asce 7-98 wind load

[bjb]

ASCE 7-98 only gives force coefficients for open buildings with monoslope roofs. What are you supposed to do for an open building with a gable roof? I am using the building code of New York State, which is based on the 2000 IBC which references ASCE 7-98.

 

[boo1]

ASCE 6.5.11.4 list using internal pressure Cp=0.8

 

[scherry]

I find the IBC (ASCE) provisions to be difficult to apply to almost anything that is not "normal", and a lot of what I am doing is not "normal" (octagonal building with a peaked roof,currently). I am using the best fit I can find and then checking it using the old UBC 97 provisions, which generally result in higher wind loads. I suspect that if I was dealing with any of the topo effects, the UBC method would NOT be conservative, and so I do not know what I will do when or if I get there, but at least the UBC method is rational...

 

[kvram]

Dear Mr. BJB,
Ref. Fig 6-3 from ASCE7-98 .Follow the coefficients with Gust factor as 0.85 in case of rigid structures or calculate G value in case of flexible structures with wind Exposure"C". ( Use 0.8 for Exposure "A or B&quot .This is applicable for all heights in case open buildings. Since the overall coefficients are summation of internal and external coefficients , GCpi = 0.0 in case of open buildings. The governing one is only GCpf.

 

[kvram]

Dear Mr. BJB,
Ref. Fig 6-3 from ASCE7-98 .Follow the coefficients with Gust factor as 0.85 in case of rigid structures or calculate G value in case of flexible structures with wind Exposure"C". ( Use 0.8 for Exposure "A or B&quot .This is applicable for all heights in case open buildings. Since the overall coefficients are summation of internal and external coefficients , GCpi = 0.0 in case of open buildings. The governing one is only GCpf.

 

[chichuck]

I think that there are two parts to this question:

1) wind loads on the gable roof, and 2) wind loads on the open structure supporting the roof.


For 1) ASCE 7-02, in the commentary to chapter 6, on page 289 is mentioned for Figure 6-18 (monoslope roofs over open structures) that for other conditions not specified in the code, some guidance may be found in their references C6-3 and C6-36.

The former is that famous classic ASCE reference on wind forces, Wind Forces on Structures, Final Report of the Task Committee on Wind Forces ao fhte Committee on Loads and Stresses of the Structural Division. Paper NO. 3269. Transactions, ASCE Vol. 126, Part II, 1961, p. 1124.
This ought to be available from a good university library or other source. My copy is old and tattered, not good for copying and sending to otheres. In that reference is included a rather large array of tables that include force coefficients for structures and buildings. In table 4(d) (on page 1160) Wind Pressure Coefficieints for Shelter Roofs, is shown the condition referenced: gable roof over an open building. Not a lot of different geometries are included, but I suggest using these directly in ASCE Eq. 6-25. In that equation,for the roof, use Af = the appropriate panel area, along roof slope, not the projection on a vertical plane. (In other words, calculate as though the wind is perpendicular to each roof panel.)
The latter is a paper from a German source, and is written in German, so I bet neither of us can read it.


for 2) you could use ASCE figure 6-21, or you might go to the ASCE publication Wind Loads and Anchor Bolt Design for Petrochemical Facilities by the Task Committed on Wind Induced Forces (available from ASCE.) That procedure is intended for relatively large industrial structures, and is rather long calculation. I'm not sure which give larger wind forces, but I suspect it is the former, and I further suspect that if you took the time to calculate with the latter, you would find little advantage (ie decrease in loads), in fact that decrease might not be worth the time spent doing the calculation.

I'm not familiar with British, European, Japanese or Austrailian codes, these might offer some help. But if you can get hold of any of these, make sure you are aware of differing underlying assumptions in those codes, eg height and exposure categorys, the time interval used to measure the basic wind speeds, limitations on size and/or shape of buildings included in their tables, etc.) Also, if you manage all that, I don't know how you would go about convincing local building authorities to accept such an analysis as a basis for your design. If it comes to that, perhaps you'd be in a better position than I to deal with that.

Good luck.

chichuck

 

[bjb]

Thanks all for your replies.

boo1: Section 6.5.11.4 references Figs. 6-3 and 6-4, which are for enclosed or partially enclosed buildings. Also, ASCE 7 covers monoslpope roofs for both open and enclosed/partially enclosed buildings, with significantly different force coefficients for the open case. I do not believe that 6.5.11.4 is applicable for my case.

Kvram: Fig 6-3 is for enclosed and partially enclosed buildings, so I don't think it can be applied for an open building with a gable roof. Fig 6-3 also applies for monoslope roofs on enclosed/partially enclosed buildings. If you compare the force coefficients from Fig 6-3 with those of Table 6-9 for monoslope roofs with open buildings, it is seen that there are significant differences. I would expect that if ASCE 7 addresses open buildings with monoslope roofs, it should also specifically address open buildings with gable roofs.

Chichuck: Thanks for your guidance, I will try to find that publication. I find it very annoying that ASCE does not include this information directly in the Standard, because with the schedule I have it is very difficult to find the time to track it down.

 

[mjohan]

Why not consider the gable as a wall and use windward and leeward forces on it?