Nowadays, long-span slabs are highly desirable as building owners demand spaces without columns. The selection of floor slabs such as flat plate, flat slab, ribbed slabs, slabs supported on beams, and prestressed slab is governed by many architectural and structural factors.
The architectural factors include floor-zone thickness, services, and large penetrations through the concrete slab. These parameters can affect the floor slab's aesthetics, structural safety, and economy. That is why they should be considered at the design stage, and the best floor slab that meets all the requirements should be chosen. The architect and structural designer should collaborate to select the most suitable slab type for the building under consideration.
What are the Architectural Factors Affecting the Choice of Long-Span Concrete Floor?
1. Floor-Zone Thickness
The thickness of the floor zone is a crucial architectural factor that may control the selection of long-span concrete slab types. The floor zone consists of slab thickness, finishing, and depth required to accommodate services below the floor. The services below the floor may include air conditioning ductwork and fancoil units, ventilation and exhaust ducts, sanitary floor traps and waste pipes, stormwater waste pipes, hot and cold water, fire water and sprinklers, smoke detectors, data, and electrical cabling, lighting and many other specialist services. The floor-zone thickness is greater than slab thickness and needs to be carefully studied while the slab type is selected.
Minimizing the floor-zone thickness is significant economically as it maximizes the number of storeys. If the number of storeys is fixed, the total height of the structure can be decreased, in turn, reducing the total cost of the building.
Services in buildings should be considered while the long-span concrete floor is selected. The services are commonly laid along and across the floor system and move vertically from one floor to another without any obstruction. They should be arranged so that the architectural view and structural safety of the building are preserved.
The architect, structural designer, and service designer should maintain proper communication to make sure that services like cables, ducts, and penetrations for services do not compromise the safety of the floor system or the operation of the building.
At the design stage, the placement of services in the floor slab should be taken into account. For instance, placement of two or more ducts closely in a slab can reduce the bond and effectiveness of reinforcements. This can be avoided by placing one duct in a thickened portion of the slab or locating services below the floor system.
Fixing services below the floor system may create other issues. For instance, it may be required to pierce beams or create holes in the slab. These actions should not be taken unless the designer is consulted to ensure that the safety of the structure is not endangered.
Piercing web beam should be avoided if the size of a duct is greater than 0.25 times the effective depth of the beam. Otherwise, the shear and flexural capacity of the beam is reduced significantly. Therefore, the selection of slab type is greatly influenced by the placement of services as they affect the capacity of the floor slab.
Large penetrations through floor systems are another factor that structural designers should consider while choosing a slab type. Commonly, sizable penetrations through slabs are required to place major ducts, construction of non-load bearing stairs, and lift shafts. Beams carry loads due to large non-load bearing ducts, shafts, stairs, and lifts ane loads of surrounding floor around the perimeter of the penetration. Table-1 presents different types of slabs along with their suitability for a long-span floor system.
Table 1: Types of Slabs and Their Suitability for Long Span Floor System
|Flat slab||It has minimum structural depth. Not suitable to support brittle portions. Avoid vertical penetrations around columns.Drop panels could interfere with large ducts.Deflection at the middle of the slab is critical.The maximum is around 9 m.|
|Flat plate||Provide great flexibility for the placement of horizontal services under the slab.Provides minimum structural depth, hence reduce floor to floor height.Need simple formwork and suitable for a direct fix or sprayed ceiling.It may not be suitable for supporting brittle partitions.Economic span is 6-8 m.|
|Beams and slabs||Large penetration through beam soffit is challenging to deal with, which complicates service placement.The depth of the floor zone is large compared with the flat slab and flat plate.Floor to floor height is large.Economic spans are large.|
|Ribbed (waffle) slab||Soffit is aesthetically appealing if it is exposed.Floor to floor height is large compared with flat slab and flat plate, but small penetrations between ribs are allowable.Sizable penetration is challenging to deal with.Economical if formworks are reusable.The economic span is 15 m.|
The span of concrete slab is controlled by the imposed loads and the depth and reinforcement of the slab. For residential buildings with moderate live load, a span of 7.6 m can be used, whereas for heavy loadings such as in the case of commercial buildings, a span of slabs could be between 12 m to 15 m.
Generally, the minimum thickness of a slab is controlled by the slab span. ACI 3188-19 provides a table to calculate the minimum thickness of a slab to check deflection. So, if a smaller thickness is selected, the deflection of the slab needs to be estimated.