Rear wall loading

Rear wall loading is normally used only to determine the net overall frame loading. Because the rear wall load is opposite in direction to the front wall load, its inclusion tends to reduce the overall lateral blast force. Rear wall effects are many... 

The shape of (he rear wall loading is similar to that for side and roof loads, however the rise time ami duration arc influenced by a not well understood pattern of spillover from the roof and side walls and from ground reflection effects. The rear wall blast load lags that for the front wall by L/U, the lime for the blast wave to travel the length, L, of the building. The effective peak overpressure is similar to that for side walls and is calculated using Equation 3.11 (Ph is normally used to designate the rear wall peak overpressure instead of P,). Available references indicate two distinct values for the rise lime and positive phase duration. 

M 5-1300 provides criteria computing the rear wall load as though it were an extension of the roof. Though graphs are provided to determine the rise time and duration, for most typical control builidngs, the positive phase will have a rise time of approximately S/U followed by a duration of td (Figure 3.1 Ob). 

The rear wall is proportioned the same as the front and side walls, spanning vertically from foundation to roof. Because the highest loads are on the front wall, a rear wall analysis would only be. necessary to determine a net loading on the overall building. The analysis will be for a wall segment 1 foot wide. 

The above lateral forces were determined from the combination of front and rear wall peak reactions neglecting any difference in time phasing. A more numerically complex approach would have been to determine lateral and vertical loads at each time step, and use the maximum value for foundation design. Such an approach would have resulted in lower toads. 

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