Wood Roof--> are rafter ties necessary?
Wood Roof--> are rafter ties necessary?
(OP)
are rafter ties necessary?
assume:
-- a plywood-roof-diaphragm is designed to carry the largest possible in-roof-plane loads.
-- the top-plate of the wall is designed to be an adequate chord for the diaphragm.
-- the gable-end-walls can transfer the the diaphragm-shear at the ends-of-the-diaphragm to the top-plate below then to shear walls below.
assume:
-- a plywood-roof-diaphragm is designed to carry the largest possible in-roof-plane loads.
-- the top-plate of the wall is designed to be an adequate chord for the diaphragm.
-- the gable-end-walls can transfer the the diaphragm-shear at the ends-of-the-diaphragm to the top-plate below then to shear walls below.






RE: Wood Roof--> are rafter ties necessary?
Neither collar ties nor hurricane clips are normally required as part of the lateral design. Where eve blocking is omitted, sometimes roof deck shear will be transmitted to the shear walls through something akin to the hurricane clips.
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: Wood Roof--> are rafter ties necessary?
BA
RE: Wood Roof--> are rafter ties necessary?
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: Wood Roof--> are rafter ties necessary?
Thanks, again.... 3 weeks of thinking, it is very helpful to have the ideas of others.
RE: Wood Roof--> are rafter ties necessary?
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: Wood Roof--> are rafter ties necessary?
Probably there will be 2-cross-ties for a 50 foot long wall, so it will not be all diaphragm transfer. Also, it may end up with partially supported ridge. Also, it is trickier as the 'gable ends' are really 4: 12 sloped roof planes. Rectangular pyramid, with 4:12 pitched roof planes, with partial ridge along long axis.
As far as I can tell the tricks are:
--adequate chord nailing at splices... looks to be about 50- 10d common at any splice, might make sense to add LSTA12 or LSTA18 at splices, exterior side of top plate
--adequate nailing at hips--- LVL and PSL can't be used (unless blocked, see below) as nail spacing is a minimum 3" OC for PSL and LVL top edges... so DF and probably minimum 3x lumber or blocking, parallel and nailed to hip. I like the idea of shaped 2x blocking, as the top edge can be ripped so sheathing fits flat to block top.
-- The hips act as collectors. It is probably reasonable to assume that collector-force from the long-wall-diaphragms can be transferred to the end-walls thru to the sloped end-wall-roof diaphragms, if it is designed for required capacity; i.e. collector force transfer and shear capacity needs to be adequate.
-- it might make sense to add wall-corner-straps as the calculated force without-transfer-of-diaphragm-shear-to-'Gable"-end-walls (6000lbs tension, no diaphragm transfer) (hips are pushing corner apart). Wrapping strap around corner is a bit questionable as wood might compress under straps, so there might be steel plates at the corners, under the straps, or angle iron at the corners, under the straps. Corners would have to be slightly rounded so as not to over stress (cut strap).
Also (without complete diaphragm transfer) the hips will be pushing out at the corners (6000lbs no diaphragm transfer) , so the hip has to have enough bearing at the corner so as not to exceed compressive strength of hip or top plates.
The above (other than top-chord splicing) seems like it becomes safety factor, or redundancy, if one is confident of transfer of shear thru diaphragms. So... I would be working with 4000 lbs at corners is probably adequate and safe.
Diaphragms are designed Dead + Live for 1.15/1.6 (ratio of Cd.s) times table values for allowable shear for diaphragms.
I would be happy to hear any other ideas you might have.
Cheers, Case580b
RE: Wood Roof--> are rafter ties necessary?
RE: Wood Roof--> are rafter ties necessary?
The things what gets one self into!
RE: Wood Roof--> are rafter ties necessary?
RE: Wood Roof--> are rafter ties necessary?
Alternatively, an exposed glulam truss at the third points could support a ridge beam at 17' centers and would have an attractive appearance (the architect may not agree).
BA
RE: Wood Roof--> are rafter ties necessary?
RE: Wood Roof--> are rafter ties necessary?
RE: Wood Roof--> are rafter ties necessary?
Is the structure to be exposed?
RE: Wood Roof--> are rafter ties necessary?
a few thoughts however..... some frivolous but...
-- i have attached an APA document I just got from APA... Folded Plates.... notice the water stamp 'Use at Your Own Discretion'
-- sags might be kind of cute, if calculated appropriately one would have a 'classical japanese style roof' ... a bit of a joke but i have wondered about the possibility for many years... does the style assume sagging or are a few curves just more elegant? probably the latter, but where did the idea come from... sagging?
-- post tensioned cables? just a wild idea... but next to the top plates, maybe one could add a 2 x4 on top of double 2 x6 top plate the building is rectangular... post-tensioning is not expensive,.... basically a perimeter band to keep everything locked together...
Thanks again for your experience. Cheers
RE: Wood Roof--> are rafter ties necessary?
I was in Jamaica earlier this year and a noticed a number of outdoor patio structures built in a manner somewhat similar to what you've proposed. They would have collar ties but only every 2-4 rafters instead of at each one. It was fun to guzzle Redstripe and contemplate the implications/intent. They seemed to be performing fine. Of course their only real environmental load would be the occasional typhoon.
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: Wood Roof--> are rafter ties necessary?
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: Wood Roof--> are rafter ties necessary?
So this is for gravity loads, right? So your top plates will act like a tension ring? Seems scary, but would be very interested in the design and details.
EIT
www.HowToEngineer.com
RE: Wood Roof--> are rafter ties necessary?
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: Wood Roof--> are rafter ties necessary?
That been said one can still add a lot of extra strength, for not a lot of extra cost... a fancy sink and fixtures probably costs more.
adding xtra nails at top plate splices (as existing is dried fir it probably needs to be predrilled for new nails to avoid splitting)
adding stuff to increase strength of corners (strap around corners).
If this was new and i was trying to do folded plate... one could use Framing anchors like HDU crossed at each corner... For the maximum conceivable tension try to open the corner connection I calculated about 6000 lbs DL + LL. This would require an HDU-11. One board of a double 2x6 top plate (a 2x6) could carry the shear load if the the HDU is installed close as possible to the corner 1 3/4" bolt-CL from-corner. NDS 3.4.3 Shear Design allows for a substantial shear force reduction. on the exterior side allowable compression is 625 psi... so a plate with 10 square inches is probably appropriate. This is also the area of the face a 2x6 so if the top-plate boards have alternate overlaps i.e. one top-plate board extends all the way to corner in one direction, and the the other in the other direction than top plate has adequate compression.
The other tricky connection is the ridge
I have attached a PDF of the structure with a detail of the ridge connection..
Thanks all for your thinking about this....