Wind Loads for Open Buildings of All Heights

[Structural Matt]

Designing a standalone Porte cochere as an open structure and in going through Chapter 27 of ASCE7-16 I am getting stuck at determining external pressure coefficient in step 6 of Table 27.2-1. My structure has a pitched roof, so I am using Fig. 27.3-5, and this is where my problems begin.

First problem: the figure specifies that h/L shall be between 0.25 and 1.0 but my structure has a ratio of 0.20. I don't see any information on what to do if you fall outside their determined range.

Second problem: What are the two load cases given in the figure? I can't find where it specifies what the Case A and Case B are for this particular figure.

 

[JStephen]

Second problem- bottom sentence in that figure says "All load cases shown for each roof angle shall be investigated", so I think the intent is to check with Case A then check with Case B.
First problem- I've run into this in Figure 29.4-5, where it says "0.25 <= H/D < 4.0". I'd say if you're close to the range, use the max/min in that range or extrapolate out to your point.

 

[Structural Matt]

1) So how would you extrapolate? The data from the table seems to be independent of whether you are on the high or low side of the h/L range

2) It is pretty clear to check both, but I guess I'm struggling with finding what Case A and Case B are. Where are they defined? Are they the different directions the wind can come from? I.E. coming perpendicular or parallel to ridge? Like Chapter 28, figure 28.3-1 clearly shows what Case A and Case B are, but that seemingly only applies to Enclosed and Partially Enclosed Buildings

 

[Agbsh]

First question, you may refer to BS 6399-2 Wind British Standard (I added few screenshots).
Second question, the two cases are coming from enveloping wind tunnel results. ASCE 7 copy values from Australia/New Zealand wind code, British standard gives very close results. (Edit: remember those values for one direction only, for 90 and 270 degree, ASCE 7 shows different table FIGURE 27.3-7, the best to do to envelope results from both sources in each direction).