2012 ibc 706.6.1

[fatboy]

OK, Stepped Buildings, Exception 1.........says you can terminate a fire wall at the underside of of the roof sheathing, if;

The lower roof assembly within 10 feet (3048 mm) of the wall has not less than a

1-hour fire-resistance rating and the entire length and span of

supporting elements for the rated roof assembly has a fire-resistance rating of

not less than 1 hour.

So, I read this that, the rafter or truss carrying the 1-hr assemble needs to be supported to equally protected bearing walls, regardless is it exceeds the minimum 10'. I have a designer saying that, no, he only needs to protect the required 10' of roof rafters/trusses, after that it can be unprotected. I think the code is pretty clear.............. What say ye?

 

[steveray]

Protect everything supporting the rating (truss, joist, column, etc)....all the way to the foundation....I have fought that fight....If the truss burns at 11' what good does the rating do?

 

[fatboy]

Bingo, thanks...........others?

 

[steveray]

What???...You don't trust me?...... .....Have a good weekend!..... It basically kicks you back to the same thing as supporting construction for fire barriers. It does not say "immediate" or "directly" supporting elements.....

 

[George McGerd]

I have encountered a similar scenario on a few other jobs. The decision was made that the entire bay of structure carrying the lower roof, which is required to be rated to 1 hour must also be 1 hour. This includes, beams, joist, rafters, columns, bearing walls (both sides) etc, basically anything that supports the 10 feet of structure adjacent to fire wall. Also, the 1 hour rating was only allowed for the lower structure if it had break away clips at the fire wall. If the structure was physically tied into the actual fire wall then they wanted 2 hour rating for the entire bay of structure attached to the firewall (same rating as firewall).

 

[fatboy]

Thanks George........my thinking is that the exception specifies 10' because that is the minimum roof span needed, if you only have 5' or 8.5', the exception does not come into play, not that only the first 10' must comply.

 

[jhperez]

The 2009 IBC has the same wording. According to the 2009 IBC Commentaries, 706.6.1

"It must be noted that all structural elements, including beams, columns and bearing walls that provide support for the fire-resistance-rated roof assembly, must also have a 1-hour fire-resistance rating for their entire length or span in order to maintain the effectiveness of the rated roof assembly. The provisions of this section are similar to those contained in the section for the protection of the exterior walls of exit stairways. The provisions for stepped buildings apply only where a fire wall is required in between the buildings, and do not apply to party walls on real property lines. "

It seems to me that the intent of the code is to maintain the 1hr separation even if the lower level burns away. If the supports burn away in less than 1hr, the roof protection is eliminated by the collapse even if the roof structure does not burn for 1hr.

 

[mark handler]

Protect to the foundation... yes

 

[Builder Bob]

To be truly effective in application of this section, you would really need a structural engineer to determine if the trusses are simple span or compound spans. how would the failure of one section of trusses affect the other....

In my simple mind, the entire roof support would require rating without documentation of bearing points and potential areas of roof collapse in a fire or other emergency conditions.... remember steel elongates a 100-foot-long steel beam nearly 10 inches at 1,000°F (1), sometimes causing wall failure