cover required to obtain 1, 1.5, 2, 3 and 4 hour fire rating to slab & to beams 1

[ajk1]

1. Does anyone have the NBC of Canada required concrete cover to principal reinforcement in reinforced concrete beams, to obtain 1.0, 1.5, 2, 3 and 4 hour fire rating? I have only a very old supplement to the code that gave these numbers, and it may have changed.

2. Also, the same question as applied to reinforced concrete slabs.

3. Do these values apply only to the bottom bars in slabs and beams (and skin reinf side face bars in beams)? i.e. there is no minimum top cover required for top bars to achieve the fire ratings (although of course there is minimum top cover required for other reasons).

4. Does it mean that these values do not apply to stirrups, since stirrups are not principal reinf.?

 

[Agent666]

1. Can't help on this sorry, not familiar with the Canadian requirements

2. Generally the minimum cover requirements are not dependent on the member type. The covers are generalised to any structure. Fire action doesn't differentiate the member type.

3. Applies to all bars (i.e. top reinforcement is required for strength/stability in negative moment regions for example)

4. Applies to all reinforcement, shear reinforcement is principal reinforcement for resisting shear forces.

 

[jayrod12]

It is laid out quite clearly in the national building code structural commentaries. I don't have mine with me today but I believe for a 2 hour rating slabs need 25mm.

I also believe that it does differentiate between beams, slabs, columns and walls. And I disagree that it applies to top bars. The protection required is for adjacent spaces beside and above the area under consideration. Fire goes up, not down.

 

[ajk1]

To Agent666: I found the required covers for slabs and beams in the CAC Handbook, of which I have the latest copy, so I have answered that part of my question myself. Jayrod is right that it does differentiate between beams, slabs and columns.

Regarding what is defined as principal reinforcement, if the stirrups are part of the principle reinforcement, then they would have been better to just say all reinforcement. So far I have not found the definition of principal reinforcement. So I am still looking for that as well as where it makes clear that in slabs the required cover for a given rating applies equally to top and bottom slab bars (which seems illogical for the reason Jayrod says).

So I am still looking for:

- definition of "principal reinforcement"
- a published definitive statement that the required covers apply to only the bottom reinf. (or equally to top and bottom reinf. (it seems to me that life safety consulting firm told us many years ago that it applied only to bottom reinf., but I am not certain.

If anyone else who is well versed in this wants go jump in, I would be grateful.

 

[Agent666]

Yeah I went and checked my own code and low and behold different values for slabs that I've never noticed before!

Re principal reinforcement - Without a specific definition in your code I'd tend to interpret it as a 'common sense' answer as any reinforcement that has a primary purpose for strength and stability of the structure under the fire loadcase. Reinforcement that might not be considered principal reinforcement might be for example reinforcement orthogonal to the span of a simply supported slab (temp & shrinkage reinforcement), or drag reinforcement within diaphragm that is there to resist loads only under lateral load cases. This is after all a conservative approach.

Therefore in a beam both the shear and longitudinal reinforcement would by this interpretation be considered primary or principal reinforcement. Only secondary reinforcement need not satisfy the cover/axis distance rules. However my 'common sense' interpretation is thrown out the window with my own code specifically implying the axis distance is only to the longitudinal reinforcement, it similarly make no mention at all of the shear reinforcement within the fire chapter so take from that what you will (maybe its not mentioned for a reason, maybe its an oversight). Usually at normal fire ratings (<2hr) the durability requirement usually is significantly larger than the fire requirement anyway, and you are providing this to the stirrups so longitudinal reinforcement easily satisfies the fire requirements.

 

[ajk1]

Interesting. Let's see if anyone else comments. I will try go call the author of that Chapter of the Cement Association of Canada (CAC) Handbook to see if he knows the answers. If not I can write to the Canadian Standards Association (CSA). But I am sure many experienced structural engineers have dealt with these questions and hopefully some should know the answers. I wonder if there are any American (ACI) publications that deal more specifically with this.

 

[ajk1]

I just spoke to the Cement Association of Canada (CAC) Technical representative in Ottawa and he says clearly and unequivocally that:

a) "...'principal' steel reinforcement (as use in the heading to Table D-2.9.1 Minimum Cover to Principal Steel Reinforcement in Reinforced Concrete Beams, is the tensile reinforcement and excludes other steel such as stirrups"

b) the required cover for fire rating for slabs applies to the bottom bars and not to the top bars.

Both these answers are what I had always believed to be the case.

I suggested that in future editions a definition for principal reinforecemen be added to the text, and also that an explanation about cover for fire rating does not apply to to rebar. He agreed that could be helpful.

 

[jayrod12]

Thanks for following up, nice to know my interpretation was correct all along as well.