Steel column hairpin rebar 2

[enriko12]

Hi,

We are working on the old steel building with no original prints available. After contractor removed the old slab, there were hairpin rebars wrapped around column pedestals. I understand their purpose is to provide "some" lateral resistance. Anyone had previous experience with those being damaged/removed and any adverse affects on the structure? How useful are those anyway? It's not like they are going to tie 2 sides of the moment frame together. Control joint are typically cut into interior slabs, so hairpins will rarely tie into more than 1 tile, and sometimes joint are cut right through that rebar. Also, slabs are typically poured last, so those are definitely not helping with any dead weight. I am thinking that single slab tile would add a negligible lateral resistance in comparison to that of the buried spread footing/column pedestal/wall footing.

Any input is highly appreciated!

 

[jimstructures]

For hairpins to be effective there must be a continuous slab from one side of the building to the other side of the building with appropriate amounts of rebar or welded wire fabric. i.e. no trenches or deep cuts. Somebody is going to a lot of work if they are cutting the slab all the way through. The slab may crack all the way through but the rebar or wwf should remain continuous across the width or length of the slab. The hairpins are there to tie the kickout at the base of the columns together, using the continuous developed rebar or welded wire fabric.

Jim

 

[hokie66]

seriko12,

You have discovered the reason I never rely on hairpins for base restraint. A slab on ground should be considered a working platform, not a structural element.

 

[enriko12]

I guess old slab did have wire wwf, but not continuous, several sections were poured at different times, with separate, non overlapping wwf and some had expansion joints all around with no wwf across. How much of a kickout is really there and any reason footings/foundation would not be able to handle it?
The structure is similar to this:

The way its planed now, there is no easy way to continuously connect hairpins on both sides with a new slab, as there is a huge service pit being excavated in between.

 

[hokie66]

The existing footings may or may not be adequate to resist the thrust. Not much way of knowing without access to the as built documents.

By that one photo, the steel column has been removed. Are you replacing the whole steel building?

 

[enriko12]

The picture is from another project, just found something similar in my archive. All we are replacing is the slab, adding a service pit and some thickened machinery foundations. No original design documents exist anymore. Will tie hairpins into sections of the new slab, however there is no way to add a continuous fully developed reinforcement between 2 bases of the moment frame

 

[dhengr]

Seriko12:
You might consider continuous grade beams from col. to col., with sufficient tension steel, at each grid line (rigid frame line), across the bldg., to tie the cols. together. They could span over the top of any trenches and through any thickened machine foundations. I agree with Hokie66 that parts of the slab may be removed from time to time, but pretty soon we will be designing for every other beam and column being removed at someone’s whim, and the bldg. should still stand and function as intended.

 

[jimstructures]

Seriko:

Do you have a complete Anchor Bolt Layout as supplied by the Metal Building Manufacturer, a complete set of reactions also as supplied by the Metal Building Manufacturer and a complete foundation design for the building?

If you don't have those items they can be rebuilt using the building size, loads and layout. The picture you supplied shows the top of the pier for a similar project and what you need is a foundation drawing showing at least the pad sizes as designed and built by the general contractor and engineer of record.

Tie beams are the next more expensive solution after hairpins. Pad and pier foundations could be designed to resist the foundation loads applied by the building to the existing foundations. These would be another acceptable solution.


Jim

 

[enriko12]

Those would have to be recreated. Building is 40 years old, the guy who built it has passed away, and local AHJs don't have any plans or records that old. I would have to measure the steel columns and calculate reactions. Of course it was designed based on 40+ year old building codes, so would need to find out what load combinations they used.
Good idea on the pads/piers, I am thinking now about making a new slab L shaped profile (with a thickened portion on 1 side) so it "hooks" into the ground to provide additional resistance.
As slab is typically added after the building is put up, hairpins would only resist snow/live loads, so I could assume building is definitely in no danger until next snow.
Alternatively, instead of recreating the original design, we can calculate capacity of 2 #5 bars and design lateral resistance system to be equal to that.

 

[Ron247]

There is more than one theory on how hairpins work. Years ago I asked different foundation designers about them. I got a huge variety of answers. Of the answers I recall, here they are:
[ol 1]
[li]Hairpin transfers the lateral load into the slab and the slab works as a tensions tie to the opposing column. Problem is when you cut the slab or do anything that disrupts the continuity. Lack of continuity of WWF can be a problem. This also means you cannot use fiber reinforced concrete in lieu of WWF. The calculation they used years ago was related to WWF size and strength. The calc yield the length of the leg of the hairpin needed to transfer the load to the slab via the hairpin.[/li]
[li]Hairpin transfers the lateral load into a section of slab that is large enough to resist lateral load. The "section" of slab is large enough that you cannot "drag it" across the ground under the lateral load. Problem is that the smoother the subgrade the lower the resistance.[/li]
[li]Hairpin allows the load to distribute the force laterally enough that lateral soil bearing against the grade beam ultimately resists the load. If that were true is seems you would use wider angle on the bend.[/li]
[li]Hairpin passes the load into the slab, but no load path description after that. "It just passes it into the slab"[/li]
[li]"I don't know how they work, I just know I have to use them" (problem is that was the most common answer, about 35% as I recall)[/li]
[/ol]

The tension tie method is more structurally sound but remember PL/AE can get large because "L" can be large unless you keep steel tensile stress real low.
If you are in a shade and shelter application (20 psf Roof Live, no snow) and the building is less than 70' wide, it is probably not a huge problem with the concepts noted above.

Wide metal building 80'+ and large roof loads can be a real problem with hairpins. Also remember that taller metal buildings have much less thrust than short metal buildings of the same width under gravity loads. I have seen the rational that this size worked on a 24' tall so it should easily work on my 12' tall one of the same width.

 

[Ron247]

What are your building parameters? Width, Height, clearspan, bay, RLL, RSL and code/yr Some of us may have an old design with similar parameters. The actual manufacturer should not matter as far as magnitude of reactions.

 

[jimstructures]

Seriko12,

I would like to expand the requested building parameters.

Width, Length, actual eave height, roof slope, bay lengths and clearspan or modular frames with spacing, Frame setback on endwalls, endwall section lengths. Full list of code parameters/loads. Building Code. I only have access to USA codes. i.e. IBC, ASCE 7, etc.

From that I can supply controlling loads/reactions.

Jim

 

[r13]

Anyone had previous experience with those being damaged/removed and any adverse affects on the structure? How useful are those anyway?

If the column pedestal is built monolithically with grade beams on its sides, I'll have least concern for short duration exposure without the hairpin. For stand-alone pedestal, provide lateral support throughout the construction period. In either case, the hairpin shall be restored as it is part of the design, unless you can prove it is useless, or find other way to make up the lost shear capacity.

 

[JLNJ]

They don't provide "some" lateral resistance, they are likely supposed to provide *all* the lateral resistance.

Column thrust forces are real, especially in PEMB frames. These forces stabilize the moment frames. The larger the span and the shorter the building, the larger the thrust. Even under just dead load.

This is not an "if the wind isn't blowing there isn't much force here", type deal, this is a "we have seriously compromised the structure and better do something about it, quick" type situation.

Get a structural engineer on board, preferably one who knows at least a little about PEMBs, before your building collapses.

 

[enriko12]

Width: 60ft
Length: 150ft
Bay length: 25ft
Year constructed: 1983, location: Indiana. No idea what code they used in 1983
Single slope, eave height 16ft, slopes down at 0.25"/ft
Column pedestals are built monolithically with grade beams (wall foundation), just like on the picture.
Front side is backfilled all the way up, rear side drops 4ft (for the loading docks), so there is additional load on the rear foundation wall due to soil pressure. Rear foundation wall is 12" thick and 8ft tall: 4ft above ground (on the outside), then it extends 4ft underground. It does rest on a footing, so I assume it was designed as a retention wall.
Building walls are made out of CMU, surprisingly unreinforced and unbraced! I guess it was OK in 1983
Columns are set back 8" (just enough to make room for 8" CMU)
Ground snow load: 20psf (but it might have been different in 1983)
Calculated dead load due to roof/purlins/insulation ~3psf
Purlins are spaced 5ft apart, 16Ga, 8” (height) x 3-1/8” (top flange) x 3-3/8” (bottom flange), lap is about 2ft. Correct me if I am wrong, but from the purlins data alone, it looks like they can support around 85plf (translates to 17psf) D+L

 

[jimstructures]

Here are some preliminary reactions based on the loads and dimensions you provided. I assumed non rigid, braced, half-load end wall frames with 6" setback which may be incorrect based on the 8" cmu block you mentioned. I assumed nothing about the foundation or the cmu walls.

 

[jimstructures]

I haven't figured out how to link a pdf file or attach the pdf file to my post. As soon as I do I will attach the preliminary reactions.

Jim

 

[r13]

jims,

Use the "attachment" feature, at bottommost of the reply window, to upload the PDF. After upload, just copy the address and paste using the "link" feature (on the tool bar above).

 

[Ron247]

I think you can also use the "Image" Icon (a camera) to add a PDF.

 

[Ron247]

In 2002, most of Indiana was 20 psf ground snow. The following counties were 25 psf ground snow.
[li]Elkhart[/li]
[li]Lake[/li]
[li]Marshall[/li]
[li]Porter[/li]
[li]Starke[/li]

There were a few counties that were only 15 psf.

St. Joseph and Laporte counties required a Site specific study.

While not 1983, this info is closer than 2020.