Metal Building Foundation Repair

[j19]

I'm looking for some ideas on how to "repair" a new foundation for a metal building. The building is wide with a low eave height so the kickout forces exceed what can be resisted by hairpins so I designed tie-rods that run from footing to footing. The footings are spread footings with about a 5' high pedestal and the top of the slab is about 4' above the surrounding ground. The contractor "knew" the tie-rods were overkill so he put some rebar, that was 1/3 the area of the tie-rods that were shown on the drawings, in the thickened area of the slab and he tied the ends of the rebar to the anchor bolts with, you guessed it, tie wire. He did add #4 hairpins because they were being used at the endwall posts and he had some left over. Note: These are assumptions because he only poured half the slab so I was looking at the unpoured area. He thinks he's ready to make the second pour and that's the reason for my assumptions.

So, I consider his rebar to be completely ineffective and I'm looking for some options before I meet with the owner, who is aware there is a problem.
1) My first idea is to saw cut the 4" slab at the thickened area all the way to the edge of the slab and jack hammer or whatever it takes to remove the concrete and do the tie-rods right.
2) My second idea is to add a pipe column at approx. midspan beneath the metal building frame to reduce the kickout and see if maybe the #4 hairpins will work. The owner is going to have to make a lot of concessions for this to be a valid option.
3) Third idea is to pour some kind of stem wall outside of the building that would help the footing resist the overturning. There's a lot of details that would have to be worked out with this idea, like how to anchor the stemwall to the footing and column pier. Also, the horizontal load is being applied at the bottom of the column base plate so how can that force be spread out so that I'm not depending on the anchor bolts bearing against the concrete?

I'm looking for any other suggestions or things I need to consider for any of the options that I mentioned. Thanks for any help you can provide.

 

[SteelPE]

what was the area of steel you called for based upon? Was it based upon the kick force only? Meaning, if you had 30kips of tension you provided enough steel to meet this requirement or did you provide more steel to reduce the elongation of the rods and prevent movement of the footing base?

I design about 6 metal building foundations/year and have been told to avoid tie rods and hairpins at all costs. I have had some chances to scope out the designs of the competition too and I can never get how someone can justify a 4x4 footing under a column with 4' of frost protection and the column on a 4' knee wall, no center column and a 60' clear span. This is probably where your problem originates.

 

[HouseBoy]

By "do the tie rods right" do you mean they would cut a trench all the way across the building?

I wonder if they could grind the slab in a groove and use CF in some way. Seems like it would be taught to rationalize the traffic wear on the CF if it is located at the surface. Just "thinking out loud".

Alternatively to excavating and removing concrete from under the column (guess you could shore the frame for temp loads) could you fabricate a steel bracket that could connect the column BP directly to the new tie rods?

 

[jimstructures]

j19

You need to provide us with some load numbers and dimensions so we can make some educated suggestions. First we need to know the width, length and eave heights of your building as well as roof slope and single or double slope roof profile(symmetric or non-symmetric). We also need to bay lengths. Then we will know controlling loads at the column bases.

I am currently working on a 90' wide x 90' long x 17' actual eave height building with a 1/2:12 symmetric roof pitch and 3-30' bays. It has 47 kips gravity load, 8 kip uplift and 45 kip horizontal kickout. Snow load is the controlling load case when combined with the other appropriate load cases. I wanted to add another column at midspan but was overruled by the owner who wants a clearspan building. So I am using tension ties across the building.

If the owner of you building wants a clearspan then a demo and reinstallation of the tension ties seems the most appropriate solution. I won't make any other suggestions until I have more information to go on. Is the concrete contractor, who didn't follow the plans, going to pay for the re-work?

Jim

 

[txeng91]

To expand off of Houseboys post, could you add footings at the top of the slab under the columns? They could shore the frame at the ridge, cut the slab and chip away at the pedestal a little bit to roughen it up, and cast a new footing around it. Not sure if it would be cheaper, or if it is even an option, but I think it is harder for the contractor to screw up. Unless he decides to "re-size" the footings because the ones you gave him were "overkill".

 

[jimstructures]

HouseBoy,

Is CF short for Concrete Floor?

Jim

 

[thaidavid40]

Do you have the freedom to raise the finished floor level? If so, then just putting the missing ties on top of the existing slab, and then pouring a new slab on top (with all of the mechanical anchorages needed) will likely be cheaper than all of that demolition and retrofitting that would keep the same floor elevation.
Dave

Thaidavid

 

[HouseBoy]

I like Thaidavid's idea!

CF = Carbon Fiber

 

[Ron]

Or....since you have some vertical room on the outside, have you considered providing buttresses at each bent location?

 

[wannabeSE]

Saw cutting the 4" slab and adding the ties looks like it may be the most cost effective solution compared to the design and construction cost of adding columns mid span or buttresses at the foundation.

 

[j19]

Wow! Thanks for all of the responses. I'm away from my notes but here's some info:
The is a gable building, 120' wide, clear span with either 10' or 12' eave heights. The reactions from the MB manufacturer show that under Dead + Collateral + Live Load the kickout is around 40 kips.

By "do the tie-rods right" I did mean to cut through the slab that has been poured, which is about 60' length. There are only two frame lines so that would mean cutting about 120 lf of trench.

This building is an add on to an existing building so I can't raise the slab. By the way, the existing building was built about 10 years ago and the contractor complained about the tie-rods but in the end he did what was called for on the drawings.

As far as buttresses, I was calling these stem walls but buttresses are a better description. This could be done but the forming and tieing into the existing I'm thinking the cost would be much more than cutting the slab.

I am not familiar with the carbon fiber solution that was mentioned. Where can I find some info on this?

 

[txeng91]

Carbon Fiber Reinforced Polymer (CFRP) is basically a thin material with high tensile strength that you can adhere to concrete to increase its tensile/moment capacity. I think it is mostly proprietary, companies like Sika Corp and Simpson manufacturer it. Like Houseboy mentioned, if you were to use it, you will probably need to still embed it in the slab since putting on top of the slab would subject it wear and tear due to traffic from machinery like forklifts. Also you would still need to figure out a way to achieve a positive connection from the CFRP to the metal building frame, which might prove difficult. I think your first proposed option is probably the best to go with at this point. Regarding your third option, you would not only need to make sure that your anchors have the shear capacity to transfer the kick out force into the concrete, you would also need enough concrete around the anchors to resist breakout, which depends on the size of the pedestal. Also that's 200 k-ft you are putting on the pedestal and the footing, both of which need to resist that force, I'm not sure a stem wall will do much for the footing overturning.

 

[hokie66]

The first step should be to fire the contractor.

 

[structuralsteelhead]

Truly depends on how much time and effort you are prepared, willing and able to spend fixing the contractor's mistake. Owner most likely didn't order a building with buttresses in mind as well. If the tie rods weren't on the drawings or not clear, I would try to convince the owner that "all super cool buildings have buttresses",...tell him,.."at least that's what they are doing in Paris".

But seriously, I would push to have the contractor trench the slab and put in the tie rods if this was on your drawings and part of your design. With regard to shear on the bolts, shear lugs are always a possibility,....for next building.

Sure creates a mess. You'd almost need to cut a decent 18" to 24" wide or more section of slab out each column line in order to have room to be able to dowel back into the slab on the sides. Pretty much looking at a existing slab trench repair detail.

Can't see trying to just remove enough concrete to set a bar in and then patch/cover the bar. Maybe there are some good high strength polymer patching solutions for this possibility by Sika or others.

Hope this helps.

 

[OHIOMatt]

You could look at adding battered helical screw piles to take the thrust.

 

[KootK]

You could place tie rods at the level of the eaves to eliminate much of the gravity thrust. That would, of course, require:

1) Another permutation of usable space encroachment and;

2) A re-evaluation of the portal frames.

Like others, my gut feel is that reinstating the SOG tie rods will be the way to go. That could get quite ugly though if it winds up being necessary to demo the piers as well in order to robustly connect the tie rods and the column bases.

For such applications in the US, I believe that the code requires tension tie reinforcement to be continuous or spliced with welds or mechanical couplers. Irrespective of other shortcomings, that alone may render your contractor's DIY alteration 100% code ineffective.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[HouseBoy]

"demo the piers as well in order to robustly connect the tie rods and the column bases"
Ugly is relative I suppose.
I'm thinking maybe a steel "boot" can be welded to the base plate and bottom of column that would accommodate such a connection.
Some eccentricity to deal with...

 

[darthsoilsguy2]

the tie-rod strips to complete the loop seems the only solid path. filling outside probably can't be done for reasons and buttressing will cost more.... not that it matters but why is the slab 4 ft higher than adjacent grade? floodplain issues? i don't think i've ever seen one with retained fill holding up the slab. i would bet that your contractor hasn't seen too many metal bldgs like this either. the retained fill just exacerbates the need for lateral restraint.

 

[j19]

darthsoilsguy2: The building is on a sloped lot and on the other side of the building the slab is a little less than 1' above grade.

 

[HouseBoy]

Wondering if you could underpin the footings at the column locations and get the dead load eccentricity to counter-balance the thrust?
That mechanism (eccentric footing at the column) is how I've seen new foundations designed when a stem wall is needed.