Roof bracing of a large room in a timber framed building

[Cruezr]

I have a building project that comprises a large 11m x 11m (33' x 33') room, trussed roof, 4m (12') high timber framed walls and 0.9m (3') elevated timber floor.
I have designed the wall bracing system for the rest of the building and am now at this room.
Current Australian timber framing codes limits the spacing of bracing walls to 9m (27') so relying on the plywood/plasterboard ceiling is not strictly possible.
The ceiling may also have expressed joints throughout so continuity of the ceiling diaphragm is not provided.

I am wondering what options I have for providing a horizontal transfer of wind loads to each side wall - parallel and perpendicular to the span of the roof trusses ?

 

[CBSE]

Any drawings?

 

[msquared48]

I guess I don't understand the problem here...

Don't you have a plywood roof diaphragm? Seems a 1:1 aspect ratio should be no problem considering this.

Do you know the rationale for the 27 foot limitation?

Any chance of getting a variance?



Mike McCann, PE, SE (WA)

 

[Cruezr]

The 27 ft limitation is a simple limitation imposed on our Australian timber-framing code (so no chance of a variance)...I don't know the specific basis for this limitation.
Generally, a ceiling (plaster, plywood, etc) solidly fixed to the underside of the roof trusses via battens is fine...I agree that the 1:1 ratio is generally not a problem, but the walls are too far apart to comply with the code limitation.
Structural plywood could be provided as a ceiling lining...I would just need to go back to first principles to calculate the plywood stresses and design the connections to the bracing walls each side...not something I have ever had to worry about before.

 

[KootK]

Is this perhaps a prescriptive residential code provision that can be circumvented if the appropriate calculations are performed? It's hard to see how you'd work a practical, duscrete bracing system into a light frame wood diaphragm.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Cruezr]

KootK...that is exactly what it is - a prescriptive limitation applied to relatively simple buildings.

 

[HouseBoy]

Some sort of drawings will help but what I'd be looking for is... are there adjacent building elements that might be effective to provide bracing.
Also, might the timber frame walls have knee bracing that could be utilized? Might need flitch reinforcing.
Just trying to imagine what things really look like (for a work-around).

 

[CBSE]

According to the OP and KootK, this is a prescriptive path method. First question...are you an engineer or a builder?

If you are a builder: then I would suggest trying to shorten the room and follow the prescriptive path. Just because someone on here may give you suggestions on how to accomplish what you want does not mean you "can" do it without an engineers approval...and we aren't approving anything.

If you are an engineer: calculate your forces that need transferred and then detail knee braces to accommodate. Or check your forces to ensure they aren't exceeding limitations. You can block and strap around openings/discontinuities (similar to force transfer shear walls) and move forces around that way.

 

[Cruezr]

Ok, so now I'm really stuck on this...need help please...it really is giving me sleepless nights :-(

Here is the roof framing plan for the building.
The main area in question is the large square(ish) box at the top (north end)...trusses spanning 11.4m at 900mm centres.
Design horizontal racking force at top plate level is 4.14 kN/m width.
Each of the "side" walls have openings for windows or large doors.
I started going through the EWPAA manual for designing a nailed plywood diaphragm, calculate shear flows etc and select a nailing pattern.
Seems ok to this point, but the supporting information tables in the manual suggest a maximum "joist" spacing of 600mm (how does this apply to trusses ?) and then designing chords but give no indication of where (what centres ?) to locate the chords.

All other areas of the building comply with the code regulations for spacing of bracing walls and have been designed accordingly
(ie. maximum 9m spacing of bracing walls using plaster ceiling fixed to battens fixed to bottom chord of trusses to distribute horizontal racking forces to bracing walls at sides).

 

[jayrod12]

Trusses at 900 centres make zero sense since your board goods are in 1200mm increments. And the published diaphragm values for board goods have a maximum allowable spacing of 600mm I believe.

 

[Cruezr]

Jayrod - does that imply that all your roof trusses are generally spaced at 600mm (24") max centres, regardless of span (I presume plywood roof diaphragms are common practice in the US). Being in Australia, typically we use 900mm centres but plywood roof diaphragms are rare (usually rely on plaster ceiling fixed to battens fixed to truss bottom chord). This is a problem I have not come across before for such a wide room - room dimensions cannot be altered.

 

[jayrod12]

Yes, wood roof trusses are at 600 o/c around here and diaphragms are typically wood based panels