Fibers in composite metal decks 1

[swadeshi]

I am researching possibilty of replacing WWF with steel+synthetic fiber reinforced concrete. Major concerns are with the diaphragm floors and deck over the beam-web-openings. Code requires additional steel in these locations. Would the additional volume of fibers meet this requirement?

 

[jike]

I would think the negative bending moment over beams and girders is also an issue.

Jim K.

 

[JAE]

Fibers are never used for structural purposes other than to serve for temperature/shrinkage crack control. If you use fibers, you would then add reinforcing bars in areas where you needed strengthening such as at openings.

 

[Lutfi]

I agree with jike...
You will have negative bending moment over the supports.

A contractor tried to sell us on accepting fiber mesh on multi-story building. They presented research data to show all kind of things. However, they could not provide one document to substantiate that fiber mesh can replace the steel for negative bending.


Regards,
Lutfi

 

[WillisV]

For standard composite deck and recommended WWF layouts teh WWF is not adequate to resist negative bending over the girder supports and some cracking is assumed to occur (check any manufacturers deck manual and they will state this somewhere). Typically many engineers places crack control type reinforcing over girders (say #4 at 24" oc) to mitigate this problem.

Therefore replacing the WWF with a proper amount of steel fibers would not present any different of a problem as long as the same type of crack control reinforcing is used as well as additional reinforcing as required over web openings.

In May 2003 the steel deck institute issued the following statement regarding this:

"Changes to the SDI Design Manual as of May 2003
At the May 2003 meeting the SDI Board of Directors voted to amend the Steel Deck Institute Design Manual (publication number 30) to permit an alternate means of providing temperature and shrinkage reinforcement for composite steel floor deck. Section 5.5 of the SDI Specifications and Commentary for Composite Steel Floor Deck was amended as follows:

5.5 Temperature and Shrinkage Reinforcement: Temperature and Shrinkage reinforcement, consisting of welded wire fabric or reinforcing bars, shall have a minimum area of 0.00075 times the area of the concrete above the deck (per foot or meter of width), but shall not be less than the area provided by 6 x 6 - W1.4 x W1.4 welded wire fabric. Cold-drawn steel fibers meeting the criteria of ASTM A820, at a minimum addition rate of 25 lb/cu yd (14.8 kg/cu meter) and possessing an average residual strength of at least 80 psi (550 kPa) when tested in accordance with ASTM C1399, may be used as a suitable alternative to the welded wire fabric specified for temperature and shrinkage reinforcement.

Commentary: If welded wire mesh is used with a steel area given by the above formula, it will not be sufficient as the total negative reinforcement; however, the mesh has been shown that it does a good job of crack control especially if kept near the top of the slab (3/4 inch to 1 inch cover, 20 to 25 mm). If fibers are used for shrinkage and temperature reinforcement, the fibers will not be sufficient as the total negative reinforcement. If the service loads and slab response require lateral distribution, distribution steel may be required in addition to the steel fibers or welded wire fabric. Concentrated loads are the most common example of this service requirement. "