Pre-engineered buildings 1

[mattshak33]

I am designing a foundation for a pre-engineered building and I am wondering where to find a good example of a thickened floor slab as a footing.

 

[ARIFSIDDIQUI]

i do not see any specific thing in that it should be designed as normal industrial structure.

The concrete footing should be designed correspoding to the forces as per ur analysis wind or whatever itmay be. The manufacturer will give the base forces to u for the design of the footing.

Once u have the forces u candesign the foundation accordingly

 

[whymrg]

Book "Design Floor Salbs on Grade" by Boyd c. Ringo and Robert B. Anderson (published by "the Aberdeen Group"-1-800-323-3550,ext 219 - this phone number is 8 years old). This book has chapter 5 "Slab design for column or wall loadings". This is wery good information and examples. Also, I would use thickened slab on grade for interior columns with vertical load only and regular spread footings for exterior columns with horizontal loads and moments.

In addition I have used hairpins for large horizontal loads at exterior columns. Hairpin is a U-shaped bent rebar which goes around exterior column or embedded in pedestal and transfers the horisontal load into the floor slab.

Hope this helps.

 

[mattshak33]

Thanks whymrg!!!
I posted this question because I am unable to design a standard spread footing at this project location. My client is placing a pre-engineered building between existing equipment and the "room" isn't there is place spread footings. I felt that the next most economical option would be a thickened slab design; however, I have never designed this type of system. I am learning!!!! The vertical loads are somewhat minimal (24.5k for an 15' bay and a 10.9k uplift) therefore I felt that this foundation system will work. The area that I am unsure of is the hairpins. The max horizontal factored force is 14.6k.
Any suggestions?

 

[mattshak33]

whymrg - I found the "Design Floor Slab on Grade" on the net. Is this book a good reference for general building design?

 

[akastud]

mattshak33,

In the preman. bldng. ftgs. I have designed, the footing size is generally based on the uplift force. As I understand both the '97 UBC and the 2000 IBC you need to apply a factor of safety for the uplift of 1.5 so your net uplift that you have to resist with your DL is 16.35 kips. With a concrete weight of 150 pcf you need a concrete volume of 109 cubic feet. That volume would be difficult to acheive with a thickened slab, and if you do find the mass, you'll need to check your slab for both bending and punching.

 

[NigelB]

Matt,
Don't forget to check for uplift! Metal buildings are very light, and the uplift force on a column can be as large as the gravity load. I design for a 1.5 factor of safety against uplift, as I see this as comparable to overturning.
Also, check for horizontal loads at the columns. It is common practice to install "haipins" around the anchor bolts and extend them 6 to 10 feet back into the slab.
Nigel

 

[rlewistx]

I don't know where you are so let me just tell you what is common in Texas. A turned down slab foundation is very common. Since frost is not a problem I usually turn down the slab edge 24". I thicken the slab the full 24" under the column, primarily for wind uplift as noted in other posts. You use your "judgment" to include weight of surrounding slab beyond the thicken column footing in your uplift resistance. Since the perimeter of the slab is turned down all the way around the building you can use an extension of it in your dead load resistance to uplift too. Again, it is a "judgment" call.

Other considerations - I always use rebar in the slab instead of WWF. I want good tension ties and I am more comfortable with rebar than with WWF. Use hairpins and lap then sufficiently with you slab rebar. This also let’s me use “judgment” to include some of the slab area in my uplift resistance. Slab construction joints are a potential problem. I never have a construction joint perpendicular to column lines, only parallel to them. Only shrinkage joints perpendicular to column lines. Shrinkage joint location is also a concern. Typically in steel framed building joints are located on column lines. I don't do that in a MTL building. I locate them 1/2 the distance between columns. Reason is that at the joint you expect the most movement. Well, at the column line I have a huge slug of concrete. That's an anchor so the slab won’t move. 1/2 the distance between columns I have a thin slab. That is the weakest point and that's where the crack will most likely occur. This seems contrary to standard practice, but if you think about it and look for the slab weak points I think you will agree. I recently detailed all the joints on a building like I explained here and the contractor thought I was some stupid engineer who didn’t know how to joint a slab. He cut it right on the column line. In my opinion he wasted his money. I somewhat hope the cracks appear where I expect them to so I can say “I told you so”. Same with the joints in the opposite direction. The turned down slab is an anchor. I always space my first transverse joint 1/2 the typical distance parallel to the exterior wall because as far as the slab is concerned it is twice as big with the perimeter turned down slab.

HTH