Reinforced Concrete Construction Low And Medium Rise Residential Engineering Essay

Masonry is referred to brick and masonry construction started twenty thousand years ago in the “Stone Age” and through history people used clay to build their houses by drying the clay in the sun till it is hardened and form clay masonry.

They used masonry in building bridges, domes, and churches. In the eighteenth century a great improvement was made to masonry construction especially in bridge design and the parabola was then invented, moreover eighteenth century saw a great output of iron and new methods of production were invented. Cast iron columns and beams were made to support a brick floor, but masonry construction was taking the lead in huge construction projects and buildings. (Crozier, 1999)

During this time understanding of materials was their main objective and mathematical analysis of structures. Studying the thrust line and making sure it remains in the middle third of the masonry element cross section. In the early of the nineteenth century masonry bridges and viaducts were constructed to support the railways expanding. (Crozier, 1999)

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The latter of nineteenth century reinforced concrete was developed and engineers found that it is easier to build bridges using reinforced concrete than masonry and gives them more flexibility in shaping the bridges. Masonry buildings was still used in buildings but due to its’ huge wall thickness was not preferred in high buildings, for example a building can be six stories with a wall of two meters thickness which was not economical and takes a lot from the building area. (Crozier, 1999)

At this time masonry was used in other ways as for example cladding of huge area of steel frames and decorations for the outer Skelton of a reinforced concrete building. Later engineers started to learn more about masonry and started to develop its’ structural design, by this development the wall bearing masonry was used to develop residential buildings with smaller thickness with number of stories up to fifteen. Engineers understood the concept of forming plastic hinges and how important are joints between elements. (Crozier, 1999)

1.2 Types of loads subjected on masonry structures

Masonry walls are similar to concrete which is designed and built to carry only compression forces and they are weak on the tension side, that’s why lateral loads cause a huge threat on masonry building (wind load and earthquakes) there are two main types of loads; permanent like dead load, semi permanent as live load and transient load as earthquakes and wind load.

Engineers developed the reinforced masonry construction to lower the risk of tensile forces subjected to masonry buildings, moreover designers studied the arrangement of bricks in planar walls to increase the strength of the bricks in resisting permanent and semi-permanent loads. (Crozier, 1999)

Loads transferring in masonry elements is a main objective for the designer to develop a durable and safe structure, therefore a designer must ensure a number of load paths through the structure to ensure safety if accidently any supporting element is damaged for example, when a column accidently destroyed a new load path must be offered for transferring loads through beams. (Crozier, 1999)

1.3 Examples on masonry structures

Brown County Jail

Figure 1.3.1: The brown county jail (1902) in USA

picsBridge [Alcantara].gif

Figure 1.3.2: Bridge at Alcantara in Spain (105 AD) (Crozier, 1999)

picsVault.gif

Figure 1.3.3: Masonry Vault (Crozier, 1999)

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Figure 1.3.4: Giza pyramids (2560 BC)

1.4 Advantages and Disadvantages of masonry construction

Advantages

Disadvantages

When well designed provide attractive, permanent and durable structure.

Masonry need a lot of very skilled labour to finish the job which might not exist unlike concrete where very skilled labour is not important.

When constructed there is no need for finishing with plastering or scraping.

Load bearing partitions and floors must all be done on the same time that is why engineers are always on a very critical path unlike steel and concrete.

Great choice for cladding as it provides a good weather protection, and great sound, and thermal insulation and fire resistance.

Masonry can be cracked easily through settlement of foundation on unstable soil.

People prefer masonry because it is warm and friendly unlike concrete.

As mentioned before masonry is weak in tension and cannot handle strong lateral loads like wind and earthquakes

Masonry has varieties of units and sizes depending on the masonry structure needed to be built.

Masonry is hard to clean unlike concrete.

Masonry provides an economical alternative to concrete in load carrying and provides cladding to large area such as factories made of steel frames.

Masonry is very critical in detailing to ensure safety and to be economical.

Masonry is fast in building and with skilled labour it can be finished faster than concrete.

Table 1.4.1: Advantages and Disadvantages of masonry construction (Crozier, 1999)

2) Materials and forms of masonry:-

2.1 Structural forms of masonry

-There are various types of masonry structural forms as piers which are designed to carry only compression force and they form as a part of the wall. Single leaf wall which is a planar wall made of hollow or solid masonry in different alignments and thicknesses and has different types as straight and zigzag leaf wall, steel are aligned horizontally in bed joints and vertically in cores of the hollow masonry. (Crozier, 1999)

Figure 2.1.1: Planar wall type (Crozier, 1999)

-Diaphragm walls which consists of two brick walls with a crossing beam and can be reinforced or unreinforced depending on the subjected loads. (Drysdale, Hamid, Baker, 1999)

Figure 2.1.2: Diaphragm wall (Drysdale, Hamid, Baker, 1999)

-Cavity walls are two same or different brick walls spaced and connected to each other wall ties made of steel and the cavity is then filled with grout and reinforcement to form the wall. (Crozier, 1999)

Figure 2.1.3: Cavity wall (Crozier, 1999)

-Veneer walls which mainly resists lateral load are formed to brick wall connected from its’ back with concrete or timber frame using steel ties. (Drysdale, Hamid, Baker, 1999)

Figure 2.1.4: Veneer wall (Crozier, 1999)

-Columns are vertical member built with either clay bricks or concrete blocks, they are designed to carry compression forced and built as a separate member from the wall. Column can be unreinforced if it is only subjected to vertical loads, but in case of bending on any of its’ principal axis reinforcement is essential. (Drysdale, Hamid, Baker, 1999)

Figure 2.1.5: Masonry columns (Drysdale, Hamid, Baker, 1999)

-Masonry beams are horizontal member used to carry loads upon an opening or a lintel which will be discussed later. Masonry beams are always reinforced to be able to resist tensile forces which come from bending moments as in concrete, lintels are constructed using special types of concrete masonry units or concrete hollow blocks and then grouted over the reinforcement. (Drysdale, Hamid, Baker, 1999)

Figure 2.1.6: Masonry beams (Drysdale, Hamid, Baker, 1999)

2.2 Materials and masonry units used in masonry construction

-There are different types of masonry units used in construction depending on the structure and the load applied from the structure or from nature. Starting by basic which is the clay masonry which can be solid with mortar or cored with reinforcement and grout, clay masonry is clay dried by fire and left to harden which relatively produce high compressive strength. Clay bricks can be found as solid or as hollow. Clay brick may be reinforced to increase its’ capacity to resist compressive loads. (ECP 204, 2005)

Compressive strength for solid and hollow clay bricks

Type

Solid clay brick

Hollow clay bricks

dimensions (mm)

Length

Width

Height

Length

Width

Height

Mm

mm

mm

mm

mm

mm

250

120

60

235

120

250

250

120

65

250

120

95

250

120

115

250

120

130

Compressive strength (N/mm^2)

Average five bricks

one brick

Average five bricks

one brick

Load bearing bricks

Not less than 8

Not less than 7

Not less than 2.5

Not less than 2

Non load bearing solid brick

Not less than 4

Not less than 3.5

Non load bearing vertical hollow brick

Not less than 4

Not less than 3.5

Non load bearing horizontal hollow brick

Not less than 3

Not less than 2.5

Based on the Egyptian code of practice (ECP 204, 2005)

Table 2.2.1: compressive strength for solid and hollow clay bricks

-Concrete units which is simply made of sand, aggregate, cement and water and its’ compressive strength can vary depending on the raw materials used, and size of the block and it is relatively high. It can be found as solid or hollow, and can be reinforced to carry tension loads or to increase the compression resistance. Concrete unit has high compressive strength but by adding mortar the compressive strength of the whole prism decreases. (ECP 204, 2005)

Compressive strength for solid and hollow concrete blocks

Type

Solid concrete brick

Hollow concrete block

dimensions (mm)

Length

Width

Height

Length

Width

Height

mm

mm

mm

mm

mm

mm

250

120

60

400

100

200

250

120

120

400

120

200

250

120

200

400

120

250

400

200

200

400

250

200

Compressive strength (N/mm^2)

Average ten bricks

one brick

Average five bricks

one brick

load bearing solid units

7

5.6

7

5.6

load bearing hollow and cored units

5

4

5

4

Non load bearing solid units

2.5

2

2.5

2

Non load bearing hollow and cored units

2

1.6

2

1.6

Based on the Egyptian code of practice (ECP 204, 2005)

Table 2.2.2: compressive strength for solid and hollow concrete blocks

-Mortar is a very important type of materials used in masonry construction; mortar is made of cement, lime, sand and water. It is used as a bonding material between the masonry units and also it is a good weather insulator. Mortar has different thicknesses depending on the design, but it is mostly to be used with ten millimetres thickness. Mortar is very weak in tension therefore under tension loads exceeding the tension capacity of both the bricks and mortar (prism) failure occur in mortar by splitting of mortar joint way before a failure occurs in the brick. (ECP 204, 2005)

Mortar components percentages and compressive strength

Mortar number

Mortar components percentage with respect to volume

Average compressive strength after 28 days (N/mm^2)

Portland cement or Blended cement

Hydrated lime

Sand (measured when hydrated and not compacted)

1

1

From 0.0 to 0.25

From 2.25 to 3.00 times the total volume of cement and lime

15

2

From 0.25 to 0.5

10

3

From 0.5 to 1.25

5

4

From 1.25 to 2.5

2

Based on the Egyptian code of practice (ECP 204, 2005)

Table 2.2.3: Mortar components percentages and compressive strength

-Grout is composed of cement, sand, aggregate and water, grout fills the cores and cavities of the masonry units to increase its’ gross area, and increase strength of masonry. Grout also may be used to form a bond between masonry units and steel reinforcement and it has a minimum compressive strength of fourteen (N/mm^2). (ECP 204, 2005)

Grout types, components percentages and compressive strength

Grout Type

Grout components percentage with respect to volume

Cement percentage with respect to mass

Hydrated lime

Aggregate percentage

Size of the biggest aggregate (mm)

Minimum compressive strength

Small

Big

(N/mm^2)

Grout for small thickness till (50 mm)

1

0.0 to 0.1

2.25 to 3.0

–

5

Bigger than 14

Grout for large thickness bigger than (50 mm)

1

0.0 to 0.1

2.25 to 3.0

1.0 to 2.0

19

Based on the Egyptian code of practice (ECP 204, 2005)

Table 2.2.4: Grout types, components percentages and compressive strength

Reinforcement form a great importance in masonry construction process, it can highly increase the strength of masonry, highly resist tensile stresses that may occur from non concentric loads and also controls masonry cracking in suitable conditions. Reinforcement is placed on the hollow units and then filled with grout to form the bond. The only concern from combining steel with masonry is corrosion, therefore it is highly recommended to plate the steel bars with anti corrosion material to prevent this process from occurring. The maximum diameter of steel bars used in masonry construction is twenty five millimetres. (ECP 204, 2005)

2.3 Lintels

masonry structures is different form reinforced concrete structures; the main elements carrying load in concrete structures are columns, beams slabs and foundation, brick walls in concrete structures act as a covering materials between columns and they do not participate in carrying load with the mentioned elements in opposite of masonry structures where loads are mainly carried through the load bearing walls, therefore openings (lintels) in these walls may reduce the ability of the wall to carry applied load and may lead to collapse of the structure. To get to a conclusion loads applied on the lintels must be well taken into consideration.

A lintel beam is the solution to carry the loads coming from the upper own weight of the wall load, loads applied from the upper roof and floors and the lintel own weight. Lintel beam is mostly designed as a simple beam and which transfer loads to the supports. Lintel beams does not carry the whole load upon it due to the transfer of loads through the wall in an arched direction to be more specific the arching action, therefore lintels carry a triangular load upon it of forty five degrees, if the vertex of the load triangle is located within the wall, therefore the loads of the masonry side will be neglected and the only loads carried are the lintel own weight and the triangular load of the wall above, but if the vertex of the triangle is upon the wall height and extended to the roof therefore the lintel must carry the full load above it. (Drysdale, Hamid, Baker, 1999)

Figure 2.3.1: Lintel load distribution (Drysdale, Hamid, Baker, 1999)

2.4 Types of masonry construction

2.4.1 Unreinforced masonry:-

Unreinforced masonry is simply constructing masonry elements without using neither reinforcement steel bars nor steel stirrups. Unreinforced masonry is mainly used in low seismic regions where lateral loads are small moreover unreinforced masonry can be only used in the construction of low and medium rise buildings to lower the effect of wind loads subjected on the structure as the case of the current research paper. The thickness of the wall of the unreinforced masonry is important for the location of the thrust line. Unreinforced masonry are mainly subjected to compressive vertical loads and this rely on the high compressive strength of different types of masonry units on the other hand tensile stresses may occur which may cause the masonry to crack and then fail, therefore tensile strength of masonry must be higher than applied tensile load. (Drysdale, Hamid, Baker, 1999)

2.4.2 Reinforced masonry:-

Reinforced masonry is simply adding reinforcement bars and steel stirrups to masonry units. Reinforcement is mainly used to resist flexure tension and shear therefore the alignment of steel bars is important, usually vertical steel bars are used in masonry wall to resist tension force produced from bending moment and horizontal steel bars are used to resist shear. Reinforced masonry is mainly used in medium to high seismic regions due to steel that is strong in resisting tension forces. Grout is an important element is reinforced masonry, and it is poured in the cores of masonry units to surround reinforcement and perform a strong bond between steel bars and masonry unit. (Drysdale, Hamid, Baker, 1999)

3) Behaviour and design methodology of masonry:-

3.1 Physical properties of masonry units

Masonry units have different shapes and sizes depending on the need of construction, but mostly they are rectangular shape with length, width and height and they are laid on its’ length dimension parallel to the in plane of the wall and it is called stretcher, the vertical front side is called the face and the two vertical side are called ends and the side which is used for laying masonry units on it is called the top. Length and height of masonry are always standard and the only variable dimension is the width of the brick, mortar used for bonding bricks together mostly has a standard thickness of ten millimetres and is placed on the top, bottom and ends side of the brick and is called mortar joints, Areas used for mortar placement are called beddings and has two types; face shell bedding which is the covering of only the face shell with mortar and full bedding which to cover the total gross area with mortar. Not all masonry bricks are solid; bricks are found as cored or hollow units as in the case of concrete hollow blocks which consists of cells which are the hollow units, face shell which is the parts of the opposite faces of the cells, end webs which are the parts at the end of the block and central web which is between the cells. A block may be called solid if the degree of solidity is seventy five percent or more and are called hollow if it is less than seventy five percent, degree of solidity is the net area of the block divided by the gross area multiplied by hundred. (Drysdale, Hamid, Baker, 1999)

Figure 3.1.1: Masonry unit labels (Drysdale, Hamid, Baker, 1999)

3.2 Behaviour of masonry prism under axial compression

3.2.1 Prism characteristics and behaviour

As to understand the behaviour of masonry elements under axial compression loading as the case of the current research paper, going through laboratory tests is the best and effective way to illustrate various behaviours. Masonry elements are composed of masonry units such clay or concrete, bonding element such as mortar, grout and when needed reinforcement these elements form an orthotropic behaviour which means that mechanical properties are different through the axis of symmetry of masonry element unlike concrete which have the same properties on each axis and this is called isotropic. (Drysdale, Hamid, Baker, 1999)

Axial compression test is made on prisms which is one unit length and thick and can be built at several heights of two, three or four units and as mentioned before with mortar joints of ten millimetres between the units, usually it is aligned in stack bond which means that units are placed on the same direction. Prisms are tested to determine the axial compressive strength of masonry on a smaller scale which is then used for creating tables used for design, and prisms is tested on a machine that applies vertical load to the prism till failure occurs and then compressive strength is then measured. (Drysdale, Hamid, Baker, 1999)

Figure 3.2.1.1: Masonry prism (Drysdale, Hamid, Baker, 1999)

Prisms under axial vertical compression have two modes of failure relative to the height. The first mode of failure is conical shear and this happens when the prism height is two times the thickness of masonry unit, but this type of failure does not happen in real masonry element such as a masonry wall because the height of the wall to the thickness is different.

The second mode of failure is vertical cracking through masonry prism and this happens when using four to five bricks height which will be the same mode of failure that happens in a masonry wall. Masonry unit under vertical compression usually tries to deform laterally and therefore it will be subjected to biaxial tension, but mortar under same case of loading tries to expand from all direction but masonry unit of higher strength than mortar prevent it from deforming and performing triaxial compression which leads mortar to burden more compressive load than its’ capacity. When the compressive load increases mortar forms tensile stresses on the masonry unit leading to vertical cracks on masonry unit and this is typically what happens on load bearing masonry wall with full bedded area. (Drysdale, Hamid, Baker, 1999)

Figure 3.2.1.2: Forces acting on masonry prism (Drysdale, Hamid, Baker, 1999)

Adding grout to masonry hollow units increases the strength because of grout carrying part of the compressive load applied to the prism, on the other hand grout put a great threat to prism strength due different material properties between masonry unit and grout, incomplete grout compaction which will create gaps between grout and plastic shrinkage which is the fast water loss of grout surface before placing which leads to cracks and this is due to low skilled labour and lack of measurements taken before and through grout placement. These factors can lead grout to produce lateral forces on the unit which will lead to the failure of the prism before it reaches the full compressive capacity. (Drysdale, Hamid, Baker, 1999)

3.2.2 Prism and factors affecting strength

There are several factors affecting the strength of the masonry units and therefore the strength of the prism. These factors depend on the height of the specimen, dimensions and strength of the unit and bonding material (mortar), and strength of the grout being used. These factors may result in increasing or decreasing the strength of prisms of different types of units under vertical compression loading, therefore taking into consideration these factors will lead to a precise measuring of the compressive strength and capacity.

The height of the unit plays an important role in determining the strength of prism, as mentioned before there are two modes of failure which depends on the height of the prism that depends on the unit height therefore tests showed that increasing the height of the unit decreases the lateral deformation of it and this due to the increase of weight and dimensions of the unit which will make it more stiff, more over when the unit strength increases the prism strength increases and therefore the prism compressive strength shows an increase, on the other hand high strength clay bricks does not effect much the strength of the prism unlike concrete blocks. One may consider that the only important parameter of the masonry unit is the compressive strength, unfortunately this is not the right case; tensile strength is of the same importance because as mentioned before failure through vertical cracks in prisms and walls occurs due to tensile stresses formed on the unit due to mortar expansion, therefore tensile capacity of the unit limits the failure of the prism. (Drysdale, Hamid, Baker, 1999)

Hollow and cored units made of concrete and clay influences the unit strength and therefore the prism strength. Solid units’ compressive strength is higher than hollow units and this due to the uniformly distributed compression stresses on the total area (gross area) of the solid unit unlike hollow units of similar dimension; stresses due to vertical compression are distributed only on the net area (area of cores subtracted from gross area) which leads to a lower compressive capacity of the unit, more over mortar bedded area which carries part of the compressive load of the prism will decrease in case of hollow or cored units and therefore the prism compressive strength will decrease, tests showed that a decrease of fifteen percent of the bedded area will influence a decrease of forty five percent on the vertical sections resisting compression stresses. (Drysdale, Hamid, Baker, 1999)

Bonding material is one of the most important parameters affecting the prism strength. Mortar strength influence the strength of the prism by increasing it to a limit, on the other hand using mortar of high strength more than needed for ensuring a durable structure will not effect much the prism strength and will affect negatively on the durability and strength of the structure. high strength units usually covers over the mortar strength, mortar is always formed on site of construction, therefore low skilled labour may not ensure the precise workability needed of mortar used in construction. (Drysdale, Hamid, Baker, 1999)

Mortar thickness as mentioned before is usually ten millimetres and increasing the thickness of mortar usually happens on construction sites due to low skilled labour, therefore clay masonry prism is affected by decreasing the compressive strength due to different material properties, on the other hand concrete prisms is not much affected by the increase of mortar thickness due to their similar material properties, therefore to overcome this conflict a standardised thickness is used. Face shell bedding in hollow or cored unit increases the compressive strength of the prism due to the lower tensile forces produced on the masonry unit. (Drysdale, Hamid, Baker, 1999)

3.2.3 Stress and strain of masonry

Figure 3.2.3.1: Stress strain curve of masonry materials (Drysdale, Hamid, Baker, 1999)

The upper figure shows the vertical stress and strain behaviour for types of masonry materials which are units, mortar and prism. It is obviously seen that masonry units can handle large amounts of vertical compression stresses due to the applied load with small amount of vertical strain and then a sudden brittle failure occurs when reaching the maximum strain.

Mortar can handle smaller amount of vertical compression stresses than the unit and a larger amount of vertical strain. To get to a conclusion mortar vertical deformation is bigger than that of the unit for the same value of vertical stress as illustrated in the above figure. When combining both mortar with units a testing prism is formed, the figure above shows that a prism can handle smaller amounts of vertical compression stress than the unit itself and this shows why the unit compressive strength is larger than the prism. (Drysdale, Hamid, Baker, 1999)

Tests on prisms showed that concrete and clay masonry have a nonlinear behaviour on the stress and strain curve. Tests showed that concrete masonry strain in bigger than that of clay masonry and studying the behaviour of masonry showed that for different types of mortars the range of strain found is close to the strain range of concrete which is from (0.002-0.003) and this shows that masonry behaviour is the same as concrete under vertical compression load. To get to conclusion masonry is a strong and economic replacement for concrete with respect to bearing of compression loads. (Drysdale, Hamid, Baker, 1999)

3.3 Modes of failure of masonry walls

3.3.1 Failure of masonry wall under vertical compression load

As discussed before in vertical compression prism test, failure of masonry prism is similar of that of masonry wall. Under vertical compression load exceeding the ultimate compression capacity of the masonry element, vertical cracks occurs due to the expansion of mortar performing tensile stresses on the masonry unit, vertical cracks is the first step of failure, and when the masonry element reaches beyond the ultimate capacity vertical splitting occurs on the masonry wall leading to a complete collapse or failure. (Crozier, 1999)

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Figure 3.3.1.1: Failure of masonry wall under vertical compression (Crozier, 1999)

To be more practical this type of failure occurs due to concentric vertical loads which means that compression load passes through the centre of gravity (CG) and this case does not ever happen in real practice, therefore due to the edges of the slab rotation on the wall face an eccentricity of load occurs and the load becomes eccentric which means that there is a horizontal distance between the load and the centre of gravity of the masonry wall called eccentricity (e). The eccentricity induce bending moment on the masonry wall creating a compression failure on the masonry element inside face, and when the tensile capacity of the wall is exceeded on the member outside face a failure occurs which is buckling. This shows that not only the compressive strength of the masonry element is important, but also the tensile strength, therefore a minimum eccentricity is always put into consideration by the engineer to overcome such failures. (Crozier, 1999)

3.3.2 Masonry shear wall failure

To be more specific this discussion is on unreinforced concrete masonry shear wall subjected to lateral loads such as wind load. A masonry shear wall act as a cantilever that is mainly fixed from the base and its’ ends are free and subjected to uniform lateral load, to study the failure modes of a shear wall, a behaviour of the shear wall must be studied with laboratory tests on specimens, mainly shear failure occurs due to the combination of compressive loads and shear stresses induced by lateral loads, more over bed joints are the only subjected joints to shear stresses in the masonry shear wall, therefore failure always occurs in the bed joint. When the compressive stresses normal to the bed joint due to vertical load is smaller than shear stresses parallel to the bed joint due to lateral load, therefore slipping of bed joint occurs and therefore shear failure occurs, on the other hand if the compression load is increased relative to shear stresses, slip and splitting of bed joint occurs. (Crozier, 1999)

Figure 3.3.2.1: failure mode of a prism under shear (Drysdale, Hamid, Baker, 1999)

When lateral loads (wind) are subjected to a shear wall it performs tension force on the close side of the wall which