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Theory of Structures_ ARCH 310
Lecture Topic: Structural Floor Systems
Name: Lawrence Ogunsanya
Email : [email protected]
IntroductionFloor systems are the horizontal planes that must support both
live loads and dead loads. A group of components fastened together make up this
horizontal frame and it require support at two sides opposite each other.
Floor systems must transfer their loads horizontally across space to either beams and columns or to load bearing walls
Floor systems may be composed of a series of linear beams and joists overlaid with a plane of sheathing or decking or consist of a homogenous slab of reinforced concrete.
The depth of the floor system is directly related to the size and proportion of the grid network it must span and the strength of the materials used.
Floor systems can be in 3 main forms: Concrete, Steel or Timber.
IntroductionConcrete:Concrete floor slabs are supported by columns, beams and loadbearing walls
Steel:Steel beams support steel decking or precast concrete planks or slabsThese beams may be supported by girders, columns or load bearing walls Beam framing is typically an integral part a steel frame system.
Timber/wood:Wood beams are also supported by girders, columns or load bearing walls
What drives the choice of a floor system?
Speed of constructionServicesAdaptability of spaceDay lightingAestheticsAcousticsFire Resistance
What drives the choice of a floor system?
Speed of constructionFor some projects the need to reduce to a minimum the
construction time (on site) may play a determining role. Indeed, time is often one of the key drivers for choosing a structural steel framework solution.
The Ark Hotel, ChinaBuilding with steel frame
What drives the choice of a floor system?
ServicesWhen a lot of service ducts are to be accommodated it may be
beneficial to adopt a floor solution that provides a flat soffit in order to maximise the flexibility in routing these ducts beneath the structural floor.
It will also be easy to remove and/or replace these ducts to meet future needs.
Solutions that provide a flat soffit don't also allow long spans. So an alternative in a building that is both highly serviced and requires long-span floors is to integrate the services within the beam depth, so that the total depth of structural floor plus service zone is minimised
Another option is to use webbed or castellated beams
Services intergraded into the floor system
Need for adaptable spaceThis is dependent on the spanning ability of the floor materialThis spanning ability allows the number of internal load bearing
walls and columns to be minimised.Open floor spaces can be created, or non load-bearing
partitions (that are easily moved) used to form (temporary) individual areas.
What drives the choice of a floor system?
Estimate span ranges for different floor slab systems
Floor slab spans
Floor slab spans
Day lighting requirements 'Deep' floor plans (floor plan widths that are more than15 metres)
may mean that, occupants of a building are not exposed to natural lighting.
Long span solutions may not then be the most appropriate solution for certain situations, rather a short span design (for example using shallow floors ) with an internal atrium may provide a more appropriate internal environment.
The shallowness of the floors is achieved by placing the slabs and beams within the same zone. An added benefit is that a flat soffit is achieved.
What drives the choice of a floor system?
What drives the choice of a floor system?
AestheticsIf false ceilings are used then the aesthetics of the soffit of a
given structural floor system are clearly irrelevant. However, a number of clients have recently been looking for
exposed soffits, exposed primarily so that the thermal mass of the floor is exposed.
The soffit must also then be visually appealing.
What drives the choice of a floor system?
AcousticsIn terms of resisting airborne sound a massive floor is beneficial,
and when considering impact sound avoiding stiff structural connections between components is helpful.
Good detailing and installation of sound proof membranes are needed.
Acoustic floors
Under screed isolation materials
Floated wood and composite floor systems
Fire resistanceThe most common solution adopted to provide fire resistance is to protect the steel members so that they remain at a sufficiently low temperature). Use a lot of concrete on the building.Intumescent coatings, which are applied off-site and expand with temperature to provide an insulation layer, are often used. If the steel elements are embedded in concrete this can provide the necessary insulation against fire. Other options include board protection and the use of a cementitious spray.
What drives the choice of a floor system?
Fire protection
Application of thin film intumescent coatings offsite
Board protection systems
Concrete floor slab systems
Flat slabFlat plateBeam and slabRibbed (waffle slab)Hollow core slab (precast)Solid slab (precast)
FLAT SLABA flat slab is a one-way or two-way system with thickenings in the slab at the columns and loadbearing walls called 'drop panels.Drop panels acts as T-beams over the supports. They increase the shear capacity and the stiffness of the floor system under vertical loads, thus increasing the economical span range.
Concrete floor slab systems
FLAT SLABAdvantagesSimple formworkNo beams—simplifying under-floor services outside the
dropsMinimum structural depthUsually does not require shear reinforcement at the columns.Disadvantages:Medium spansGenerally not suitable for supporting brittle (masonry)
partitionsDrop panels may interfere with larger mechanical ductingVertical penetrations need to avoid area around columnsFor reinforced flat slabs, deflection at the middle strip may be
critical.
Concrete floor slab systems
FLAT PLATEA flat plate is a one- or two-way system usually supported directly on columns or loadbearing wallsIt is one of the most common forms of construction of floors in buildings. The principal feature of the flat plate floor is a uniform or near-uniform thickness with a flat soffit which requires only simple formwork and it is easy to construct. The floor allows great flexibility for locating horizontal services above a suspended ceiling or in a bulkhead.
Concrete floor slab systems
FLAT PLATEAdvantages:Simple formwork and suitable for direct fix or sprayed ceilingNo beams—simplifying under-floor servicesMinimum structural depth and reduced floor-to floor height.Disadvantages:Medium spansLimited lateral load capacity as part of a moment frameMay need shear heads or shear reinforcement at the columns or larger columns for shearLong-term deflection may be controlling factorMay not be suitable for supporting brittle (masonry) partitionsMay not be suitable for heavy loads.
Concrete floor slab systems
BEAM AND SLAB•This system consists of beams framing into columns and supporting slabs spanning between the beams.•It is a very traditional system. The relatively deep beams provide a stiff floor capable of long spans, and able to resist lateral loads. •However, the complications of beam formwork, co-ordination of services, and overall depth of floor have led to a decrease in the popularity of this type of floor.
Concrete floor slab systems
BEAM AND SLAB
Advantages:
•Traditional effective solution
•Long spans.
Disadvantages:
•Penetrations through beams for large ducts
•difficult to handle
•Depth of floor
•Greater floor-to-floor height
Concrete floor slab systems
RIBBED [WAFFLE] SLAB•The basic construction method begins by digging a trench down to undisturbed mineral subsoil. •Ribbed floors consisting of equally spaced ribs are usually supported directly by columns•They are either one-way spanning systems known as ribbed slab or a two-way ribbed system known as a waffle slab.•This form of construction is not very common because of the formwork costs and the low fire rating.•Ribbed slabs are suitable for medium to heavy loads, can span reasonable distances, are very stiff and particularly suitable where the soffit is exposed.
Concrete floor slab systems
Concrete floor slab systemsRIBBED [WAFFLE] SLAB•Advantages:•Savings on weight and materials•Long spans•Attractive soffit appearance if exposed•Economical when reusable formwork pans used•Vertical penetrations between ribs are easy.•Disadvantages:•Depth of slab between the ribs may control the fire rating•Requires special or proprietary formwork•Large vertical penetrations are more difficult to handle.
HOLLOWCOREHollow core floor planks are precast, pre-stressed units produced on long-line casting beds using slide forming or extrusion methodsDuring manufacture, cores are formed throughout the unit, reducing its self-weight. Planks are usually 1200-mm-wide, though some manufacturers can produce 2400-mm-wide units. These wider units may require increased crane capacity but offer greater speed of placement, less joints, grouting and sealing.
Concrete floor slab systems
SOLID SLABSSolid precast pre-stressed floor slabs are typically wet-cast on long-line beds in unit moulds or by hollowcore equipment using slip form or extrusion methods.Thickness is usually 150, 200 and 250 mm.Solid slabs are chosen where the loading results in high shear or there are heavy point loads;the environment is aggressive, for example in splash zones or where condensation may occur over water, dry arid areas.High cover or special concrete is required;
Concrete floor slab systems
PRECAST CONCRETE SLABSAdvantagesDurability and low maintenance. Corrosion and impact resistance, Fire resistanceAble to control the quality of materials and workmanship
better than onsite Manufactured to various sizes and thicknessDisadvantages:Handling and transportingStresses induced during handling and erecting units may
cause significant structural design concerns
Concrete floor slab systems
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