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Structural Frame Construction And Cladding Of Superstructures

This study covers structural frame construction along with the cladding and finishes for this particular element of design. The areas of structure and cladding technology included in this study are as follows:

Design and detailing of wall structure and cladding.

Durability

Life span and maintenance

Problems associated with use

The design of the superstructure for a given commercial or public building depends on the function of that building. Broadly, superstructure design for commercial and public buildings can be divided into two principal categories;

Large span, low rise buildings

Multi-storey structures.

The design of the superstructure of large span, low rise buildings is principally concerned with the roof. The design of the superstructure of multi-storey buildings is principally concerned with the design of skeletal frames. In this particular study the later is the case. Often these buildings and the approaches adopted for their superstructure are completely distinct one from another, however, in many modern commercial buildings there are areas where long span roofs and multi-storey construction are found on the same site or even in the same building. Examples include roofs above courtyard areas between buildings or roofs above large atria spaces. In these cases, multi-storey construction methods and long span construction methods need to be joined together.

This study looks at the superstructure design of multi-storey, skeletal frames and investigates wall designs and systems and the technologies that are available to produce cost effective and structurally efficient multi-storey buildings. There are a number of approaches to providing multi-storey, skeletal frame buildings.

The use of framed structures results in a requirement for a separate external envelope so that the internal building environment is effectively insulated from the external environment. In the case of framed structures, this is almost always done via the use of a cladding system. This is essentially a wall structure that is not required to carry any structural loads apart from its own self weight and any imposed wind loadings.

A significant advantage that comes from releasing the external wall from structural load carrying duties is the freedom we can have in the selection of materials and the flexibility that is available in terms of their application to a framed structure. In some respects the range of approaches to cladding framed structures is limited largely to our own creativity. Provided we meet all of the necessary performance requirements the options are limitless.

Another significant substructure is concrete construction, whether it be monolithic, Precast, Insitue or any other of the numerous concrete construction combination choices. This construction method offers a high thermal mass element to the building that is chunky and durable but also has the added drawback of a longer construction time than skeletal frame construction.

Objectives

The objectives of this study are summarised as follows:

To produce a report on the analysis and performance requirements, and problems associated with structural frames.

Utilising the information available and the results of this study, to review the design and detailing monolithic concrete construction.

2.0 Literature Review ?

3.0 Structure.

The structural approach used for the art gallery is monolithic concrete construction. This helped achieve a high thermal mass (which is extremely helpful for the conservation of art displays) it also provides a strong durable structure. The functional requirements of this framed structure are:

Strength and stability

Durability and maintenance free

Fire safety

Strength and stability

The strength of the structural frame depends on the strength of the material used in the construction of the members of the frame and also on the stability of the frame, which is dictated by the way in which frame is connected and on the way the joints are constructed.

Durability and maintenance free

On exposure to air and moisture, unprotected steel corrodes to form an oxide coating, i.e., rust, which is permeable to moisture and thus encourages progressive corrosion, which may in time adversely affect the strength of the material.

Fire safety

All load bearing structures should be designed so that they do not fail prematurely during a fire. Providing the structure with the necessary fire resistance helps to reduce the risk posed by falling debris to building users, pedestrians and fire fighters.

3.2 Discussion/ Findings

The construction methods used in this structure, is all in situ concrete constructed with on-site formwork, the need for columns and beams has been practically eradicated because the walls and floor themselves act as the structure only requirering 4 centrally located concrete columns for support. This creates a very repetitive construction, particularly the technique used to create the two leaves of exposed concrete, required repetitive site practices. This shuttering technique required temporary formwork to be constructed and the inner leaf of concrete to be cast, followed by removal of the formwork. This process is then repeated for the outer leaf. Although this method of building is proven, there are more cost and construction efficient techniques in concrete construction.

Sequence of construction.

The inner leaf of the concrete wall is constructed first where the internal 250mm exposed concrete wall is cast from floor to floor using formwork on top of the substructure. The 280mm concrete floor is then cast using formwork and is connected to the wall via steel reinforcing bars. Shear pin connectors are fixed to the concrete wall which ties into the external leaf.

The 120mm polystyrene insulation is then fixed to the external face of the concrete. The external leaf of 250mm exposed concrete is cast using formwork. Then the desired finnish is applied to the external face of the concrete. 40mm impact sound insulation is laid on top of the concrete floor and edge insulation is placed in the recessed area where the wall meets the floor. The 80mm cement and sand screed is poured on top of the insulation.

Functional Purpose and Performance Requirements

The wall structure and cladding must achieve the following performance requirements. In preparing design details we must consider how each of these will be achieved.

Load Transfer

Wind loads must be supported and transferred back to the structural frame and subsequently to the building foundations.

Thermal Performance

The external wall/roof must keep heat in the internal environment particularly in the winter. The cladding system therefore must present a barrier to thermal loss. Additionally, the external wall must assist in preventing excessive solar heat gain in the summer and may be required to assist in cooling the internal environment depending on the ventilation strategy that is selected for the building.

Temperature Movement

The wall must be capable of coping with expansion and contraction forces that will be set up by cyclical changes in external temperature.

Relative Movement

The wall must be capable of coping with relative movement between itself and the beams and columns that comprise the frame that the wall and roof is attached to or suspended from.

Exclusion of external elements

The wall must prevent the ingress of moisture, either as a result of surface dampness and run off or as a result of wind driven rain, and it may have to prevent the admittance of external air.

Sound

The external wall and the roof should prevent sound transfer in both directions between internal and external environments.

Ventilation

Depending on how the building’s internal environment is to be managed with regard to air quality, the wall/roof may be required to admit ventilation or alternatively to prevent air infiltration.

Openings

The external cladding must incorporate windows for the purpose of admitting daylight and providing a view in almost all circumstances. However, with this in mind the wall should also prevent excessive solar glare on surfaces.

Construction.

The main structure is constructed using external cavity wall is built up as far as the column head level in courses corresponding with every course of precast ashlar panel equivalent to two courses of block work.

The intersection of the roof unit and the external wall is now dealt with where a steel UB is fixed to a steel UC on either side and located in the cavity wall and acts as a support for the roof unit. A course of blockwork is now laid on top of the UB and butted against the roof unit which acts as a support for the window fixings. An aluminium drip flashing is fixed on top of the blockwork.

The next level of blockwork and insulation is carried up to the cavity tray level. The steel channel support for the external ashlar panels is fixed between the two columns.

The cavity tray is then laid on top of the block work and carried down and rested on top of the steel channel. The precast panels are then continued up from the steel channel support.

Critical Analysis

The use of the precast concrete columns and roof panels as the structural type is fitting as they form an essential part of the building detail and finish. The use of large amounts of precast elements in the detail provides good buildability. The use of these precast concrete elements results in an economic and efficient way of putting the detail together. The design of the detail facilitates the ease of construction and improves accuracy. 16mm dia. dowels in the column head are used to fix the column to the precast roof unit. This hidden detail allows a seamless connection between the column and roof for the observer.

The craftsmanship required for this detail are minimal as the skills and finishes necessary for the polished precast columns, ashlar wall panels and roof units are prepared off site. This reduces potential for accidents, and addresses the on-site skill shortage. The detail performs sufficiently through excellent structural design with the precast elements that are fixed with minimal effort. The thermal performance was also carefully thought out as the building is fully insulated. Rainwater is taken away via a rainwater inlet and pipe concealed in the insulated ply deck. The standard achieved is superb with a high quality white polished concrete finish along with detail shown to the vertical and horizontal modules where the 450mm cladding panels used, correspond to the floor to floor height and generate a horizontal module of 1200mm to produce a 7.2m x 3.6m structural grid of circular precast concrete columns. These cladding panels along with the flooring materials and their modules run effortlessly through the building. This technique forms the visual character of the building. The roof panels contain louvre slots, which were designed to minimise their thickness, allow the passage of light and they wash the walls with light. A maintenance problem is clearly evident in the roof construction where the lightshade and associated electrics are built into the roof construction. If a problem occurred here, access to the wires or the lightshade would only be possible through tearing up the roof construction.

Case study

PERSISTENCE WORKS, SHEFFIELD

Construction technique: monolithic concrete contruction.

The persistence works building in shefeild was designed by “Feilden Clegg Bradley, and is constructed using monolithic concrte construction. The new arts building had to offer 68 high quality and affordable studio spaces for crafts people and artists – jewellery designers, metal bashers, sculptors, potters, painters, weavers and illustrators. Some studios had to be large enough to contain 6 m-high sculptures, while others were the size of a spare bedroom for working with jewellery items. The ground floor has the high ceilings that contain the large studios, while the upper and lower levels of the taller six-storey building facing away from Brown Street contain the smaller studio spaces.

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