ASCE 7-05 Simplified MWFRS & CC Parapet

[Lungshen]

Dear fellow structural engineers,

This question is more real work experience related than exam related.

I am curious to know what do you do when using ASCE 7-05 simplified method to calculate the MWFRS wind and Component & Cladding wind on the parapet wall?

I have looked every where and come to the conclusion that ASCE 7-05 do not specifically address the parapet wall loading when using the simplified method.

For Figure 6-2, MWFRS, if you take the positive horizontal wall pressure from zone C and add to the absolute value of negative vertical roof pressure of Zone G, will that be a conservative approach?

In Figure 6-3, Components & Cladding Wind, vice versa, if you take the positive wall pressure and add to the absoute value of negative roof pressure, will that be a conservative approach?

Or does it mean if you have a parapet wall, you just have to use Method 2 Analytical procedure and cannot use simplified procedure?

I think I have very rarely run into any simple diaphragm building without parapets - most building structure will have parapets of some sort to hide RTU's.

In ASCE 7-10, Section 27.6.2 Directional Procedure, you take 2.25 times the wind pressures given in table 27.6-1 (depends on which exposure) that will give you the parapet wall pressure. I suppose you can further multiply that by 0.6 to get the pressure down from strength level down to service level and compare that value with ASCE 7-05. Will it be safe to use the same approach in ASCE 7-05, you just multiply the simplified wall pressure by 2.25 for parapet wall load on MWFRS?

In real life if you underdesign or overdesign your MWFRS by let say 5 psf - its not going to hurt your design or make your building collapse. So I am leaning toward to take that 2.25 factor and apply it to ASCE 7-05, when using simplified method and call it good.

What are your opinions?

Thanks.

 

[TehMightyEngineer]

In my opinion the intent of ASCE 7-05 is to have you use Method 2 for parapets as it's the only section to address it. This is backed up in my mind by the limitations of section 6.4.1.2 which call for a "flat" roof. Obviously the roof can be flat behind this but in my opinion this throws out roofs with significant parapets.

In the commentary see section C6.5.11.5 Parapets. This talks about the history of the Method 2 section and how it was only introduced in ASCE7-02 and revised in 05. Thus, it's quite new and I imagine Method 1 was kept as similar to prior simplified wind load calculation methods and thus doesn't address the new research.

As for ASCE7-10, I'd compare ASCE7-05 method 2 with your simplified method from ASCE7-10 and see how the loads differ. That may be sufficient to prove that your simplified method is conservative compared to ASCE 7-05's Method 2 approach.

Remember, we're not just trying to provide safe, collapse free designs when we engineer a building. We are also providing shielding for the client and/or owner from liability and litigation should something fail. While you might be only 5% off on your load and well within the safety factor someone could argue that your design is wrong. $$$$ = ultimate engineering standard

Anyway, this was an interesting question and I'll have to ponder this more.

 

[McEngr]

pirate is right - use method 2.

 

[Lungshen]

so If I understand you correctly, any building - even if its a straight forward box simple flexible diaphragm enclosed building - you CANNOT use simplified approach (method 1) if it has, let's say 3'-0" of parapet wall to hide the RTU's?

If that is the case, the only building that can simplified method is pre-engineered metal buildings as most commerical buildings, even single story buildings don't qualify.

 

[TehMightyEngineer]

so If I understand you correctly, any building - even if its a straight forward box simple flexible diaphragm enclosed building - you CANNOT use simplified approach (method 1) if it has, let's say 3'-0" of parapet wall to hide the RTU's?

If that is the case, the only building that can simplified method is pre-engineered metal buildings as most commerical buildings, even single story buildings don't qualify.

Sadly I would say you are correct. However, I see just as many flat roofed buildings with parapets as those without in my personal experience.

 

[McEngr]

you can't use the simplified procedure for metal buildings. they don't assume a flexible diaphragm. they are horizontal (tension-only-diagonals) trusses. you can use the method 2 for h<=60 from figure 6-10 though

 

[McEngr]

why would you design a building for method 1 for a commercial building anyways?

 

[TehMightyEngineer]

you can't use the simplified procedure for metal buildings. they don't assume a flexible diaphragm. they are horizontal (tension-only-diagonals) trusses. you can use the method 2 for h<=60 from figure 6-10 though

I don't recall and can't find any requirement that the ASCE7-05 method 1 wind design be limited to flexible diaphragms only. Am I missing something?

why would you design a building for method 1 for a commercial building anyways?

Agreed, method 2 isn't that much more complex if you're dealing with large box structures (as commercial generally tends to be).