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When choosing a frame for a building, there are factors that need to be taken into account before a suitable choice can be made. These factors include; how big the building needs to be, in terms of both floor area and in height, another factor is what the building is going to be used for, as this may determine how much of the floor area needs to be clear uninterrupted space or where columns maybe placed for support. Factors such as cost, speed of construction, shape of the building, type of ground conditions on the site will also play a big part in deciding which type of frame should be used for constructing a structure.
Options for Frames
The main choices of frames constructed of steel for buildings are essentially a skeleton cage frame, cantilever frame, propped cantilever frame. Each of these systems have their advantages and disadvantages. A skeleton cage is the simplest of the frames, and most likely to be used where a simple form of building is required. Each floor with this type of frame is almost structurally the same. This frame consists of beams and columns spanned at maximum widths to ensure the cost of the frame is kept down, as well as to minimise the self weight.
A cantilever frame has some different advantages over the skeleton frame, the main being that the floor area of the upper floors can be larger than that of the footprint of the ground floor. This feature can be used where there is a small site, or for example where the site has a steep slope on part of it, the foundations could then be placed on the flat and the building could over-hang the slope slightly. The cantilever should only project one fifth to one quarter of the span between beam and columns. This type of frame is lighter due to the smaller footprint it requires, and will also require less material for foundations, meaning with regard to materials this frame may be a more economic choice. One disadvantage is that the structure is more complicated and there may therefore be more problems that the structural engineer encounters to ensure this frame works.
A propped cantilever is technically not really a cantilever since it is supported at both ends even through the supports are lightweight, however this type of frame is very useful in certain situations. In some circumstances it may be necessary to span a distance that is longer than a fifth to a quarter the length between the beam an column of a building, this is why the lightweight props are required to ensure the building does not collapse. The disadvantages of this frame is that the stress that the beams will be under may require the depth of the beam to be larger than usual, meaning the self weight of the building will be greater, leading to greater costs for materials.
The points made regarding the properties of steel frames can be found in the works of Foster.J.S and Harington.R (1988 p.190-194), H.W.University Naismith.N. (notes)
In situ Concrete
There are various choices of frames for in situ concrete. Some are very similar to the steel frames like the skeleton and cantilever, others are different and incorporate a floor which in some case is a fundamental part of the frame or in some situations is used in combination with beams to form different type of in situ frames.
Flat slab construction for in situ concrete is structurally the simplest frame. One major difference between steel and in situ concrete construction is the need for formwork with the concrete. Flat slab construction removes the need for beams by using the slab as beam and having the columns supporting the slab, this in turn simplifies the formwork and can increase the speed and cost of construction. Incorporating the beam and slab means the overall floor depth will be reduced, which will therefore lead to a reduced height of building overall. The disadvantage to this type of frame is that it is not as strong without a separate beam. Another drawback is the force put on the columns by the slab can push the columns through the slab by a method called punching shear, causing the floor and ultimately the structure to fail.
A solid slab with drops is another alternative frame. It is very similar to the flat slab, but has extra support (drops) in place where the support of the columns are required. The benefits of this effectively iron out the drawbacks of the flat slab, such as not being as strong or having the ability to carry large loads for the problem of punching shear. The downside to this is that it undoes some of the advantages of the flat slab. It requires slightly more complex formwork to create the 'drops' and therefore increases the overall depth in addition.
Another alternative frame is the ribbed slab, which there are two types, with beams or band beams, band beams accommodating larger spans. Theses types of frame are very strong for the amount of concrete required to make them. This therefore makes theses frames a lighter-weight, which can reduce construction costs with regards to foundation types used. The drawbacks are similar to the slab with drops, more complex formwork is required, which can lead to longer construction times.
Troughed slabs with integral beams are another in situ concrete frame to consider. Basically the same idea as the ribbed slab, but the beams are flush with the slab (within the depth of the floor). This therefore makes the floor depth shallower, which has advantages with regards to the final building height. Again the disadvantages are formwork and time of construction due to this.
Three types of waffle slabs are available for use as frames, the waffle slab on beams, the waffle slab with integrated beams and the waffle slab without beams. Similar to the ribbed slab, but has 'ribs' moving both ways and a thin floor layer of concrete. This means there is no necessity to have a floor or ceiling finish depending on preference. The three types of waffle slabs incorporate the features of other type of frames. The waffle slab with beams has mostly the same pros and cons as the ribbed slab with beams. The waffle slab with integrated beams essentially has the same pros and cons as the troughed slab and the waffle slab without beams is similar in properties to the flat slab.
Points made about in situ concrete frames come from both Chudley.R and Greeno.R (2005 p378-391) and H.W.University Naismith.N.
Pre-cast concrete frames were introduced as a 'happy medium' between steel and in situ concrete. The advantages of steel such as the speed of construction and the economy and fire resistance of concrete were primary factors. If steel were to be fire proofed in concrete, it would require almost as much concrete to coat as it would to just used a pre-cast beam and column frame, however there are more modern methods of fire proofing available to keep the weight of a steel frame to a minimum. The type of frames that can be constructed in pre-cast concrete are therefore very similar to steel and in situ concrete, with the main difference being with the fixing elements.
My Choice of Frame to Use
Due to the sandy ground conditions of the site, my choice would be to opt for a steel frame construction. More specifically since there is no need for an overhang or mention of significant slopes on the site, then a skeleton cage steel frame would be a suitable choice. Since the ground conditions are quite poor the frame will benefit from being fairly light in comparison to concrete. Appropriate fire protection will have to be applied, for example Great Northern Insulations (2008) spray fire proofing. To ensure the frame weight is kept to a minimum, castellated beams could be used. Although they are lighter weight an use less material, these beams are more labour intensive and require more time spent to cut and weld the sections, therefore making this option more expensive than conventional steel beams, however this option should prove worth while in the longer term with regards to the poorer ground conditions.
If the building were to be altered to a 30 storey structure, then there would have to be some changes made to the design and structure of the building. The main item to consider is the atrium. This has been incorporated into the original design to provide natural light flow through the building. Although this has been used as a central core for columns in my original concept, and could still be used as a core, this principle reason of providing natural light will not be nearly as effective dissipating through a 30 storey structure. Therefore it would be a better choice to remove the atrium.
The suitable choices for alternative frames for a structure of 30 storeys include,
Ÿ Shear Truss Frame in Steel
Ÿ Shear Wall Structure in Concrete
Ÿ Tube Systems in Steel or Concrete
The shear truss in steel consists of a vertical core made up steel trusses as the name would suggest. The trusses essentially work as a vertical cantilever and take advantage of the structural shape they form. X or K shape bracing, which is used to form the core structural, is used to connect the internal beams and columns. This type of steel structure can be built up to 40 storeys.
The shear wall in concrete uses shear walls as its vertical element. It is a much heavier structure and therefore a stiffer structure. The shear walls can have in situ concrete floors connected directly to them, and can transfer the lateral loads to the foundations. This concrete system can accommodate up to 50 storeys.
The tube systems move the external columns to the perimeter of the building and connect them by beams. This provides a sound structure against lateral loads. These tube systems can be combine to provide more support against lateral support. The tube in tube system is one such solution, where the internal core tube can be constructed of steel or concrete. The other is bundled tubes there two or more tubes are place side by side and each tube provides support to and from another tube.
The information used to provide the alternative proposal can be found in the works of Chudley.R and Greeno.R (2005 p.424-425) and H.W.University Naismith.N. (notes)