Fibermesh / Metal building

[bvbuf]

I am reviewing a job engineeered by another structural engineering company.

The engineer specified fibermesh as reinforcement for a slab on grade for a premanufactured metal building. He also specified "hairpins" at the columns.

The building was built in October 07 and there are significant cracks around the column footings. I'm wondering if the fibermesh wasn't sufficient to disperse the tensile load (horizontal column load) throughout the slab.

I'm wondering if anyone has any experience with using fibermesh reinforcing with a metal building.

 

[msquared48]

What kind of cracks? Are they due to differential settlement between the slab and mainframe footing, or tensile spreading due top the kick of the mainframe?

Another thought - were expansion/contraaction joints specified and used in the concrete slab?

Mike McCann
McCann Engineering

 

[jike]

Describe the building in more detail. Is it a single clear span?

What are the shapes of the cracks? What is the calculated thrust?

Has the building seen its full roof live load? (If it is a snow load area, it might have).

 

[bvbuf]

The cracks are generally semicircular around the footing in the direction of the horizontal force. They occur at the interior columns on one side of the building. (Note these columns reside on the interior of the building but are end columns for the metal building frame.) The other side of the building abuts an existing building. They do not occur at the corner columns.

I believe they are caused by tensile spreading due to the horizontal force or kick of the frame. If the cracks were due to differential sttelement, I believe they would be circular rather than semicircular.

Have you ever used Fibermesh in a slab that supports a metal building with a "kick"?

Saw cuts were placed in the slab.

 

[bvbuf]

The building is a single frame (one bay)with a "lean to" on one side.

Maximum thrust is D-5.5K , COL-2.7K, L-11.05K.

It has not seen maximum live load.

 

[cvg]

fibermesh is not intended to provide "structural" strength, but instead provides more durability and reduces cracks caused by shrinkage during initial curing. The slab should have been cured prior to erecting the building so the fibermesh likely would have had no effect on your cracking problem.

 

[hokie66]

It sounds like the slab is unreinforced, and the concrete has failed in tension due to the thrust at the base of the column. It is hard to believe anyone would depend on plastic fibers to resist this type of force. Are you sure the slab has no steel reinforcing? Are there footings which are designed to resist the thrust? My thought in this regard, giving the original designer the benefit of the doubt, is that he may have put in the hairpins as belts and braces, but that was a poor idea, as the slab had to fail before the footings could resist the thrust.

 

[msquared48]

What was the length of the long legs of the hairpins? Even without WWF in the slab, if the hairpins could develop the tensile force over a large enough area, the cracks should not have developed. If the length was insufficient, remedial measures will have to be take to maintain the stability of the frames under full design load. Were the hairpins large enouth in area to take the load?

These measures might include addining additional in-slab hairpins, longer and sawcut/epoxied in place, or addditional footings next to the existing footings to take the lateral thrust to the soil matrix. Might even install a rebar tenstion-tie link across the width of the building at each mainframe, epoxy grouted in as with the exgtra hairpins. Could be a costly solution.

Mike McCann
McCann Engineering

 

[msquared48]

Another thought here - were the hairpins placed properly - behind the anchor bolts and not in front of them?

Mike McCann
McCann Engineering

 

[LPPE]

I agree with msquared48. The hairpins should have been designed for the thrust and therefore embedded enough in the slab.

 

[shin25]

Can you describe the column footings a little bit?

 

[dik]

By fibermesh, are you referring to polypropylene fibres? These should not be used for transferring tensile forces. They only assist by retaining bleed water and providing a somewhat better concrete.

Dik

 

[bvbuf]

The maximum thrust is 19K and the hairpins at each column are two #5 with two 10'legs.

The slab is only reinforced with polypropylene fibers

The column footings are placed monolithically with the slab and are 8'-0" x 8'-0".

Are you saying that even if the slab is plain concrete, if the hairpins have adequate embedment, there will be no tensile cracks?

 

[shin25]

I doubt that within this short time frame (oct-07 thru jan-08), the building has experienced the design loads, has it?

If the cracks are right at the location of the footing perimeter, then the cracks may be due to stress concentraction at deep footing and the thin slab interface. The spread and depth can be minimized if adequate reinforcement is provided.

 

[msquared48]

bvbuf:

Do I understand you to say that there are two #5 hairpins at each mainframe and not just one? If so, that is good. If not...

I did a calc on the 19 Kip load and based on my standard 2:1 flare of the hairpin legs for one hairpin, one #5 hairpin is insufficient and two are needed. I come up with an As required of .53 Sq. In.

If two were provided, did they have the same, or different flares to the legs? They should have been different to spread the tensile load out better to the slab, and of different leg lengths too.

Can you describe the geometry of the hairpin(s) to us more precisely please?

Mike McCann
McCann Engineering

 

[Atomic25]

bvbuf, if the hairpins have adequate embedment into the plain slab you shouldn't have cracks.

Honestly, my first thought would be a problem with concrete curing and not so much the kickout forces. I mean, how does the 19kip into the slab...with the anchor rods sitting in oversized holes? Baseplate friction? Did the building see enough load to cause a problem?

 

[hokie66]

I suspect that the cracks are drying shrinkage cracks rather than due to the frame thrust. The 8' x 8' footings would have restrained the slab, and the shrinkage had to go somewhere. If the slab was not strong enough in tension to pull itself along from the first sawcut, it had to crack. And with no steel reinforcement, the cracking manifested itself as a single crack rather than a series of smaller cracks.