techrete Company has manufactured the load bearing sandwich panels for Dublin's city centre; the city centre was designed by O'Mahony Pike and for the first time the sandwich panel was used in a structural capacity on residential development. The sandwich panel provides very strong, durable, energy-efficient and fire resistant cladding systems.

All the panels are manufactured in the factory and they are 'Just in time' delivered to the site, they are enabled to provide very high quality finishes. Construction will be much faster and the load bearing walls panels they provide both structural support and external finish, the labour on site is minimised.

(Taylor.P.J, 1992)

Most of the precast concrete cladding system comprises of a single layered structural concrete panels which are manufactured in the factory and than installed on multi storey buildings with a weather resistant external finishes. Sandwich panel generally contains insulated material between the two precast layers.

Techrete

is one of the leading precast concrete manufacturers which has expanded the range and potential of the sandwiched panels. The Techrete company has introduced the precast panels in stone and bricks. The air cavity can be fixed in between the panels and they can be integrated as a part of load bearing structures.

6.2.1.1 Two-Wythe Sandwich Panels:

The precast concrete slabs are constructed by two Wythes of concrete which are separated by thermal insulation layer. Two Wythes panels are provided with strong concrete which enables both lifting and handling. The solid concrete may also have catastrophic impact on the thermal performance on the precast concrete panels. The research was mainly directed towards the development of precast concrete three-Wythe sandwich panel with the improved thermal performances. Often, both the concrete Wythes are of same thickness and the surface of the exterior Wythes may include the architectural panels. The panels with two concrete Wythes and one insulated layer are referred to as two-Wythe panels.

(Lee B.J,Pessiki; 2006)

6.2.1.2 Three-Wythe Sandwich Panels:

The three Wythe panel which usually as three concrete Wythes and two insulated layers, those are connected by solid concrete and they are staggered in location so that no concrete path extends directly through the entire thickness of the panel. In practice, the three-Wythe panels are evaluated by estimating the thermal resistance (R-value) using the finite element methods.

Three Wythe panel was developed to reduce the thermal bridges which were produced by solid concrete. Generally the thermal performance of three Wythe panel is evaluated by estimating the R-Values (thermal resistance) using finite element methods.

The benefits of three Wythe panels are:

The concrete connection between the Wythes allows improving the thermal performance over the two Wythe panel. The increased overall panel thickness may lead to increased span capability, this how which increases the usage of sandwiched panels. But the three-Wythe sandwiched panel may not be applicable for all scenarios because it increases the production time and production costs when compared to two sandwiched panels.

On the whole three Wythe's panels provide greater advantages in thermal performance than two Wythe panel but with higher cost and time productivities.

(LEE, PESSIKI; 2003)

6.2.2 Tilt-Up Panels:

The tilt up technique which combines the advantage of precast walls with other benefits of site casting, the size and the thickness of the panel is reduced. The tilt up construction had grown more rapidly with respect to the increase in the demands for more durable and economical buildings.

The use of WWR(wire welded reinforcement) in the tilt up panels is relatively new concept. According to concrete international there are inherent advantages and disadvantages to the use of the WWR. (Griffin.J, 2003)

WWR (wire welded reinforcement) mats are manufactured in the plant-controlled environment, which gives the correct number of bars that to be placed in the panels depending upon the additional drawings. In the fields the prefabricated mats give assurance that the bars do not bunch or free float together in the plane of reinforcement, "step- through" meshes are well maintained which offers the workers the ability to step between adjacent bars, reduction of labour on the site.

Tilt up panel can be reinforced in less time because of the labour reduction. This wire welded reinforcement(WWR) may be used in joining the precast concrete walls and may finish the work very quickly without the need of excess labour and has an advantage in reducing size and thickness of the walls in multi-storey structures.

6.2.3 Double wall precast panels:

Double wall building technology means that both sides of the wall and the floor components are form finished. The interior surfaces of the walls are dry and smooth, only single coat of paint may be required to achieve the look and feel the drywall finish. The exterior surfaces of the walls can be produced with variety of finishes and surface treatments.

Dukane Precast Company has used the double wall technique for the low-rise residential and non residential constructions. This company has built a plant geared for low cost production of roof and wall that created safest, durable and most energy efficient building systems. The double wall building method may offer significant energy conservation by recognising thermal mass properties.

The benefits of double wall panels:

The double wall panel design may be a good choice for the home buyers looking primarily in cost, comfort, health benefits. The high degree of insulation provided by the panels can permit the use of smaller heating and air conditioning units, thus may save monthly operating costs of the house.

(Concrete Products Staff, 2002)

6.3 Floors/Roof Panels:

6.3.1 Hollowcore Floors:

Hollowcore and prestressed floors are also commonly used as floor slabs in multi-Family housing, schools, and hotels, offices which may take an advantage of span to depth ratios, high load carrying capacity, fire ratings and speed of construction. The hollow unit reduces the self weight of the slabs. These floor units may be available in 1200mm in widths & depths from 110 to 400 mm.

The hollow core slabs for the residential buildings may have very good span capabilities (short & long span). The long span is used for the car parks and office constructions and they can exhibit upward cambers. The short spans can also be provided with a layer of the expanded polysterene on the soffit to provide the insulation for the ground floor situations.

The hollowcore slabs with reinforcement can be generally 225mm deep and 1200 mm wide Termodeck Company is more specialised in providing hollow core units.

(Borghoff.M, 2006)

The hollow core wall panels can be installed with or without insulation. The floor units can be provided with the polysterene or poly-isocyanurate (PIR) insulation material.

The benefits of using hollowcore floor units are as follows:

It may include high load carrying capacity, long spans, durability, erection speed, providing instant working, and very good thermal and sound insulation, providing the floor with fire resistant properties without the need for the fire protection treatment.

Hollow core units may be the ideal building material for the construction of ware houses, manufacturing plants, schools, retail stores, office buildings and administration buildings. The use of pre stressed hollow core units and the solution which may enables fast construction and it is cost effective because the secondary fire resistance treatment is not required

Precast hollowcore floors are designed with up to 4 hours fire resistance by using tabulated data that gives minimum dimensions for the depth of concrete cover to the prestressing strand or wires as well as overall depth of the floor slab to be used. (Norman E Brown, Head of Engineering Services – British Precast
Secretary – Precast Flooring Federation)

6.4 Transportation:

The precast concrete panels can delivered to the site over the highways by semi trailer trucks. A few can be shipped by rail or other modes of transportation depending upon the feasibility of resources. The precast plants may not restrict the size and weight of the precast panel production if there is ease of transportation. The use of light weight aggregate concrete panels can minimize the impact of weight on shipping, handling and erection operations.

The cross wall building units are very easy to place and install them from the factory components like wall panels, bathroom pods etc. The construction procedure is done in 5 stages.

In the first stage, the flat packs are delivered on to the site directly from the factory precast. In the second stage the components are lifted from the vehicles and they are delivered where required. Third stage, bathroom pods are placed into the rooms.

Fourth stage, floor slabs are rested on the rooms. At the same time the stair cores and lift cores are erected.

This is the way; all the floor and slab units are installed in medium and high rise buildings.

The above figures which clearly shows how the floor units are transported and lifted from the vehicles. The bathroom pods can also be manufactured with the precast concrete panels. The structure of the bathroom pods may generally consists of very thin concrete walls and floors.

The electric wires or pipes can be installed, after the concrete has been caste, the above fig. shows that bathroom is fully fitted with all the requirements and it will be ready for use. The precision engineered in factory are incorporated with the bathroom pods. The pods are fixed together with few joints for quick sealing.

The precast panels should be delivered to the site as indicated in the erection drawings, with a date of production. The precast panels must be selected from storage, loaded and delivered in a proper order to meet the predetermined erection sequences. If the panels are shipped horizontal, they should be well supported at two points with the supports located at the fifth points of the long dimension to avoid the excessive stresses which may be induced by twisting, racking of the trailer. The manufacturer should determine that a protective covering is given to the precast concrete panels or not before it is rested on the vehicle. The factors such as size, shape, type of finish, type of aggregate, the method of transportation, road conditions, and distance should be carefully inspected. The panel sizes mainly depend upon the plant capability, distance of the job site, highway conditions. For maximum economy, panels should be limited to a height and width of 8ft and a weight of 20,000 lb to allow two moulds on the truck. In some areas they allow height up to 13 ft 6 inch is allowed without special permits. (ACI Committee, 2000) Initially the balcony units may be temporarily supported until the floors as been placed and the floors may offer sufficient strength to withhold. Balcony units are also incorporated with the drainage facility and provided with the tiled floors and other fittings for the balustrades. The precast panels can also be used for terracing. The precast concrete floors may provide very strong, durable and versatile terrace unit that can be quick and very easy to install

6.5 Installation:

The floor units, bathroom pods, staircase are precasted and carefully erected in the required positions. During crosswall construction, the risks encountered are low while erecting the precast slabs and floor slabs and other equipments. Health and safety regulations are more taken during the work progress.

6.5.1 Staircase:

On time delivery of the precasted stairs and approximately zero wastage of materials keeps the site neat and tidy, more cost effective and reduction of unwanted materials on the sites leads to more space free on the congested sites.

The above figure shows the staircase after erection. . The precast concrete stairs in multi-storey buildings may be cost effective because the greater the number of units lesser may be the price. The precast concrete lift cores may provides safe access routes during the construction process. They remove the on site shuttering need and instead provide very quality finish.

6.5.2 Slab Panels/Floor slabs:

The floor slabs are transported from the factory and erected on the site.

By transporting concrete from a conical chute to fill the joints between the precast floor units is very difficult, intensive labour and wastage of material and can be risky. The problem was solved by the Aat van Baarsen, a crane engineer from Dutch as invented a robust site operating joint filling cart. The rubber wheeled steel cart which is known as 'kiervuller' which has a third wheel for filling the grooves. (Holdsworth.B, 2002)

The cart is generally shaped side plates to utilise the concrete flow. It is provided out with a simple hand controlled gate flap at the end of the nozzle which allows the concrete mix to be precisely positioned into the gaps and fills the voids. Site safety is optimised and the crane mobility problems are solved and there is 40% increase in profitability.

This is the way the 'kiervuller' is being used for filling the joints on the prefabricated concrete deck in building construction. (Holdsworth.B, 2002)

6.6 Quality Control and Assurance:

Quality assurance and control programs can simplify and improve the interrelation among the owners, engineer architects, contractors. In this program, the reinforcement, forms, embedment should be thoroughly inspected before placing the concrete panels. The face finish of the panel should be inspected after the panel is properly cured. The panel finishes should be thoroughly inspected with respect to the project needs for colour and texture before shipment. The people who are involved in the production and inspection should be fully familiar with the specifications, this helps to avoid confusions and complicated plant procedures. (ACI Committee, 2000)

7.0 Cross Wall Multistoried Building:

It consists of flooring and load bearing walls, the walls supports the floors and also the structure above.

In multi storey buildings, lateral stability is provided transversely across the building by the cross wall systems and longitudinally by the stairs and the lift shaft cores which are also formed by precast wall panels. This type of construction is ideal for buildings of cellular structures for example apartment blocks. The use of crosswall in apartment construction has been increasing day by day.

The ASHRAE Transactions gives information about the use of concrete frame panels in residential construction such as houses when compared to wood frame panels, how they are optimised in the innovative constructions and how they are far better than the other wooden frames. The transactions explain about the applications of equivalent concrete wall method to compare the thermal performance of concrete construction against wood frame systems.

Balconies can be also provided in crosswall multi storey buildings mainly in apartment complexes. These balcony units may have steel reinforcing bars which are projecting from the back and tie in with the steel reinforcement in the concrete floor structures. (DENMAN.C and BUDGE.C; 2006)

In the crosswall multi-storeyed structures the wall are usually designed for the means of primary support. The longitudinal stability to the building is obtained from the external concrete panels, floor diaphragm, roof diaphragms and they are connected to the staircases which may also be formed from the shaft units. According to concrete centre, the above system may provide structurally efficient building with the main walls which offers very high degree of sound insulation between the adjacent rooms or cabins. The multi storied buildings can be provided with the hollowcore floors and prestressed floors. (GIBB.A, 1999)

7.1 Hotels:

Hotel construction may use a technique of Flat Pack Building Components which is referred as cross wall construction. It comprises of vertically cast division walls with sandwiched panel constructions.

Pre cast structures limited may have good experience in hotel construction covering the broad products and services of direct supply, including

  • Thin coat decorative wall and ceiling finishing.

  • Bathroom pod supply.

  • Service distribution.

  • Cladding and rendering finishes.

All the floor units are tied together using a series of hidden joints so all the joints are covered and made only visible from outside the structure. Roof stresses, external cladding, brick layers, windows are installed as soon as the ceiling at the roof level is erected.

8.0 Armature Crosswalls

1)

Male and female demolition workers on collapsed RC infill building in Bhuj,

2001 one month after the Gujarat Earthquake. Women work alongside men in heavy construction tasks in India.

2)

Bare Frame of incomplete building next to partial collapse in Bhuj, 2001. Bare frames, even if weak and poorly constructed, often do better than expected in earthquakes that happen before the infill is installed because the buildings are lighter than when finished, and frame action can take place.

3)

Collapsed steel frame infill wall building in Bam, Iran, after the 2004 earthquake. Many light frame buildings with infill masonry collapsed in the Bam earthquake largely because of defective welding and poor layout that resulted in torsion.

At the 13th World Conference on Earthquake Engineering in August, 2004,Fouad Bendimerad Director of the Earthquakes and Megacities Initiative reported that approximately 80% of the people at risk of death or injury in earthquakes in the world today are the occupants of reinforced concrete frame infill-masonry buildings. Concrete frame buildings with masonry infill-walls (RC infill) are commonly constructed with brick or hollow block masonry partitions and exterior walls.Thousands have died in this type of building inearthquakes in different countries around the world, including recently in Turkey and Taiwan in 1999, India in 2001 and Morocco in 2003. In Iran, where light steel frames are used instead of concrete, these infill wall buildings also fell down in Bam in 2004. How can it be that a technology of building construction based on these promising new strong materials of steel and reinforced concrete could ultimately be connected to such deadly catastrophes? Is it sufficient to explain such pervasive and repeated calamities by pointing a finger only at bad quality construction practices Instead, it may be possible to find the precursors leading to this problem in history.

8.1 Steel Skeleton Frames:

Modern skeleton frame construction evolved from iron frame construction of factories and exhibition buildings that originated in the late 18th century and developed through the 19th century, but it was not until the first efforts to construct tall office buildings in Chicago and New York City that the iron frame was extended into the masonry exterior walls of multistory buildings. The early skyscrapers were infill- frame buildings with the exterior and interior masonry walls enclosing and infilling the steel frames . Carl Condit(1968) states A puzzling feature of the first skeleton frame "skyscraper," the Home Insurance Company Building in Chicago was the total absence of wind-bracing, an omission that the designers may have defended on the ground that the masonry bearing members and the heavy masonry cladding on exterior columns and girders proved sufficient rigidity for the whole structure. As a matter of fact, the view persisted for nearly a decade that buildings with relatively extensive horizontal dimensions needed no internal bracing."This was a practice that would later meet with criticism, and steel braces quickly became common, but the masonry infill and cladding also remained, even though it was not included in the engineers' calculations. The first earthquake test of the capacity of the early steel skeleton frame buildings was the 1906 San Francisco earthquake and fire. A significant cluster of Chicago skeleton frame buildings had been constructed in San Francisco in the decade before 1906, and post-earthquake reports prove that their performance ranged from good to extraordinary. None collapsed from the earthquake, and damage in many was minimal. The masonry of the partitions and façades, as would be expected, showed the most disruption, but the steel frames were rarely found to be damaged

Flatiron Building in NYC under construction showing masonry started at 7th floor level and at base at same time. (1901)Chicago Frame Bldgs in San Francisco after 1906 earthquake. All standing buildings shown are still extant today.To understand why this history is significant, it is important to take note that the design effort for wind forces was then, as it is now, aimed to provide enough resistance to avoid damage. For large earthquakes, however, the forces are too large to have any reasonable expectation to be resisted without damage. Even today, code conformance is based on an expectation that damage will occur, and so the damage that was found in the infill-masonry from the 1906 earthquake means that the masonry must have helped protect the underlying steel frame from damage that it otherwise would have sustained had the masonry only been dead weight. Even in the case of high winds, this was later proven to be true. A 1938 report on the brick and stone clad 1931 Empire State Building in New York City, reveals that the infill masonry at the 29th and 41st floors cracked during a storm with wind speeds of 90MPH. Strain gages that had been placed on the steel only began to register strain in the steel frame after the masonry cracked, thus showing that it was the "non-structural" masonry infill that bore 100% of the lateral loads until cracking. (Rathbun (1938) reported in Eldakhakhni, 2002) In many histories of the modern skyscraper from the Home Insurance Co. Building to the glass curtain wall towers today, one reads a story of seemingly marvelous progression until, in all its glory, the skeleton frame emerges out of its antiquated masonry jacket. As Sigfried Giedion, the principal historian of the Modern Movement, said the ornamentally accentuated play of load and support [in the] nonsupporting exterior walls [of early]…American Skyscrapers…is an embarrassing farce. Giedion, 1928 Here Giedion is focused on the establishment of a new architectural style for low and mid-rise building that is to be based on the open skeleton frame.Again, Giedion in 1941 it took more than half a century before the importance of the iron skeleton for apartment houses was recognized. The conclusion to be drawn here from theconstruction is: fixed interior walls are senseless in this type of construction (Giedion, 1941) Domino frame as ideal structural form by Le Corbusier1915. A massive RC frame in Golcuk, Turkey under construction at time of1999 earthquake before installation of infill masonry walls. Much greater damage or collapse would have been likely had the infill walls been installed by the time of the earthquake.Thus "Modern Architecture," embraces skeleton frame construction as the basis for radical changes in architectural expression Giedion did not recognize, and few engineers of his time and since have given credit for, is that it may have been the non supporting masonry walls that may be the reason why most of the pre-1906 steel frame buildings in San Francisco are still there today despite being hit by the earthquake and then totally burnt out. To fully understand this conceptual shift, it is worth placing the early skyscraper infill frame construction into its true historical context. Rather than seeing it only as a transition to a new form of honesty in architecture, it is more accurate to place it into the context of more than 2000 years of construction history, where timber frame with masonry infill has been used from well before the Roman Empire right through history until the present. Regional manifestations of the timber and infill masonry construction have been called colombage in France, fachwerk in Germany and of course half-timber in Britain. In Turkey it has been called himis

But This form of construction relate to the Chicago Frame buildings in San Francisco in 1906 The answer is best found in considering the example of Lisbon after the great earthquake, fire, and tsunami of 1755. In planning for the rebuilding of Baixia, the completely destroyed area of central Lisbon, the Marquis of Pombal gathered a group of engineers to determine the best manner of earthquake-resistant construction to use for the rebuilding. The type of structural wall construction selected has become known as the Pombalino system. It is essentially a well-braced form of half-timber construction, the use of which was probably inspired by what Pombal's engineers could see had survived the earthquake. The significance of the Pombalino system lies in the fact that it was deliberately developed and selected as earthquake-resistant construction for a major multi-story urban area Pombalino walls in Baixia, Lisbon, 2003 House in Golcuk showing himis construction. This house was abandoned and in ruins at the time of the Kocaeli earthquake in 1999, but remained standing with little additional damage from the earthquake.In recent Portuguese Government sponsored lab tests on actual wall sections removed from a Pombalino wall, the wide hysteresis loops show that the walls were able to dissipate large amounts of energy over many cycles, without losing their structural integrity. The sample remained largely intact despite having been cyclically pushed beyond what would be expected from an earthquake .Sections from a Pombalino wall that was cut out and tested in the Portuguese Govt. lab. The hysteresis loop from the test of the Pombalino walls showing extensive ductility and reserve strength. In Turkey, 20th Century examples of himis, another regional form of half- timber construction, survived on average much better than the modern reinforced concrete buildings around them (Langenbach

,

2000) In other earthquake areas, variations on this construction system can be found with a history of resilience in past earthquakes. The way these pre-modern infill wall buildings responded to the earthquake vibrations is not unlike the way that the Chicago Frame structures must have behaved in the 1906 earthquake – with the masonry confined by the steel frame, working together with the frame and also protecting it by cracking and dissipating energy without falling out.

While steel frame construction emerged in the 1890's for highrise construction, reinforced concrete construction made its debut for low to mid-rise construction. It was reinforced concreteof which Giedion speaks when he describes how Le Corbusier develops the new housing function from the ferroconcrete skeleton frame with a thrusting boldness that has enriched all of architecture The shells fall away between interior and exterior Air becomes a constituent factor! There is only a single, indivisible space. (Giedion 1928) Freestanding columns supporting slabs of concrete became the basic structural system for housing that spread, first through Europe, and then over the rest of the world. As long as the placement of the infill walls was to be left up to the architect, then the engineer thought they could be ignored in his calculations. The problem is that masonry walls did not actually disappear. In reality, even the most open residential floor plan required many walls, and the most practical fire resistant material for them

is masonry. From the robust walls of the early skyscrapers, the walls evolved into weak and loose single-wythe membranes of brick or hollow clay tile. One is forced to wonder why the RC moment frames, rather than shearwall design, became the default system for residential buildings in most places because, in earthquake areas, they are so unsuitable for buildings that are permanently divided into small rooms anyway. The added cost of shearwalls is only part of the explanation. A more compelling reason may be the force of the Modern movement that began in the 1920's and '30's that led first to its firm establishment in areas where earthquakes are not a risk, such as France and Germany, and then its dispersion to areas that have suffered from pervasive problems in construction quality together with great earthquake risk.

This story brings us back to the present, with the vast problem referenced by Fouad Bendimerad at the beginning of this article. This problem is expanding daily, as more vulnerable buildings continue to be constructed in cities around the world. The proposal outlined below for "Armature Crosswalls" is founded upon a simple practical premise: that it is necessary to recognize and accept the fact that defective RC infill buildings will continue to be constructed, despite the best efforts by both engineers and public officials to improve RC design and construction practices,so additional safety measures are required.

8.2 Armature Crosswalls:

Instead of the existing method of constructing the infill walls totally out of hollow clay tile or brick, the Armature Crosswall proposal is that they be constructed with studs and cross-pieces of timber, steel or concrete. These studs and cross-pieces (the 'armature') would be securely attached to the primary frame of concrete, and the bricks would be tightly packed into the 'armature.' The mortar to be used for this construction would be a high-lime mix intended to be less strong, stiff, and brittle than ordinary cement mortar. When finished, the wall would be plastered as it would normally. The intention is that these walls would have less initial stiffness than standard infill masonry walls, and the studs would also serve to reduce the development of the equivalent diagonal strut.This system is based on an approach where all parts of a building's fabric are to be regarded as"structure," so that the ductile behaviour that cannot be assumed to exist in the underlying concrete frame can be achieved through the energy dissipation provided by the controlled degradation of the infill walls. The danger of a soft story can be reduced or avoided using the Armature Crosswall system because (1) the crosswalls can be extended to the ground more conveniently than shearwalls because they do not have to follow such a rigid system of lining up with the walls above, and (2) the reduction in the initial stiffness of the walls at all floor levels allows frame action to occur in the superstructure frame because it can sway within its elastic range before the crosswalls begin to bind. This sway is then restrained when the crosswalls begin to shift and crack along the interface with the 'armature,' which dampens the building's response and dissipates energy. Then, as they yield and degrade, they shed load to other crosswalls so that all parts of the building function to support and supplement the frame. This can be termed a "sequential degradation" approach to earthquake resistance design. (Langenbach, 2003) Thus, 'Armature Crosswalls' are intended to address the initial stiffness, diagonal strut formation, out of plane collapse, and energy dissipation issues that exist for RC infill buildings. The purpose is to make the infill walls into a productive part of the overall structural system, in a way that turns what is now a problem into an advantage. This approach to mitigation is based on the assumption that low to mid-rise buildings will continue to be constructed with the same palate of materials as are currently used, and that the RC frames themselves are most likely to continue to be unreliable.

9.0 Case Studies:

9.1 Case Study 1:

Buchan has a 'cross-wall' solution that offered Carrillion contractors to construct student accommodation at University of west of England, Bristol. The University of west of England has invested Ł70 million for 'student village' houses in a series of accommodation blocks. The university wanted to reduce the operatives on site, they have chosen offsite prefabrication. The offsite production has offered various components such as bedrooms and the quick installation of the components has given the opportunity for the external fittings and decorating the walls which balanced the time for precast cladding panels. The factory pre cast was delivered to the site 'just in time' and than all the components were fixed at the rate of approximately 4000 to 6000sq.m per week. The Carrillion has used a new type of solid unit precast flooring for the accommodation project. By giving zero camber the 200 and 225mm thick units were prestressed uniformly. The electric floating to the floors in the factory was given which resulted in giving very smooth finish to the floors. The savings to the project was by neglecting the cost and time associated with the screeding.

The floor units were pushed down by 100mm for placing the bathroom pods on it. After placing the walls, floors, bathroom pods than the electrical and mechanical work as started. The precast was delivered onsite in April that was behind the schedule by two week. This delay in construction was compensated by swift installation and the project was ahead of schedule by two weeks.

By using crosswall in this project the wet trades were completely eliminated in the project. The crosswall construction has provided safety and good quality solutions, fire resistance, high thermal mass and good acoustics on the congested site. (The Concrete Centre, 2007)

9.2 Case study 2:

Bell and Webster has a 'cross-wall' solution that offered Jarvis construction to construct ibis hotel in Wembley. The hotel involves 14 storey and 210 bedrooms above the in situ concrete foundation. The client has selected crosswall because it provides excellent finish and the client has experienced the precast in his previous hotel constructions. This crosswall system has allowed delivering the components on specified time and to an estimated budget. The first floor of the hotel was laid by in situ concrete and than the rest of the floors include lift cores, wall panels, floors, were incorporated with precast concrete. On whole 1265 precast components were delivered directly into the structure.

The wall units were positioned and the bathroom pods were delivered to the place where it has to be installed. The bathroom pods were delivered into the structure with all internal fittings such as shower, toilet and vanity units. As soon as the wall panels, bathroom pods, floor slabs were installed, the mechanical electrical work has been carried out. After the internal fitting have been done the cladding work was started. Since the structure had the external walls, the only penetration was through the top slab (vertical risers). The cladding components were installed into the structure by transporting through cranes.

The structure had a high thermal mass, and the entire concrete surface in the rooms was highly fabricated and they were ready for minimum decorative treatments. The decoration to the concrete panels was started from the first floor using the spray and it was applied on the concrete floors directly.

The crosswall in this hotel construction has brought speed in erecting components, higher quality components, high thermal mass, acoustic performance was proved and enabled to complete 210 bedrooms in only 26 weeks. This cross wall system has given the ability to finish the precast components by bringing major benefits in time and cost to the client and also minimised the maintenance cost of the hotel. (The Concrete Centre, 2007)

9.3 Case study 3:

Bison concrete products have a "crosswall" technique that offered bullock construction to build Residential Apartments consisting seven-storey block of 77 flats. The crosswall construction was chosen for constructing apartment project because the crosswall provides excellent part E performance and sound transmission benefits. The cost and value assessment was done and they found crosswall wall was more favourable. According to architect Simon Oakley the timber frame was not competitive as concrete frames for seven storey structure. On whole 7246 sq meter of crosswall and 6640 sq.meter of hollow core floor, precast concrete staircase was optimised in this residential project.

The precast concrete components which were delivered to the site were perfectly scheduled to fit in with the construction sequences. The crosswall team has worked very fast and accurately craned 488 prefabricated wall units and safely fixed to the final position in 16 weeks. Optimising crosswall, the seven story building has achieved external cladding insulation and insulated concrete panels. Cross wall helped in speeding up the construction process, saved time, and increased project productivity, excellent internal finishes. Cross walls has reduced the wet trades and produced very early dry boxes for quick construction.

(The Concrete Centre, 2007)

9.4 Case Study 4:

City and Industrial Development Corporation Ltd (CIDCO) have the crosswall technique that offered to construct twelve multi-storeyed buildings for Maharastra Housing and Area Development Authority (MHADA) at Jogeshwari in city of Mumbai by covering an area of 11,148sq.m (12000sq.ft). These prototype buildings are constructed for the low income category houses under slum rehabilitation authority.

The crosswall technique was used because of its remarkable short completion period compared to other cast in situ systems. The crosswall system was used because very little site work is involved and small workforce is required at site such as labour camps, godowns for storage of material and equipment. The precast wall panels for this project was made of M20 grade concrete with reinforcement provided as per requirements. The wall panels are cast using the long line method in 150 m long precasting bed. The long panels are cast in horizontal positions and than lifted from the casting beds after the concrete as attained the minimum strength of 10MPa.The roof panels or the floor panels were 2m wide and spans up to 4.5m.

Utmost care was taken for handling the precast units during transportation and erection. The precast panels were transported through trailers and trucks and were supported by 'A' frame during transport. The floor and roof panels were transported in horizontal position in trucks and trailers. The polymer-modified cement mortar bed was provided before placing the precast elements in their correct positions.

The construction time with this crosswall system was almost 50% less when compared to conventional system because the large panel precast construction requires no scaffolds or no forms or no construction material like shuttering, concrete reinforcement at the job sites.

For this project very few subcontractors were required at the site and this crosswall system has allowed other trades to follow immediately while the precast assembly was still under way.

(A.B Shah, 2005)

10 Summary & Recommendations:

The crosswall construction is widely used in repetitive structures such as mass housings, hotel, and prison constructions. This produces several benefits such as the reduction in construction time and produces good quality buildings. The crosswall construction is used both in India and U.K. While it is used only for the construction of mass houses in the former, it is used for hotels, apartments, student accommodation and prison construction. The precast panel system provides economy, speed & quality in massive projects.

Studying the newly launched technique in India which is being practiced extensively in the United Kingdom will give an insight onto how the process can be improved at its beginning stage. Due to the high sustainability of the technique, this study will pave way for expanding the technique to other countries where constructions of mass buildings are on the rise.

Precast wall panels were used in the past and present in various areas starting with the prefabricated wall and floor units.Some of the authors have explained how the crosswall technique is more advantageous than other conventional methods.This literature review gives clear understanding of the use of sandwich panels, hollowcore slabs, and tilt up panels. The case studies of crosswall construction in India & U.K give the broad view and similarities in the use of this cross wall technique.To make the large panel precast system widely acceptable in Indian construction sector, the government tenders should provide alternate designs with the large panel constructions. This might encourage the use of this system through new entrepreneurs.

The continued creation of bad buildings is not so much an engineering problem as it is a socio-economic problem in the "building delivery" process. As long as this is the case, efforts to solve the problem only with the dissemination of technical reports for engineers may best be described as a "Marie Antoinette approach to Earthquake Hazard Mitigation" from the quote "Then, let them eat cake" attributed to her when there was a bread shortage in Paris. If money, materials, or trained engineers necessary for proper detailing, good quality steel and concrete, or tools for mixing and vibration of the wet concrete, or water for hydration, are simply unavailable, then the chances for safe reinforced concrete frame construction will continue to be remote. Armature Crosswalls are not intended to displace good design, but rather add to it – and to be there when nothing else works. They are based on an empirical wisdom that has been passed down through the ages and will probably defy most attempts to turn them into a system that can be fully calculated. One architect said to me recently: For many engineers, if it cannot be calculated, it does not exist." Here is one example where the time has come, after years of collapsing buildings, to borrow an idea from the past that was not meant to be calculated, and tohave the humility to accept the fact that nevertheless it may work now and in the future.

11. References & Bibliography:

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