ASCE 7-16 Wind loading Figure 27.3-1 and Figure 27.3-8 1

[BulbTheBuilder]

I need help understanding this and correct me if I am wrong. I have been trying to get deeper understanding of wind loading in ASCE 7-16.
When wind blows, we have the windward, leeward, and vortex (at the side of a structure). Is that what Figure 27.3-1 is trying to illustrate? From the image we can see the wind spreading perpendicularly to the wind direction.

Figure 27.3-8 shows the load cases to be used for wind designs. From Figure 27.3-8, load cases 1 and 2 show wind forces to act along wind direction only (without the suction on the sides). Is it because we don't expect wind load to act in the X and Y directions simultaneously?

I don't see the wind action illustrated in figure 27.3-1 in figure 27.3-8 (basically I thought the wind will be behave as shown in 27.3-1).

Also from figure 27.3-8, we get 12 set of load cases.

2 from case 1, 4 from case 2 (due to +/- ex and ey), 2 from case 3, and another 4 from case 4​

Does the sign convection affect M_T in cases 2 and 4? E.g. From Figure 27.3-8 M_T is counter-clockwise when e is + and clockwise when e is -ve?

[SOrry for typos. It's late here and my sleepy self reading to understand all these is making my head spin.]

 

[ProgrammingPE]

When wind blows, we have the windward, leeward, and vortex (at the side of a structure). Is that what Figure 27.3-1 is trying to illustrate?

You're correct that Figure 27.3-1 is showing that the wind produces a positive pressure on the windward wall and a negative pressure (suction) on the side walls as well as the leeward wall.

I don't see the wind action illustrated in figure 27.3-1 in figure 27.3-8 (basically I thought the wind will be behave as shown in 27.3-1).

The purpose of Figure 27.3-8 is to show two things:
[ol 1]
[li]How to combine pressures when wind is blowing in two different principal axis at the same time.[/li]
[li]How to account for torsional wind loading.[/li]
[/ol]

While Figure 27.3-8 does not explicitly show the side wall pressures, I would not interpret that to mean that they disappeared, since that would be counter to what Figure 27.3-1 shows us. They were just not included in the figure to make it easier to understand the intent of each load case. If the diagrams also showed the side wall pressures, then Cases 3 & 4 would have two linear loads applied on all four sides. Cases 1 and 2 would have pressures on on four sides which would make it hard to see what direction the wind direction is blowing.

Also, side wall pressures in most cases end up cancelling out (except for buildings with expansion joints), so they don't really have an impact on the global analysis of the MWFRS, which is what Figure 27.3-8 is used for. Side wall pressures do still need to be considered, though, when you have a MWFRS element that braces the building that also receives wind pressure directly like a shear wall that also transfers wind pressure up and down to the diaphragms.

Also from figure 27.3-8, we get 12 set of load cases.
2 from case 1, 4 from case 2 (due to +/- ex and ey), 2 from case 3, and another 4 from case 4
Does the sign convection affect MT in cases 2 and 4? E.g. From Figure 27.3-8 MT is counter-clockwise when e is + and clockwise when e is -ve?

I see 24 load cases (same as the cases you figured, but with twice as many to account for wind in the opposite directions as well). Sign convention for MT shouldn't matter since it tells you to use plus/minus for the eccentricity, so just be sure to check all cases. (For Case 4 there is plus/minus for both ex and ey. For the 24 load cases, I'm assuming that you are combining ex and ey so that they add together to increase MT.)


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[BulbTheBuilder]

Thanks for your response ProgrammingPE (Structural).

Couple of questions.

While Figure 27.3-8 does not explicitly show the side wall pressures, I would not interpret that to mean that they disappeared, since that would be counter to what Figure 27.3-1 shows us.

To clarify, taking Load case 1 from Figure 27.3-8, I will have windward and leeward (q_zGC_p) and in addition, my suctions on the side(q_zGC_p) which applies to all four main load cases. If there cancel out then I just ignore them

Also, side wall pressures in most cases end up cancelling out (except for buildings with expansion joints)

Can you explain a little on this? I can't visualise it.

Side wall pressures do still need to be considered, though, when you have a MWFRS element that braces the building that also receives wind pressure directly like a shear wall that also transfers wind pressure up and down to the diaphragms

Assuming I have masonry walls in-between shears (taking wind forces), is it right to use C&C wind forces to design only the masonry walls (in this scenario I am not considering the masonry wall to be part of MWFRS)

Thanks a lot!

 

[ProgrammingPE]

Can you explain a little on this? I can't visualise it.

For the side wall suction to cancel out, you have to have a diaphragm that is continuous between the two loads. If there's an expansion joint, then you have multiple diaphragms and each receives the load from only one of the side walls.

Everything else you said appears to be correct.

 

[BulbTheBuilder]

I get it now. Thanks a lot for your help!