Fire Reduced Capacity of Wood Members

[CANeng11]

I am wondering if anyone has any experience with calculating out structural capacities of members based on a specific fire rating? I found the Technical Report no. 10 put out by the American Wood Council that seems to be a good reference but I still have questions. One specific question I have is whether you able to apply any reduction to the live load during fire conditions?

 

[Agent666]

One specific question I have is whether you able to apply any reduction to the live load during fire conditions?

In this part of the world (NZ/Australia) the fire load case is typically taken to be the longer term serviceability load case. For NZ/AU this is typically G+0.4Q. Similar logic probably applies in the USA. It is meant to represent the likely distribution of live load on the basis of an "everyday" case. In reality load might be higher, but some of your live load is running out the door during the fire as well.

 

[msquared48]

And some is walking in... Do you really want to lower the live load? Really?

Mike McCann, PE, SE (WA, HI)

 

[r13]

During a life threatening event, the multiplier of live load factor could be higher than 1.0, as people clustered compete for exit. The multiplier is zero, when the building is burned to ground.

 

[Agent666]

I'm happy reducing live load, as is our national codes. Why? Well for the same reason you don't check deflections under full live load or factored live load with load factors greater than one, or assess seismic load cases with full live load, etc. This might not be the case where OP is, but it's certainly the code prescribed method in these parts.

The probability of an office or retail being at full live load is small and at full load factored live load even smaller, sure its probabilistic so sometimes higher, sometimes lower than the long term cases (which are established and recognised by historical investigations and studies into actual real loading intensities for typical occupancies). These are the basis of the live load reduction factors (LLRF), sometimes also called area reduction factors in modern codes.

The fire load condition is considered after maximum charring has taken place for wood (typically based on consideration of some time based rating you are trying to achieve). This is considering the reduced capacity afforded by this loss of section. It is meant to represent the live load present during fire fighting and post fire conditions. It is not for consideration of people rushing to exit. That is effectively covered by the normal ULS design.

 

[CANeng11]

So I think I've found some information on this, at least from US codes:

From ASCE 7 - 2.5.2.1 Capacity - For checking the capacity of a structure or structural element to withstand the effect of an extraordinary event, the following gravity load combination shall be considered0.9 or 1.2)D + Ak + 0.5L + 0.2S (2.5-1)in which Ak = the load or load effect resulting from extraordinary event A.

I would think a fire is an extraordinary event, so you would use 1.2D + Ak + 0.5L + 0.2S. Although I'm not sure how Ak would be utilized for a fire situation.

Does anyone know if Canada has something similar? I haven't been able to find anything in the NBCC.

 

[dik]

Have to be careful... with the new TJI members, their residual strength may be nothing... I've had several firefighters comment on the danger of these in a fire.



Dik

 

[CANeng11]

This particular case I am looking at is an old, heavy timber construction building.