Height limits for nonbuilding structures

[joeHSE]

I have a nonbuilding structure that is similar to a bldg. As I look at ASCE7-05 in Table 15.4-1 at the Ordinary steel concentric braced frames I am limited in height to 35' for SDC=D. Just below I note "With permitted height increase" with lower R, Omega and Cd and a height limit of 160ft. My structure is 63' and not a pipe rack...where-oh-where do I find criteria for a "permitted height increase"? No luck in AISC341 or in the rest of 15.4. Thanks

 

[MiketheEngineer]

Is this an industrial application?? I have seen many structures over 300' tall using many differing methods.

 

[joeHSE]

Yep. It is a loadout bin above a railway. Basically loading coal into rail cars. The more I look at this thing the more I'm thinking of going with nonbuilding NOT similar to a building and looking at Elevated bin/hopper. R=2 and a 1.5 importance factor...should be interesting.

 

[chrislaope]

Hi, joeHSE:

Just curious, why do you take I=1.5? Why not I=1.0? Is there any reason for that?

Thanks.

 

[joeHSE]

Well, in 15.4.1.1 of ASCE7-05 you get three options for the importance factor and have to choose the highest from the following:
1. Applicable reference document listed in Ch 23.
2. The largest value as selected from Table 11.5-1.
3. As specified elsewhere in chapter 15.

I didn't have any luck with 1 or 3 so I think I'm stuck with 2, thus the 1.5.

 

[azcats]

My understanding (and I'm no expert) of Table 15.4-1 is that you can use the 'permitted height increase' up to the limits in the table provided you use the more stringent design values on that line. That's what makes it permitted.

Regarding 15.4.1.1, is your structure in occupancy category IV? I could perhaps understand III for power generation if that's where your coal is headed. Item (b) in 15.4.1.1 is oddly worded, but I don't think they intend for you to always select I=1.5 unless your structure is O.C. IV.

 

[joeHSE]

Thanks azcats...I hadn't thought of it from that angle. That makes senseand that will definately help. Thanks again.

 

[chrislaope]

joeHSE:

Regarding 15.4.1.1, I have the same feeling as azcats. I have seen a design calculation report of a grain storage hopper bin one or two years ago, in which, the designer (an SE) applied the ACSE7-05 table 1-1, and take the hopper bin as an Agricultural facilities, thus belong to O.C.I. I guess it's reasonable because Table 1-1 applies not only to buildings but also to "Other Structures".

Just my 2cents.

 

[chrislaope]

With more close study of ASCE7-05, my interpretation of ASCE7-05 for the importance factor of this kind of structure is:

if the hopper/bin is in a hospital, a police station, an emergency shelter, etc., then it belongs to O.C.IV.

if the hopper/bin is in a power generation station, a water treatment factory, a university, etc., then it belongs to O.C.III.

if the hopper/bin is in a farm, then no matter what content it has (coal, sand or grain), it belongs to O.C.I.

Please correct me if I am wrong.

Thanks.

 

[joeHSE]

chrislaope:

I'd say you are dead on and that is the approach that I settled with...

In my rush to get the thing done I took the ASCE text at face value...reading "largest value" and moving on. I simply hadn't considered the intent of the code.