Maximum absolute deflection in a ridge beam
Maximum absolute deflection in a ridge beam
(OP)
I have a project with an existing ridge board and rafter ties, where the client wants to cut out the ceiling/rafter ties to vault the ceiling. The plan is to install a ridge beam that spans approximately 30 ft to support the existing roof. I'm just wondering if anybody typically limits the deflection above just the standard code maximums per length of the member to limit the thrust on the exterior walls. The roof pitch is 10/12.






RE: Maximum absolute deflection in a ridge beam
RE: Maximum absolute deflection in a ridge beam
RE: Maximum absolute deflection in a ridge beam
So I did a Google search with the words "A-frame roof collapse". Of course numerous flat roofs and those of relatively little pitch showed up. A few older roofs with apparently boards as the sheathing showed up. None of the more modern residential residential or commercial buildings with 1:1 pitch +/- and with some form of collapse showed up (other than from fire).
A review of design methods (on Google)for pitched roofs showed all the typical info about trusses and rafter design, but none giving any benefit to the "beam action" within the plane of a roof sheathing on a decent pitch with plywood sheathing.
So for the subject question about removing some of the struts between rafters, it would appear that nothing significant will happen. The need for a ridge beam then boils down to the fact it is needed mainly for construction purposes. Thereafter it serves no significant purpose.
RE: Maximum absolute deflection in a ridge beam
From what you have said the roof is coupled and thus forms an A frame, once you cut these ties the roof become a non-coupled room and the ridge beam WILL take the load. If the ridge is just a non-structural framer (as is common for coupled (*Edit) roofs) it will sag and your roof will spread and compromise it's structural integrity.
Design a structural ridge beam to take the loads appropriate load width and install before they decouple the roof.
Deflection - this is definitely an important visual criteria and excessive sag can cause cracking in finishes. I would use a deflection limit of L/400 but no greater than 12mm for dead load
RE: Maximum absolute deflection in a ridge beam
RE: Maximum absolute deflection in a ridge beam
RE: Maximum absolute deflection in a ridge beam
RE: Maximum absolute deflection in a ridge beam
Thanks for the suggestion about the consideration of wall bracing at the gable end, always a concern with a vaulted ceiling, especially vaulted after the original construction.
RE: Maximum absolute deflection in a ridge beam
msdmoney - I do not usually check for a maximum absolute deflection. However, I am also fairly conservative with the applied loads. I usually try to keep the total load deflection above L/360. When your spans get to be greater than 25 feet or so, I find that going to a steel option for the ridge beam may be preferable. Depending on your local source for lumber, large ridge beams can be cheaper in steel versus wood. Also, you won't have creep issues.
RE: Maximum absolute deflection in a ridge beam
RE: Maximum absolute deflection in a ridge beam
L/360 for snow/live load
L/240 for total load
and also consider limiting maximum deflection not to exceed 1".
This has worked very well for me in the past in high end residential. Tract homes have somewhat less strict criteria.
I often have young engineers I am mentoring calculate what beam size they would need to meet Code minimum requirements, then calculate something more along the lines of what I have suggested above. When comparing the two and estimating the cost difference between the two beams, it becomes clear that an extra (for example) $300 to $500 for a bigger beam in a $100,000 reno project is not that significant and well appreciated by the homeowner.
RE: Maximum absolute deflection in a ridge beam
The cost difference is usually not even that much. Going to a 16" LVL from a 14" LVL, for example, gives you about 50% increase in stiffness but a cost difference of only about 15%. Similarly from a 12" to a 16" gives you about 140% increase in stiffness for about 33% more in cost. Moment of inertia is your friend.
RE: Maximum absolute deflection in a ridge beam
RE: Maximum absolute deflection in a ridge beam
RE: Maximum absolute deflection in a ridge beam
RE: Maximum absolute deflection in a ridge beam
RE: Maximum absolute deflection in a ridge beam
24FV4.
Mike McCann, PE, SE (WA)
RE: Maximum absolute deflection in a ridge beam
RE: Maximum absolute deflection in a ridge beam
RE: Maximum absolute deflection in a ridge beam
RE: Maximum absolute deflection in a ridge beam
RE: Maximum absolute deflection in a ridge beam
Mike McCann, PE, SE (WA)