Tilt Panels - Australian Fire Reg's and Ply Roof Diaphragm

[Toby43]

Hi all,

I have a job in the works with tilt panel walls (basically a single storey big box) with a timber ply ceiling. I am happy to structurally use this ply ceiling as a diaphragm, yet am not sure if I can comply with the Building Code of Australia (BCA) regulations that stipulates the panels should not collapse outwards in the event of an internal fire, if I don't provide a steel trussed diaphragm at roof/ceiling level. Further to that point, would steel cross-ties be sufficient along with the ply diaphragm to satisfy the BCA, or must the steel be integral to the trussed diaphragm? Cantilevering the panels from the footing/slab on ground level is not a desirable option at the moment.
It would be appreciated to hear anyone comments or experiences.

Thanks
Toby

 

[CRQUT]

Do a search for fire ties, then provide a braced steel framed roof. I personally wouldn’t rely on timber ply to offer lateral support to concrete precast walls. What if you get a leak in the roof and it rots out?

 

[Agent666]

Usually in NZ in this scenario the panels would need to have a footing capable of cantilevering the panels for the fire case.

Alternatively the roof needs to be fire rated to provide the stability required to restrain the top of the panels (via intumescent coatings or other means). But usually its dealt with via a suitable footing.

The fundamental test to apply is:- if an element of the building is providing stability to a fire rated element, then that stability element requires the same fire rating (even though in its own right it wouldn't require any fire rating).

 

[Toby43]

Thanks for your comments
CRQUT,
Timber rot would be an issue, but the reliance of timber roof diaphragms is prevalent in the US. They must not have the same fire regulations as Australia.

Agent666,
Yes I would normally cantilever the panels, but for various reasons was not desirable for this project.

As a side note this particular project ended up with standard steel roof bracing (trussed diaphragm) but I am still curious if fire regulations could be met with a continuous cross-tie that isn't part of the trussed diaphragm, i.e ply diaphragm with steel cross-tie as per my original post.

Thanks
Toby

 

[Agent666]

How is the structure supporting the panel fire rated in your configuration? No fire rating = no support.

 

[Toby43]

Agent666,

The roof structure for a single storey does not in itself require a fire rating, it just must be capable of staying connected to the panels, thus pulling the panels in with it as it collapses into the building, minimising the possibility of outward collapse.

 

[Agent666]

You are assuming the roof burning out pulls the panel over and pulls it inwards. Let's say it doesn't, then during the next day when the firefighters are dampening down hot spots a wind causes it to fall and kills the firefighters?
Alternatively a wind during the fire causes it to fall inwards/outwards onto something/someone due to the roof being burned out?

I'll reiterate again, if you want stability, then the elements providing the stability need to be protected in some manner.

 

[Toby43]

Agent666,

My experience is with the BCA but I believe New Zealand has similiar provisions also.

The BCA requires that the roof connections by designed to accommodate the larger of the forces associated with
(i)developing the ultimate moment capacity at the base of the panel
or
(ii) the overturning moment at the base associated with an outwards lateral displacement at the top of the panel equal to 1/10th of the panel height. (NZS3101 uses 0.5kPa face load)
Cast-in ferrules must accommodate 2x this
Drilled-in inserts must accommodate 6x this
These magnifying are there to account for reduced capacity of connections in a fire.(I believe section 4.8 in NZS3101 accounts for this by using 30% of yield)
The lateral supporting members (steel trussed diaphragm) must also accommodate these forces.
Of course real world performance may differ, but steel trussed diaphragms are commonly used with nominally pinned-base tilt panels. If experience suggests that has not performed well, then I am all ears.

Regards
Toby

 

[Agent666]

Hi Toby

Yes NZ has similar provisions as you note.

In your case you note that the lateral supporting members must also accommodate these forces. However my point was if they are not fire rated they cannot support these forces. Up to a point they can until there is sufficient loss of strength/stiffness from the heating of the structure. In a real fire are you willing to say the exact temperature of the steel to work this out? I would think that doing that would be all but impossible, sure maybe in a fire test based on ISO fire, but not in a real fire which generally burn hotter and faster followed by a cooling phase.

Code takes an all or nothing approach to FRR, rely on protected steel, do not rely on unprotected steel. Keeping in mind unprotected steel is never good for more than 15 minute FRR if you do the calculation, might get 20 minutes in heavier sections, but very rarely 30 minutes. Code approaches this aspect in a manner that with sufficient fire rating you do not exceed the limiting temperature of the steel based on the loading required to be supported.

Now having said that, reading up on the specifics of portal/panel design and so forth for the fire case, I came upon the attached document, which in NZ at least is actually referenced from NZS3101. The general gist is basically as long as you protect the connections between the panels and the main steel structure, you cross your fingers and it magically collapses inwards.

The only downside it mentions is that once things start to be pulled in you effectively loose the fire protection to neighbouring properties and the protection to those inside the building fighting the fire. But they seem accepting of this aspect. My view is it all seems a bit suspect, personally I'd like to see more robust stability than a pinned based panel and the hope it all falls inwards, but there it is referenced directly from the NZS3101 code so it might answer some of your questions.