![]() Manual Design of Beam and Raft Foundation to Eurocode 2 ![]() ![]() Provide Y8mm 300mm c/c as shear links (A sw/S = 0.335) Okĭownload the full calculation sheet from the link below We can therefore analyse the ground beam as a continuous beam as follows Įffective depth d = 1200 – 50 – 10 – (16/2) = 1132 mm (assuming ϕ16 mm reinforcement will be used for the main bars, and ϕ10mm for the links). The self-weight of the beam in this case is beneficial (favourable), and can be ignored. This gives unconservative results, especially for shear forces induced in the beam. For simplicity of hand calculations, we are going to use the formula for load transfer to the ground beams. The figure above shows the assumed load distribution from the raft slab to the ground beams. Therefore, provide H10 250 c/c TT (A sprov = 314 mm 2/m) Design of the ground beams Therefore, provide H10 250 c/c NT (A sprov = 314 mm 2/m) Therefore, provide H10 250 c/c BB (A sprov = 314 mm 2/m) Therefore, provide H10 250 c/c TT (A sprov = 314 mm 2/m) Since k < 0.167 No compression reinforcement requiredĬheck if A Smin < 0.0013 b d (187.2 mm 2/m) Moment coefficient for two adjacent edges discontinuous Short Span Long Span Mid-span 0.059 0.034 Continuous edge 0.078 0.045 Where C c = concrete cover of the raft slab, and ϕ is the diameter of the reinforcement to be used.Ī smin = (0.26f ctm b t d)/f yk ≥ 0.13bd/100 Self weight of the slab = 25 kN/m 2 x 0.15 m = 3.75 kN/m 2ĭesign pressure for the raft slab = 13.85 – 3.75 = 10.1 kN/m 2 (note that it will be improper to factor the self-weight of the ground floor slab since it is favourable in this case)Įffective depth of the slab d = h – C c – ϕ/2 = 150 – 30 – 10/6 = 115 mm For raft foundations without ground beams, punching shear is also very critical. The design of the raft slab is done just like the design of a normal suspended floor, but there is usually no need to check for deflection. Therefore, using a pressure of 13.847 kPa, we can design the raft slabs and the ground beams using the conventional methods which I believe that we are familiar with. You can convert from ULS to SLS for simple buildings when using Eurocode by using a factor of 1.37 ( Ubani, 2017). Pressure distribution calculation is carried out using SLS load, but we used ULS here. Also, due to the close values of P max and P min, instead of using a trapezoidal pressure distribution, we can assume a rectangular pressure distribution using the maximum pressure. These values are way below the allowable bearing capacity of the soil, therefore there is no need to increase the dimensions of the base. Pressure distribution P i= (R/A) ± (6Re/LB 2) The base dimensions are taken as follows Taking moment about the centroid of the base As a result, the foundation must be filled with imported earth material to the ground beam level, before the raft slab concrete is done. In the case of down-stand beams, the ground beams are constructed into the natural soil, with a height that rises above the natural ground level, in order to make up the architectural level for the ground floor slab. This is one of the scepticism of professionals on the use of downstand ground beams in raft foundations. ![]() Therefore, for the theory and assumptions made in the design of a beam and raft foundation to be valid, the slab must be in direct contact with the ground, and be stiff enough to resist the earth pressure intensity. This is a typical reverse of what is obtainable in the design of suspended solid slabs. Since the pressure load on the raft slab is coming from the ground, it follows that the top fibre of the slab is in tension in the sagging areas (span), while the support regions are in tension at the bottom fibre. Therefore, the uniformly distributed load used in the design of the ground beams is assumed to be transferred from the raft slab. The general theory in the design of beam and raft foundations is that the earth pressure intensity as a result of the superstructure load is first resisted by the raft slab, which then transfers the load to the ground beams. The beams in a beam and raft foundation may be upstand or downstand. ![]() Figure 1: Schematic representation of beam and raft slab action ![]()
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