Hairpins

[EIT2]

When using hairpins and Welded Wire Mesh in the concrete slab, does the mesh have to be present throughout the entire length of the rigid frame, from hairpin to opposing hairpin, or can the mesh terminate beyond the length of the hairpin?

The contractor, to combat the rising steel price, would like to use fibermesh in the center portion of the slab.

Thank you!

 

[DaveAtkins]

If you do not use wire mesh across the entire slab, it could "pull apart" at slab control joints (I am assuming that the wire mesh crosses the control joints). This is okay, as long as you have enough friction under the slab to resist the hairpin force. Remember that you can only use that portion of slab between the outside of the building and the first slab control joint parallel to the outside wall.

DaveAtkins

 

[JRGse]

A book written by Alxender Newman, "Metal Building Systems" discusses hairpin design. In it he correctly states that unreinforced concrete or concrete containing fiberglass is unsuitable for tension loads and that if the reinforceing is not continuous across the floor the friction under the floor slab would have to resist the load. In his book Mr. Newman also states that the reliance on friction to resist this type of load would not be prudent enginneering practice, to which I agree.
From this reference and other material published on the design of hairpins I believe that it is good practice to have the reinforcing be continous across the floor slab.

 

[EIT2]

May we conclude that the purpose of the reinforcing across the entire floor is to tie it to the opposing hairpin?

Further, to make this reinforcing effective, to what extent must the reinforcing be lapped?

In the case of a lean-to addition to an existing building, I believe the floor and reinforcing should be tied to the existing building. Any suggestions?

Thank you!

 

[DaveAtkins]

Yes, reinforcing across the entire floor transfers the thrust to the opposing hairpin. With wire mesh, I think a 6" lap is effective. With reinforcing bars, I would use a Class B lap splice. A lean to should not impart any thrust to the slab.

DaveAtkins

 

[furter123]

Doesn't placing reinforcing across a control joint render the joint ineffective?

 

[jheidt2543]

In lieu of the mesh, reinforcing bars can be installed perpendicular to the hair pins. The Metal Building Dealers Association (MBDA) published a book in the 1970's that had hair pin design equations and examples. The book is out of print, but you might contact them for excerpts.

 

[DaveAtkins]

I believe that wire mesh across a control joint can yield to allow the control joint crack to open up, and yet can still transfer tension force across the crack.

DaveAtkins

 

[furter123]

Grover1,

The author of the metal building book emphasizes that placing reinforcing across control joints will only leave you with cracks not at the control joints.

DaveAtkins,

The reference i used for slabs on grade states that reinforcing should always be stopped across a control joint. This is also the case in any ACI slab publications, or if you look at any generic details of control joints on the web. Why would this not apply to steel buildings?

Does any one else find it suspicious that the MBMA and any building manufacturer that published foundation design guides no longer offers them? I think there was too much liability in specifying what is usually considered a non-structural item (a floating slab) as a structural component. I tried contacting Butler Buildings who still advertises a design guide on their website. I spoke with three different people and each one of them was shocked that the design guide was mentioned on the website. Hmmmmmm.