Smart questions
Smart answers
Smart people
Join Eng-Tips Forums
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Member Login




Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • 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!

Join Eng-Tips
*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.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

polskadan (Structural) (OP)
28 Aug 13 9:44
Hello all,

Lately I have been working on a project involving retrofitting an existing structure where more weight would need to be added to the original system. Our retrofit puts us in the category of an Alteration 2 within the IEBC code. After investigation of this older structure it has been found that the structure was never designed for seismic loads, rather it was designed for wind loads. IEBC has a 10% trigger rule for lateral loads which states, "any existing lateral load-carrying structural element whose demand-capacity ratio with the alteration considered is no more than 10 percent greater than its demand-capacity ratio with the alteration ignored shall be permitted to remain unaltered." With that being stated, using my common sense engineering judgement I thought that this rule is considering that the structure/member was designed correctly originally. Meaning lets say that a member was designed originally so that the demand-capacity ratio is exactly 1.00 under original loads. Well then I feel comfortable taking the design to 1.1 interaction and eating ever so slightly into the safety factor for lateral loads. I feel comfortable doing this knowing that code updates have led us to more stringent requirements and it wouldn't be fair/economically viable to update/upgrade all buildings every time a new building code was adopted with more stringent guidelines...this was my understanding of why the IEBC was created in the first place.

Now here is the question/issue that has arisen from this project. After I analyzed this structure for current seismic loads, I have had interaction ratio's of 5.00 and higher at some braces due to the fact that this structure has a high mass at the top of the building and wind load obviously acts differently here. So my common sense engineering judgement/interpretation of what the code intends here is that these members need to be upgraded since we are 'touching' the structure and altering it. But here is where I have been apparently been proven wrong...since the new interaction ratio with the alteration considered is only lets say 5.05, then this structure is fine! We do not need to upgrade and bring it up to code! I have even received recent correspondence with the ICC on this and they too stated that you only need to compare the demand-capacity ratio's to each other before and after alteration...so apparently it doesn't matter what the interaction ratio is under original loads (could be 10000.000+) as long as the difference in the interaction ratios with the alteration considered is <10% for lateral loads!

Has anyone else here had experience with this portion of the code? Have you had other interpretations with this portions of the code that differ from what I have heard or would you go about it the way I originally interpreted the code? What do you guys think? It is just hard for me to swallow that the code says it is legal for something to be overstressed 1000% over as long as originally it was only overstressed 999%...this seemed to be a no brainer to me until I had other engineers arguing for this interpretation (though I bet these same engineers would not agree with using the 5% gravity trigger in the same way as this load is a 'real' load and seismic not so much...although show me where in IBC or ASCE that this is seperated in such a way ;) ). Maybe my original interpretation was flawed and bias simply because my natural engineering judgement tells me to design members to not fail...crazy concept I know.
Bagman2524 (Structural)
28 Aug 13 14:04
Use of common sense vs. something written in a book. should be a no brainer. If the building collapses under seismic load you have to live with the consequences and it's your butt on the line, not theirs.
msquared48 (Structural)
28 Aug 13 14:30
Before you say that a member was designed incorrectly, you have to know, or at least suspect, how the original designer loaded the member.

I just ran into a situation where three concrete beams were used to frame around an original opening a two way flat slab with drop panels and shear heads that was constructed in the early 20's. If you take the full possible area to the beams, the beams fail miserably in bending. But if you factor in the most probable load distribution (much reduced) from a two way slab scenario with beam and column strips, the results are reasonable and the beams do not fail.

I have found from experience that the designers of those times were pretty reliable. However, you do have to consider the implications of current code with any modifications.

Mike McCann
MMC Engineering

SteelPE (Structural)
28 Aug 13 14:52
I have had to use this section of the code a few times. My interpretation is along the same lines you discussed. If the increase is <10% then I don't worry about it.

You have to also look at it from the eyes of the building code and building owner. If you design a building today and then code changes tomorrow with an increase in the snow load (gravity load limit is 5%) and then the owner wants to put a new RTU on the building, does than mean the owner needs to update the entire structure for the new snow load? The owner of the building would not be very happy and the code would not change too often. The same argument could be made for lateral loads. Increase the mass of a roof (by adding a RTU) for a building that was not designed for seismic loads and the whole LFRS would need to be updated. That would make many existing buildings worthless.

Depending on your relationship with the owner, maybe you should discuss with him what your findings are and what the consequences if there is a seismic event in the future.
RFreund (Structural)
29 Aug 13 19:41
I interpret this the same way (right or wrong)-> just looking at the percent increase not that actual capacity.

EIT
www.HowToEngineer.com

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!

Back To Forum

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close