Presentation on theme: "2.2 STRUCTURAL ELEMENT Reinforced Concrete Slabs"— Presentation transcript:
1 2.2 STRUCTURAL ELEMENT Reinforced Concrete Slabs 2.0 ANALYSIS AND DESIGN2.2 STRUCTURAL ELEMENTReinforced Concrete SlabsRearrangement by :-NOR AZAH BINTI AIZIZKOLEJ MATRIKULASI TEKNIKAL KEDAH
2 INTRODUCTION Concrete slabs are similar to beams in the way they span horizontallybetween supports and may be simply supported,continuously supported or cantilevered.Unlike beams, slabs are relatively thinstructural members which are normally usedas floors and occasionally as roof systemsin multi-storey buildings.
3 INTRODUCTION Slabs are constructed of reinforced concrete poured into formwork.The formwork defines the shape of thefinal slab when the concrete is cured (set).Concrete slabs are usually 150 to 300 mm deep.It is usually timber but steel is commonly used on commercial projects. on-site or into trenches excavated into the ground.
4 INTRODUCTION Slabs transmit the applied floor or roof loads to their supports.Slabs may be classified into twomain groups depending on whether theyare supported on the ground orsuspended in a building.It is usually timber but steel is commonly used on commercial projects. on-site or into trenches excavated into the ground.
5 GROUND SLABS Ground slabs are those slabs that are poured directly into excavated trenchesin the ground.They rely entirely on the existingground for support.The ground must be strong enoughto support the concrete slab.Normally, a minimum bearing capacity forslab sites is 50 kPa.The ground (more correctly known in the industry as the foundation)
6 GROUND SLABS Diagram of slabs with arrow representing applied floor and roof load pointed down to the slab.The slab is supported by foundation andthe slabs transmit its load to foundation.In most cases, the foundation easily meets this minimum bearing requirement. However, where clays and silts are present in the soil, the slab may experience stresses. These soils tend to be on reactive sites which are those areas where the volume of soil changes because of its moisture content. This results in the foundation expanding or contracting depending on how much moisture the soil contains.Foundation movements can be significant enough to damage a slab and any other components it supports, such as the brickwork shown in the photo.
7 SUSPENDED SLABSSuspended slabs are slabs that are not in direct contact with the ground.They form roofs or floors above ground level.
8 SUSPENDED SLABS The way a slab spans its supports Suspended slabs are grouped into two types:One way slabs- which are supported on two sidestwo way slabs- which are supported on all four sides.The way a slab spans its supportshas a direct impact on the way inwhich the slab will bend.
9 ONE-WAY SLAB One way slabs are usually rectangular where the length is two or more timesthe width.These slabs are considered to besupported along the two long sidesonly even if there is a small amountof support on the narrow ends.
10 ONE-WAY SLAB A diagram of a concrete slab with two supporting sides is shown.The width of the slab is alsothe short span.Rule of Thumb:For ly/lx > 2,design as one-way slably = the length of the longer sidelx =the length of the shorter side
12 ONE BEND SLAB It is assumed that; one way slabs bend only in the directionof the short span, so;the main steel reinforcement runs in thisdirection across the slab.
13 ONE BEND SLAB A diagram of a concrete slab with two supporting sides is shown.Compression on the slab pushes towards themiddle of the slab which causes the slab to bend inwards.Tension is distributed across the supporting sides.
14 TWO WAY SLABTwo way slabs are approximately square where the length is less than doublethe width and the slab is supportedequally on all four sides.Rule of Thumb:For ly/lx ≤ 2,design as two-way slably = the length of the longer sidelx =the length of the shorter side
15 TWO WAY SLAB A diagram of a concrete slab with four supporting sides is shown.The pressure spans equally across the widthand length of the concrete slab.Spans equally both direction
17 TWO BEND SLAB These slabs are assumed to bend in both directions, so main steel reinforcement of equal size and spacingis run in both directions.
18 TWO BEND SLABA diagram of the compression that occurs in a two bend slab is shown.The pressure runs to the middle of the slab which causes all four sides to bend equally.Compression
19 Example:Figure shows three floor layouts of a monolithic beam and slab construction. a) State whether the floor panels are one-way or two-way spanning. b) Sketch the tributary areas for all the beamsBA1A12C3050mm7650mm7050mm
21 EXAMPLE :The beams supporting the floor panel A-A1/1-2 are 350 mm deep and150 mm thick, and the floor slab is 150 mm thick, given the density ofconcrete as 24 kN/m3.Calculate the self-weight of the beam A/1-2, considering only the rib of the beam in kN/mCalculate the self-weight of the slab in kN/m2Calculate ultimate load on beam A/1-2 in kN/mCalculate reaction force at column A/1BA1A12C3050mm7650mm7050mm
22 ANSWER : a) Self-weight of the rib in kN/m 350rib1507050A/2a) Self-weight of the rib in kN/m= x ( ) x 24= 0.72kN/mb) Self-weight of the slab in kN/m2= 0.15 x 24= 3.6kN/m2c) rib self-weight = 0.72kN/mslab self-weight = 0.5 x w x lx= 0.5 x 3.6 x 3.05= 5.49kN/mUltimate load on beam A/1-2 in kN/m= rib self-weight + slab self- weight= 1.4 x x 5.49== kN/md. Reaction force at column A/1= x 7.05/2=30.64kN
23 One-way slab DesignDesign a one way slab supported on two brick wall spanning 3 m c-c. The characteristic dead load ( excluded self weight slab) and characteristic live load supports by the slab are 0.35 kN/m2 and 2.5 kN/m2.( fcu=25 N/mm2 , fy=250 N/mm2, concrete cover=25 mm and assume diameter of main bars at 10 mm)
24 One-way slab Design Is designed as a shallow rectangular beam Consider a strip 1 m wide for designAn upper limit to the value of thelever arm, z = 0.95 dThe reinforcement area evaluated from;M ult = 0.87 As fy z
25 Figure 2: Building layout plan 3000 mm3000 mm3000 mm3000 mm21a7500 mm1AA1BAFigure 2: Building layout plan
26 One-way slab Design Fst Fcc (d-0.9x/2) a F cc x Equation As(d-0.9x/2)aF ccx0.9xConcrete compressionSteel tensionEquationFcc = 0.45fcuAFst = 0.95AsWhere:f cu - Characteristic of concrete strength (30N/mm2)f y - Characteristic of reinforcement strength (460N/mm2)A – area of beam cross sectionAS – area of reinforcement cross sectionM – Moment∑Ma = 0Fcc (d-0.9x/2) – M = 0Fcc = Fst
27 Section A-A h=125 7500 Characteristic Dead load,gk = slab self weight + weigh of services, finishing & ceiling= 24kN/m3x h kN/m2= 24 x= kN/m2Live load, qk= 2.5kN/m2
28 Factored load on the slab = 1.4 x 25.125 + 1.6 x 18.75 = 65.175 kN/m Gk= x = kN/m7500Qk= 2.5 x = kN/mFactored load on the slab= 1.4 x x 18.75= kN/m
29 TABLE: Ultimate bending moment and shear force coefficients in one-way spanning
30 Refer Table:Ultimate bending moment and shear force coefficientsin one-way spanningAs a continues beam, it is not easy to findshear force and bending moment, so we usediagram given.Use middle interior span & interior supportF= kN/m x 3.00 m = kNUseM = FL=0.063x x 3.00=36.96 kNm
31 0.45 x fcu x A x (d-0.9x/2) – M = 0 H=125 7500 Fst Fcc d= /2 = 95mm∑Ma = 0Fcc=FstFcc (d-0.9x/2) – M = 00.45 x fcu x A x (d-0.9x/2) – M = 00.45 x 25 x 0.9x x x ( x/2) – x = 0x ( x) x 106 = 0x x x = 0x = @
32 Fcc =FstFcc = x 25 x 0.9(5.25 ) x 7500= N= x fy xAsWhere fy=250 (mild steel)As = / 237.5= mm2Lets say for 35 rods; 2223/35 = mm2 (1 rod)So size rebarA = Πj2= ΠD2/4 = 48 mm2D = √ 48 x 4 / ΠD = 8 mm for 1 barSpacing = 7500 – 25 (2) / 34 = 219 mmSo use 35 R ,(35 mild steel bar 10mm dia. with 219 spacing)