×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

ASCE 7-05 Table 15.4-1 "Height Increase"

ASCE 7-05 Table 15.4-1 "Height Increase"

ASCE 7-05 Table 15.4-1 "Height Increase"

(OP)
What allows you to say that you have a system with "permitted height increase"?

I am analysing a structure used as a cooler along a conveyor system and the owner would like to install heavy item supported above it. ASCE table 15.4-1 does not permit the use of an OMF in seismic design category D unless it is a "permitted height increase" which would mean I can use an OMF up to 100ft as long as I use R=2.5, omega=2, & Cd=2.5. However, what permits me to say that I have an allowable height increase? I can't seem to find anything on this.

Thank you for helping,

RE: ASCE 7-05 Table 15.4-1 "Height Increase"

A quick search popped up this thread:

Link

RE: ASCE 7-05 Table 15.4-1 "Height Increase"

(OP)
Thank you jittles. I did read that thread before posting mine and I would agree with azcats on saying that I as an engineer permit the height to be increased as long as we design the structure for the lower R values. However, our plan checkers in our area are getting to a point where they want everything proven mathematically or academically and will not accept an engineer's judgement on simple or obvious things.

So my question still stands. Also section 12.2.5.6 & 12.2.5.7 for building structures does say I can use ordinary systems as long as the tributary load to the frame from the roof does not exceed 20psf and in light frame construction with floor or roof loads not exceeding 35psf.

RE: ASCE 7-05 Table 15.4-1 "Height Increase"

For non-building industrial structures I almost always use the lowest R value possible, preferring to take the penalty on my seismic loads rather than conform to AISC 341.

Contrary to popular belief, the cost is often in the complexity of the details rather than the magnitude of the seismic load. The idea that a smaller earthquake forces (higher R values) lead to smaller members doesn't usually hold true because members get held to tight compactness criteria and slenderness ratios that usually force you into a huge size.

That said, why not use an R of 1 per table 15.4-1 and avoid AISC 341 altogether? R=1 removes height limits altogether and only requires you to detail to AISC 360. R=2.5 still requires you to detail to AISC 341. Why make it harder than it needs to be?

To answer your original question, I believe the permitted height increase is addressed in the footnotes of Table 15.4-1 isn't it?

RE: ASCE 7-05 Table 15.4-1 "Height Increase"

(OP)
DCBII I just wanted to get away from using a higher seismic load, since I am analyzing an existing structure. I guess I'll just have to let the owner know that his structure was not designed to carry additional loads. Specially when those loads are 50 - 60% of the effective weight.

RE: ASCE 7-05 Table 15.4-1 "Height Increase"

You'll have an even harder time getting an existing structure to conform to AISC 341 if it wasn't built to 341 to begin with.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources