ACI 318-19 Reduced Shear Capacity 1

[zurch1818]

thread507-499654

Expanding this referenced thread, how have people been handling this for existing structures that weren't designed with this provision in mind? To me it seems really odd to have to say a structure that was completely fine before now is screwed over when the gravity loads (5%) and lateral loads (10%) exceed IEBC limits?

For example, I'm working on an existing pan and joist building from 1983 where some small rooftop telecommunications equipment is proposed to be installed. I'm exceeding the gravity loads by 8% from the original condition. With where I'm putting the loads, moment is checking out (although starting to reach its capacity) but shear is now blowing up (even with the 10% bump with 8.8.1.5). Shear is also blowing up in the original condition. If I use the old provisions, shear will check out in the proposed case.

Since it is a pan and joist system, there really isn't room for stirrups (even if there was some way to install them in a joist that is only 4" wide at the base).

 

[milkshakelake]

The way I think about these things is that if it's existing, you don't need to do anything. The second you touch it, you have to comply with modern codes.

A similar thing is altering an existing building that uses brick shear walls, or has no shear walls. If you extend the building by a few feet, suddenly the entire building's lateral system has to comply with modern codes, and all those old shear walls count for almost nothing.

 

[Deker]

It may seem odd, but sometimes educating clients about evolving code requirements is part of the job. Sounds like a good application for FRP wrap.

 

[milkshakelake]

Or add a corbel? Shear is taken at a distance d from columns. Decreasing the d will decrease the shear, hopefully by enough.

 

[lexpatrie]

I think the previous codes had specific treatment of pan joists. They performed better in shear (as I recall) than beams. You also have that distance d or d/2 from the face to take into account that may get it closer to pass.

If you read the IEBC there's some guidance on the overall issue - if the loads are 5% or less of the original design (or the in place load, ?) then it's considered okay without reinforcing. This aspect wouldn't require an engineer to sign off on it.

As the (replacement) engineer of record you could potentially accept it as-is regardless of the percentage of overstress. You'd be at some risk, but it would probably need to fail. I personally wouldn't cross that line but some might. FRP would be an option, if reinforcement is desired.

Usually the bars in joists were very manipulated, so there's old CRSI designs you can look at, and usually some of the bottom bars turned at an angle and became top bars toward bearing. When I had these they were always computationally intensive, but they had decent capacity.

If you get into a full on current code, the Phi factor could go upward slightly if the concrete strength is verified, I forget which code. It came up in another thread. I think it was ACI 318 there as well. Not some odd code.

Regards,
Brian

 

[zurch1818]

Thanks for the information all. I've never heard of FRP strips before. Since my client is not the structure owner, it makes performing some of these modifications very difficult. Where can I get more information on these FRP strips?

It turns out that these joists are at a lot higher of DCR than originally thought. I probably should have looked at the work a little bit closer before broadcasting it. The calcs were submitted to me with a trib width of 1.75' and in reality, it is 3'. I guess that's why it's trust but verify. Now I'm calculating that moment is near 100% in the original condition.

Yes, your memory is correct lexpatrie, joists get a 10% bump in shear capacity. In the commentary, it says that is because 1) it gets the capacity more in line with previous versions of the code and 2) Joists are a repetitive member (similar to designing wood joists).

 

[lexpatrie]

DCR though? Design-Capacity Ratio?

If you have those bottom bars that turn into top bars, I think they were sometimes used as diagonal shear reinforcement.

Also, usually the ends tapered out into a larger width, if they are standard forms.

CRSI Design Handbook, Concrete Steel Reinforcing Institute, 1959, p. 136- 186. Particularly 170 or so. And if you're in Ohio, Fling likes to do a lot of moment redistribution, and basically everybody worked for Fling at one point.

 

[zurch1818]

Yes. DCR is the Demand Capacity Ratio...and it seems original EOR economized the design (in at least the localized location of where I am checking). My structure is from 1983 where I don't have those bent bars. It's actually probably the newest pan and joist structure I've ever worked on. From my experience, it seems the hay day of the pan and joist structural system was more like the 1940's to the 1960's and then it started petering out in favor of more economical solutions. I've seen it in structures from the 1920's but nothing really older than this.

 

[lexpatrie]

If you are in the neighborhood of getting these sections to work, there's possibly a higher phi factor you can use for existing strength evaluation (Chapter 21, ACI 318). I mentioned it here (with pictures!)

https://www.eng-tips.com/viewthread.cfm?qid=510664

Regards,
Brian