Negative moment and reinforcement in steel decks over more than two supports.

[IngDod]

Greetings,

It has been my understanding in the past that for steel decks resting on more than two supports its necessary to provide reinforcement due to the negative moment that is generated over the supports (due to continuity). Now, I was reading some of the manufacturers manuals and in some of their examples they provide no steel at the continuity area and assume that because of this the deck should be considered as simply supported and designed as such. Is this common in day to day practice?, specially when the span length is small (around 4 ft)?

Thanks.

 

[hokie66]

The assumption of simple support can be made, but I don't think it is common practice. Most of us want some control over the cracking which will occur over the supports.

 

[IngDod]

Thanks, this decks always have wire mesh for temperature and shrinkage but I have not seen many with more reinforcement than this.

 

[slickdeals]

We always provide reinforcement in the decks at locations where the beams frame into girders. If you look in the SDI manual, they show a "draped" wire mesh. More often than not, they end up at the top of the deck.

 

[dhengr]

IngDod:
Support the WW mesh to stay high in the slab in the negative moment regions.

 

[IngDod]

After doing some quick calcs I get that the required steel for resisting the moment at the supports is three times lower than that of the reinforcement provided by the wire mesh (due to the very short span of the supports). However, I believe that the temperature steel should not be used for this situations correct?.

 

[IngDod]

@dhengr, @slickdeals: Sorry I had not read your comments. The WW mesh has enough area to resist the moment. I just was not sure if it was actually possible to use it this way.

 

[hokie66]

IngDod,

If the "temperature steel" is in the correct position, it can certainly be used as flexural reinforcement. That is always the case. Flexural reinforcement can always be used to satisfy the requirements for shrinkage reinforcement.

 

[IngDod]

@hookie66: Thanks. On the same subject... would it be necessary to evaluate interaction between the stress induced by loading and the stress induced by temperature changes in the reinforcement? For example, the minimum reinforcement area for temperature is x in2 and for the moment is y in2... the provided area is As... should it be so that As > x + y ? It seems to me that its difficult to estimate when this loads will truly interact and how much... Also I have read the code and my interpretation is that the reinforcement area should be above the minimum required for temperature control... It does not state that it should be sufficient to cover both load and temperature together.

 

[hokie66]

No, if your flexural reinforcement equals or exceeds the amount required for T&S reinforcement, that serves both purposes. And if the T&S reinforcement equals or exceeds the amount required for flexure, AND if it is placed on the tension face correctly, that serves both purposes. The two requirements are not additive.

 

[IngDod]

@hookie66: Thanks, most helpful as always.

Thanks to everyone.

 

[TXStructural]

T&S ONLY applies perpendicular to flexural reinforcement. (ACI 318-11 7.12.1 & R7.12.1)

And I have examined a number of projects where VE/Fear of VE removed the negative moment bars over a steel beam. Performance suffers without this detail. Always provide discrete reinforcement (rebar or welded wire) over stiff members to assure satisfactory/optimum performance.

 

[paddingtongreen]

The worst place for cracking is over the girder with composite beams, so we ran a saw cut along to control it, to make it look like we expected it.

It is an interesting condition, if the ends of the beams can resist compression against the girder, you have a theoretical full depth moment connection if you reinforce the concrete sufficiently.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin