DESIGN OF STRAP (CANTILEVER) FOOTINGS Design Steps and Equations

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1 DESIGN OF STRAP (CANTILEVER) FOOTINGS Design Steps and Eations For an example on Design of Strap Footing click here (a) (b) (a) Strap Footing with non-niform strap bean thickness (b) Strap Footing with niform strap bean thickness 1 of 6

2 P 1 P S l 1 l Strap Beam h L 1 x B 1 L x B e R 1 R L 1 L B 1 b 1 Strap Beam b l 1 l B STEP 1 DIMENSION FOOTINGS (Determine L 1, B 1, L and B ) Allowable load P = P 1 + P Ultimate load P = [1.4DL LL 1 ] + [1.4DL + 1.7LL ] Ultimate ratio r = P P, Ultimate applied pressre = a x r!m col. = 0 R 1 (S " e) " P 1 S = 0... (1)!M R1 = 0 P (S " e) " R (S " e) " P 1 e = 0... () of 6

3 !F = 0 P 1 + P " R 1 " R = 0... (3) To solve these three eations assme a vale for e, a) Find R 1 by e. (1) b) Find R by e. (). c) Check e. (3). If e. (3) is not satisfied, then repeat 1 and ntil e. (3) is satisfied. = r a ( l 1 % Footing 1: L 1 = x & e ) # ' $ and B 1 = R 1 L 1 Footing : let k = L B (1.0 means footing is sare) B = k R and L = k B Note: In step (c) above, any vale of e will do, however, the best selection is the one that gives B 1 = B STEP DRAW SHEAR AND MOMENT DIAGRAMS (L - DIRECTION) 1, L = x B 1, L = x B The general shape of the shear and bending moment diagrams are shown on the next page. 3 of 6

4 P 1 P S l 1 l Strap Beam 1, L, L Constant Shear in Strap Beam = V Strap + V(kN) " Top Steel Footing 1 Top Steel, Strap Beam, M (strap) Top Steel Footing " M(kN.m) + Moment drawn on tension side Bottom Steel Footing 4 of 6

5 STEP 3 DEPTH OF CONCRETE, d' Estimate d' for footing 1 by 3-way pnching shear nder colmn 1 and for footing by 4-way pnching shear nder colmn. (Use Eations For Pnching Shear or approximate d' by Strctral Depth of Concrete table for pnching shear failre). Note: Usally pnching shear govern over wide beam so there is no need to check against wide beam. STEP 4 REINFORCEMENT IN L-DIRECTION The calclations below are repeated for both footings. 1. Select the appropriate moments (refer to moment diagram) and estimate the moment per meter by M (x)/m = M (x)/b.. Using M (x)/m, d', f c ' and f y estimate the reinforcement A s (refer to Eations for Reinforcement or the percent reinforcement can be obtained directly from Percent Steel Tables). STEP 5 REINFORCEMENT IN B-DIRECTION For Footing 1 B1 " b1 L 1 ' = M 1 = * L ' + 1 For Footing B " b L ' = M = * L ' + Using M `-, d', f c ' and f y estimate the reinforcement A s (refer to Eations for Reinforcement or the percent reinforcement can be obtained directly from Percent Steel Tables). 5 of 6

6 STEP 6 DEPTH AND REINFORCEMENT OF STRAP BEAM Depth h of strap beam: Assme that the width of the trap = b (with of the smallest of colmn 1 or ). The shear stress,, = V strap -, c = 8 c b h.87 f'. Solve for h. Reinforcement of strap beam: Select the appropriate moments (refer to moment diagram) M (strap). Using M (strap), d', f c ' and f y estimate the reinforcement A s (refer to Eations for Reinforcement or the percent reinforcement can be obtained directly from Percent Steel Tables). A s is the total reinforcement in strap beam. For an example on Design of Strap Footing click here 6 of 6

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