Metal Bldg. Hairpins

[StruKturdg]

I have a metal bldg foundation to design in Texas. The arch. is providing slab leave-outs for the tennants. Where do I place the hairpins in the area of the slab leave-outs? I do have grade beams connecting columns that are across from one another.

 

[K2skis]

What type of span does the frame have?

The next question is if you have a grade beam from pier to pier why do you need the hairpin?

Hairpins are cheap but in many conditions are not properly designed and just thrown in. I would highly suggest against any type of hairpin design when portions of the slab are left out or if the possibility exists for future slab work trench etc.

I used grade beams on the last one I did it was around a 90' clear span.

 

[McQSE]

Are your building columns supported on pedestals? I don’t use hairpins if I have grade beams connecting the footings as long as I am not using pedestals.

 

[StruKturdg]

The bldg dims. are 100X60. SO you are saying if I am using the grade beams to connect the columns together then I don't need hairpins. My columns are supported by a 6X6 footing with a thickened grade beam at the column to accept the baseplate and anchor bolts.

 

[dgkhan]

Calculate the uplift at the base of column. work your way to see if enough weight is there to counter balance uplift

 

[K2skis]

Hold the phone here do you have a perimeter turned down slab that you are calling a grade beam? Or do you have the grade beam running parrallel with the frame tying each end together to prevent horizontal spread.

If your frames are spanning 100' I suggest using the grade beam if they are spanning 60' you may get away with mement resisting foundation design. Either way if the slab is in question don't use hairpins for horz. spreading.

I also isolate my pier from my perimeter slab or wall foundation. These metal building move all over the place and I try to avoid residual stresses in the rest of the foundation which tend to cause cracking.

I live in the North East so things may be a bit different up here with snow loading controlling the design typically.

 

[hokie66]

There have been a lot of discussions here about using hairpin reinforcing to tie the base of metal buildings to the slab on grade. Hairpins are the cheap way out, but in my opinion should not be used in any building where present or potential future use would entail removing sections of the floor slab to install trenches, equipment footings, or whatever.

It is possible to design footings, whether earth supported or on piles, to resist the shear, tension and bending from the columns, so that is the approach that I take.

 

[archeng59]

Are the "grade beams" oriented parallel to the rigid frames or perpendicular to the rigid frames? If the grade beams are parallel to the frames, the horizontal thrust from the columns can be resisted by the grade beam rebar acting as tension ties between the columns. If the grade beams are perpendicular to the frames, then add "tie beams" parallel to and between the frame columns. The rebar in the tie beams must be sized to resist the horizontal (outward) thrust. The tie beam can be located below the bottom of the slab.

 

[jwilson33]

I've read from reliable sources that hairpins should be limited to 10-15kip column thrust conditions. Anything beyond that would warrant the under-slab tie or some other method.

Do any of you use lower limits?

K2skis-Have you seen movement in buildings with large spans/design forces, or in a variety of metal building types.

 

[StruKturdg]

Thank yall for the comments. I have placed interior grade beams, with 2#5 top and bottom with #3 ties at 30"o.c. that connect the main frame columns together. I also have a perimeter grade beam that is integrated with the spread footings.