ASCE Design Wind Load Cases & Side Wall Pressures 1

[Howlyn2]

Good Morning All. ASCE 7-16 states one shall follow the design wind load cases given in Fig. 27.3-8 when designing the MWFRS of a building. Exclusive of the torsional load cases, Case (1) shows the full wind pressure perpendicular to each axis of the building and Case (3) shows quartering winds. All wind loads are based on windward and leeward pressures. Figure 27.3-1 shows Plan & Elevation Views of different types of structures. Included in Figure 27.3-1 are Roof & Side Wall pressures in accordance with the last paragraph of 27.3.1 - where side walls and roofs fall under qi = qh that must be considered simultaneously.

During design and analysis when are the side wall / roof loads considered? Shall side wall and roof pressure only be added to Case (1) of Fig. 27.3-8? I don't see how you could include them in Case (3) as you'd have to break down the load trigonometrically and that does not appear to be the intent of the code. The commentary does not give any explanation either.

All advice is appreciated.

 

[driftLimiter]

The last paragraph says that the pressures must be considered on the windward and leeward walls and roof surfaces simultaneously. Case 1 and Case 2 from Figures 27.3-8 show a load applied simultaneously to windward and leeward faces. I believe using exactly the 4 cases in this figure covers all necessary conditions.

Notice the subscripts of the 'P' terms in the Notation on this figure. There is no sidewall pressure. You simply obtain the windward and leeward pressure in each direction, then combine them according to these 4 cases.

 

[Howlyn2]

Hello driftLimiter. I read the paragraph the same as you. However, the question remains then when will side wall pressure ever be considered for MWFRS Design / Analysis? There are no subscripts for it in Fig. 27.3-8 while Fig. 27.3-1 shows it but doesn't tell us when to consider it.

 

[driftLimiter]

This is a matter of notation and semantics I believe. Whatever you are calling 'sidewall' pressure in my interpretation is just the pressure on the walls that not getting hit with the 'prevailing' wind direction pressure. What I am saying is that there is no additional 'sidewall' pressures in this part of the all heights method. Everything you need are in the 4 load cases. There is pressures for windward and leeward surfaces in the X direction, and then there are pressures for windward and leeward in the Y direction. Combine them as required in case 3 and in case 4 only. Analyze the structure for their independent applications in case 1 and 2.

 

[phamENG]

Side wall pressures will come into play for things like:

Sub-diaphragm ties resisting sidewall pressures (while the diaphragm is busy taking windward/leeward loads to the vertical lateral force resisting elements, it's also being pulled outward on both sides by the sidewall forces; they cancel out, but have to be accounted for as a tension load in either the diaphragm or framing beneath it);

And tributary loads on MWFRS elements (think of a brace in a braced frame set in-line with steel studs; the steel studs will see that sidewall pressure and deliver it to the brace; the brace will bend about the X axis due to self weight/weight of wall above, bend about the Y axis due to the MWFRS side-wall pressure, and have axial load due to the MWFRS windward/leeward pressures as delivered by way of the diaphragm, etc.)