Title 35 RCD - Wall and Isolated Footing.pdf ✅

PSAD 35: RCD – Wall and Isolated Footing 1. Effective Soil Pressure In HGE 20: Ultimate Bearing Capacity, the ultimate and allowable soil pressure is computed using Terzaghi’s formula and a factor of safety. The allowable soil pressure carries all loads, from surcharge loads, weight of materials, and the structural loads. Surcharge loads are widespread uniformly distributed loads, such as slabs on grade. For the pressure due to the weights of soil and concrete, Ws + Wc = γsAfhs + γcAfhc Ws + Wc Af = γshs + γchc Therefore, allowable pressure = surcharge + weight of soil + weight of concrete + pressure due to loads qall = qs + γshs + γchc + qeff 2. Footing Dimensions 413.3. Shallow Foundations 413.3.1. General 413.3.1.1. Minimum base area of foundation shall be calculated from unfactored forces and moments transmitted by foundation to soil or rock and permissible bearing pressure selected through principles of soil or rock mechanics. The effective soil pressure is computed from the service loads and the area of the footing. qeff = Pservice Af For a strip footing, a foundation supporting a relatively long member as in a wall or tie beam, only 1 m strip of length is considered. Thus, to compute its width, the service loads must be expressed as a linear load (kN/m). qeff = Pservice in kN/m B(1) qeff = Pservice B For a square footing, qeff = Pservice B2 Square footings are more preferred due to the uniformity of loading in both directions. However, a rectangular footing is used when there are restrictions on the dimensions of the footing such as due to property boundaries or closeness to other footings. qeff = Pservice BL
3. Design of Wall Footing (or Strip Footing) 3.1. One-Way Shear Similar to beams, wide beam shear (also called one-way shear) is computed at a critical section not always on the face of the support. 413.2.7. Critical foundation 413.2.7.1. Mu at the supported member shall be permitted to be calculated at the critical section defined in accordance with Table 413.2.7.1. Table 413.2.7.1. Location of critical section for Mu Supported member Location of critical section Column or pedestal Face of column or pedestal Column with steel base plate Halfway between face of column and the edge of the steel base plate Concrete wall Face of wall Masonry wall Halfway between center and face of masonry wall 413.2.7.2. The location of critical section for factored shear in accordance with Sections 407.4.3. and 408.4.3. for one-way shear or Section 408.4.4.1. for two-way shear shall be measured from the location of the critical section for Mu in Section 413.2.7.1. 407.4.3.2. Sections between the face of support and a critical section located d from the face of support for non-prestressed slabs or h/2 from the face of support for prestressed slabs shall be permitted to be designed for Vu at that critical section. To illustrate for wall footings, Consider a wall footing only carrying axial loads, the wide beam shear is the force lifting the removed portion, equal to the ultimate soil pressure times the effective area. The ultimate pressure can be computed using: qu = Pu in kN/m B For the one-way shear considering 1 m length of footing, Vu = qux(1) Vu = qu ( B 2 − w 2 −d) (for concrete wall) Vu = qu ( B 2 − w 4 −d) (for masonry wall) If not specified, the default consideration for the critical section is that of the concrete wall. From PSAD 34, the value of the ultimate shear capacity of concrete, Vu = ∅Vn, is Vu = ∅(0. 17λ√fc ′bwd) In footings, the shear is carried only by concrete as there are no shear reinforcements.