Roof bracing of a large room in a timber framed building
Roof bracing of a large room in a timber framed building
(OP)
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 ?
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 ?






RE: Roof bracing of a large room in a timber framed building
RE: Roof bracing of a large room in a timber framed building
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)
RE: Roof bracing of a large room in a timber framed building
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.
RE: Roof bracing of a large room in a timber framed building
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.
RE: Roof bracing of a large room in a timber framed building
RE: Roof bracing of a large room in a timber framed building
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).
RE: Roof bracing of a large room in a timber framed building
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.
RE: Roof bracing of a large room in a timber framed building
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).
RE: Roof bracing of a large room in a timber framed building
RE: Roof bracing of a large room in a timber framed building
RE: Roof bracing of a large room in a timber framed building