Multi-storey Framed Structures 6z5k48

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At the end of this class, student will be able to :◦ Understand about the multi-storey framed structures and floor system ◦ Know the types of material available for multistorey framed structures and factors that influence the choice of material to be used

What is Multi-storey Building?  A multi-storey building is a building that has multiple floors above ground in the building. 

Multi-storey buildings aim to increase the area of the building without increasing the area of the land the building is built on, hence saving land and, in most cases, money (depending on material used and land prices in the area).

Abu Dhabi Investment Authority Tower

International Commerce Centre, Hong Kong

NO. OF STOREYS  Multi-storey building consists of buildings of more than 1-storey. MATERIAL  Construction materials may differ depending on the number of storey of the building.  Timber can be used for building not more than 3-storeys high.

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Steel Reinforced Concrete Timber

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Availability of material / labour Cost Speed of erection Method of construction Transportation Possibility of standardizing the sizes of the structural Size and nature of site Modification Fire resistance required

For buildings of more than 5-storeys high, steel is the most suitable material to be used. Steel is:

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Light in weight - easy to transport; less load on foundation Inorganic - will not decay, bend, crack or peel off High strength & durability – safety structure; Quality control - prefabricated in the factory under effective quality control – less manpower, faster construction

Less maintenance; Cost effective; Not at risk to delays

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Heavier; dead load; Non-recyclable; High risk in crack & mould growth; Higher total cost; Duration of work – long, risk to delay Flexible design

Element

Steel

Concrete

Site •Made under factory condition consideration – BS/MS •Fabricated “off site”

•Strict supervision on site •Work carried on site

Cost and speed of erection

•Combination of skilled and unskilled •Site space required for storage and mixing •Cheaper in cost but longer in construction

•Erection by skilled- labour, quickly and accurately •Early completion but may be limited due to the large number of complicated connections

Design Extension – need to expose by consideration the removal of any fire resisting casing

•Flexibility in design – monolithic in nature and not confine to standard sections but limit to the cost of formwork

Fire resistance

•Requires protective cover varying in nature and thickness / sprayed asbestos

•Provide a substantial degree of fire resistance – varied by the cover thickness

Type of construction

•Dry construction •Clean site

•Wet construction •Messy site

FLOOR SYSTEM • The principal structural elements of a typical multi-storey building comprise floors, beams and columns. A wide variety of alternative forms and arrangements can be used in multistorey steel framed structures. Floor slabs: Several different types of slab can be used, in either composite or non composite form.

COMPOSITE FLOOR SYSTEM 

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The system consist of simply ed universal Beam acting compositely with a thin concrete slab (concrete topping) ed on metal deck Metal decking, which spans between secondary steel beams usually spaced at between 2.5 and 3m centres. Steel stud are welded through the decking onto the flange of the beam below to form a connection between steel beam and concrete slab Concrete is then poured onto the decking, usually by pumping, to make up the composite system. Metal decking acts both as permanent formwork for the concrete, eliminating the need to provide props, and as tensile reinforcement for the slab. Steel bars are included in the slab to prevent cracking and to provide reinforcement in the event of degradation of the decking in a fire.

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Slimdek is a proprietary shallow floor system. It is a composite system comprising of an asymmetric slim floor beam contained within the depth of the slab.

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There are many types of steel decking available. Some of the advantages of the composite metal deck system are:

◦ act as permanent shuttering which can eliminate the need for slab reinforcement and propping of the construction. ◦ The floor deck serves as a working platform during construction and as formwork for a sitecast concrete slab ◦ reduce the overall depth of structure. Provides up to 2 hours fire resistance (500°C collapse) without additional fire resistance and 4 hours with added thickness or extra surface protection.

◦ light, adaptable system that can be cut awkward shapes and can easily be drilled or cut out for additional service requirements. Reduce frame loading and foundation cost. ◦ Simple, rapid construction

NON - COMPOSITE FLOOR SYSTEM 

For shorter spans, precast concrete floor slabs can be used with steel beams. The precast units may be ed on the top flange of universal beams or on the bottom flange

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Universal beams ing pre-cast floor units have advantages over other forms of construction. This system offers: ◦ Fewer floor beams since pre-cast floor units can span up to 6 to 8 m. without the difficulty. No propping is required. ◦ Shallow floor construction can be obtained by ing pre-cast floor units on shelf angle. ◦ Fast construction because no time is needed for curing and development of concrete strength.

Disadvantages: • Composite action not readily achieved without a structural floor screed. •

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Heavy floor unit are difficult to erect in many location and require use of special machines. Small opening can be cut in the field, narrow openings parallel to slab span are preferred, engineering analysis is required for wide opening

Apart from the method of construction, the appropriate choice of building materials is amongst the important factors in the construction of multi-storey buildings. The material for structural frame can be made from steel, in-situ reinforced concrete and pre-cast concrete. Compare the choice between steel structures to pre-cast concrete frame based on the principal factors considered in their selection.