Metal building foundation design 1

[CJBHM52]

I just posted this in the foundation section of the Geotech forum, then I realized I was in Geotech and these are more structural questions.

I have a few metal building foundation projects coming up. On the last foundation I did for this company, there were 15 to 20 different base plate/edge distance/reaction combinations. For most of these, there were 8 or so wind reactions, 2-4 seismic reactions plus the dead, live and collateral reactions. This is a lot of load combinations to check by hand (I actually set up an excel spreadsheet) and it takes a long time. Is this how most of you go about finding the critical reactions? Is there a way to simplify it? I know you can eliminate some of the LC by inspection, but it is still very time consuming.

Also, I am in a high seismic area. When designing for uplift or shear, if you add enough rebar so the concrete breakout is not of concern, you can assume you have ductile failure of the rebar, correct? I want to be able to avoid having to use the overstrength factor because the seismic loads are high enough already, I don't want to have to design for double or triple the reaction. Thanks

 

[chrislaope]

We use MBS software. It will output all reactions from various load combinations, you just need pick the most critical one for downward, uplift, and horizontals respectively to design foundation, no need to do load combinations yourself.

 

[Jerehmy]

Hilti has free software out, although it is unnecessarily conservative for at least one breakout condition that I'm aware of.

Also, why does the metal building have so many different baseplate + anchor edge distance combinations? Most I've seen is like 3. Corner, endwall/sidewall, overhead door track columns.

 

[JedClampett]

You can ask the metal building engineer to provide his/her reactions in a digital format that you can create an Excel spreadshhet for. I've done that. Or you can do what I did, have the pdf digitized and enter that into Excel.
This is a task that is normally never given enough budget.

 

[PMR06]

Find an intern or new grad and have them work through this "very interesting and challenging task that will hone their mastery of load combinations"

 

[SteelPE]

Can't you eliminate most of the reactions just by looking at them? In my area we the snow load is greater than roof live load.... One combination gone. In my area seismic loads are typically much less than the wind (although I will still include these in the analysis).

I have a spreadsheet that runs through about 40 or so load combinations. I take the most critical loads and run them through that sheet.

 

[minorchord2000]

It is precisely because of the myriad amount of load combinations that we insisted that the metal building manufacturer provided the load combinations. They have the ability and the program to do it. They will cry and whine but they will ultimately do it. They can factor the loads as well. You need factored loads because the anchor rod design per ACI 318 App D is based on factored loads. We have found that the metal bldg community has ignored the changes required implicit in App D in regard to edge distance for shear.

 

[dylansdad]

I echo minorchord2000. Along with edge distance the spacing between anchor bolts is not given much thought, also.

 

[CJBHM52]

Thank you all for the replies. The last building we did was not exactly just a box, it was 'L' shaped and had three different roof heights, but there were a few different baseplate/anchor combos for similar frame lines. There are certain load combos that can be eliminated by inspection, but especially with the seismic overstrength factor coming into play (the second part of my post), I have to go through all the wind and seismic combos to find the critical cases. Thanks again.