How to determine if a slab is restrained for fire cover requirements

[tngolfer]

2006 IBC, Tables 721.2.3(1)-(5) deal with the increased cover requirements for fire protection. The tables provide separate requirements depending if your slab is restrained or unrestrained.

My case is a 7 span, two-way flat plate analysis. The end span is 30'-10" but is supported by a full length (equal to tributary width or slab analysis) 12" c.i.p. concrete shear wall. The joint will be detailed to make the slab/wall act monolithic.

Q1: Would you all consider the shear wall as restraining the slab for cover requirements?

Q2: How do you determine whether something is a restraint? Is it a thrust force applied to the top of the wall and the wall can't deflect a certain amount? If so, how is the thrust force determined and how much deflection is too much?

Thanks in advance.

 

[hokie66]

The wall would restrain the slab mainly parallel to the wall, not perpendicular to it. All structural slabs are restrained to a degree by their supports. The more rigid the support, the more restraint in that direction.

 

[ishvaaag]

Except the code or official states otherwise, I think most would agree that here restrained means rotationally restrained at ends, for the biggest usefulness of the higher cover is in avoiding the formation of a mechanism upon spalling and the direct application of heat to the rebar. A continuous bean situation demands more than a hinge formation and then the cover requirements may be relaxed a bit. Then a floor continous to each side and with it supporting wall, is likely to be considered there restrained to this effect. Yes, fire my cause at such connection a hinge but the floor needs more to fail, since not simply supported.

 

[jike]

The definition for "restrained vs. unrestrained" regarding fire protection is not the same as that regarding structural rotation.

For fire protection, a "restrained" condition is one that is prevented from expanding. An "unrestrained" condition is one that is free to expand. Generally, interior bays are restrained and exterior bays are unrestrained.

The UL book has clear definitions. Also ASTM E-119.

 

[ishvaaag]

Even if such is the case and by reference the codes are properly linked, I think stability of structures under fire has more to do with the formation of mechanisms through spalling than with linear ablity to expand or not. Furthermore, the requirement is addressing cover, which is closely related to the formation of the hinges under heat, something much more related to spalling and cover.

We can apprecciate the stabilization given by just small variation of the lengths between columns, this would had more to do with linear expansion and is still a true factor in general stability. It is scarcely affected by cover, except a mechanism forms. So, I still see, for a RC framed structure, this is of lesser impact on stability than hinge formation (and subsequent to it).

 

[tngolfer]

I was taught that a restrained system is capable of allowing rigid member arching - capable of supporting the thrust forces applied by the rigid member. I don't think anyone would argue that a column is unrestrained, but where is the line and how to do I figure out what it is? Does a 12" wall perpendicular to the span restrain a slab, a 24" wall, etc.? How is the thrust force determined that I can apply to calculate whether my support is stiff enough to be considered "restraining" the slab.

I believe this has a lot to do with the floors above and below as well. If it is a lower floor in a 10 story building, there is going to be a lot of compression in the wall that will help with the restraining capacity but may result in a snap-through bifurcation if the thrust force is large enough. If the restraint is at the roof, the wall will act as a cantilever and not be as stiff even though the wall thickness may not change.

ishvaaag, jike is correct. Restrained/unrestrained has to do with the ability to limit the expansion, not with the rotation of the ends.

Thanks guys. Judging by the answers, this apparently is another area the Code needs improvement/direction/clarification.

 

[slickdeals]

I am not in the office, but there is a paper by John Ruddy in AISC that has good calculation procedure for thrust forces and means to determine if the support provides adequate restraint to help arching.

In your case, your cover requirements for an unrestrained end span may control.

I have heard many recommendations from PTI suggesting that end spans in concrete construction is unrestrained.

AISC's research has suggested that all steel construction can be considered to be restrained, including end spans with simple connections.