Maximum absolute deflection in a ridge beam

[msdmoney]

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.

 

[BUGGAR]

On that job, I would consider creep and visible deflection. I would ignore code and go conservative.

 

[oldestguy]

What would happen if you had no ridge beam? Roof collapse? No way. Why?

 

[oldestguy]

I assume the subject roof is reasonably modern with sheathing as plywood, laid in stacked bond pattern, resulting in an integral flat plane similar to what a single sheet of plywood would be for each plain of the A frame with a pitch of each over 1:1. The beam action strength in the plane of the roof sides would be very great.

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.

 

[jwilki]

What happens if your remove roof members depends if the roof is coupled (has ties from rafter to rafter)or non-coupled.

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

 

[oldestguy]

Ridge board present or not, removing the strut action of the "ties", only places bending moment in the rafters, none on the ridge board. Adding a beam as vertical reinforcement of ridge is not going to do anything to reduce the inclined bending moments in the rafters. Since adding that 30 foot ridge beam probably requires first taking out the ties, do a detailed elevation survey along the ridge beam before and after tie removal. Likely it will not show any change when ties come out. If any shows up, install a temporary column at 15 ft. and measure the load it takes to lift that center back up. Use that as a design partial load figure for the beam, plus the snow load if any. I'd recommend not building the replacement ridge beam until the ridge downward deflection is measured. It may not show any extra support is needed.

 

[oldestguy]

More from OG. This subject deserves careful review. Supposing the rafters will deflect due to loss of struts. The rafter lengths then will shorten sightly. That's the inclined component of ridge board movement. The vertical component will be less, but the elevation change is due to rafters bending, not due to any load on the ridge. Again brings up the question of why a reinforcement there anyhow. What about a sag in the rafters thereafter? that may be more something to think about than the ridge board.

 

[XR250]

No offense to oldestguy, but I would def. put a structural ridge in there. I would keep total deflection to around 1" max. Also, if the ceiling is currently bracing the hinge at the gable end walls, that will have to be addressed.

 

[msdmoney]

Thanks for the responses, the roof is an existing cedar shake roof that they intend to leave in place with no plywood so I can't assume that the roof acts as a folded plate. Definitely going to design a new structural ridge, just wanted to get input on what others use for an absolute deflection. Would love to add plywood to the roof or underside, but that suggestion probably won't be well received. I will likely specify a gypsum diaphragm minimum on the underside of the existing rafters.

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.

 

[BS2]

I agree with XR250 - you must install a structural ridge beam. Without a structural ridge (or some other structural system), you are asking for noticeable deflection and a possible collapse. The only thing worse than a client complaining about the cost of an upgrade/repair is finding out you designed it wrong and it will cost even more to fix.

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.

 

[oldestguy]

This Old Guy is anxiously awaiting a conclusion of this thread with a statement showing what size ridge beam you finally will install. In designing it,would your ridge beam be any different if it was a flat roof with no peak, same building size, same dead load.? If so why?

 

[SkiisAndBikes]

I would look at limiting the deflection to

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.

 

[XR250]

This Old Guy is anxiously awaiting a conclusion of this thread with a statement showing what size ridge beam you finally will install. In designing it,would your ridge beam be any different if it was a flat roof with no peak, same building size, same dead load.? If so why?

I would use the same beam as code does not differentiate from flat to sloped roof's (as long as the deflection is reasonable enough to prevent ponding). Also not taking into account the added snow load on a flat roof.

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.

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.

 

[oldestguy]

XR250: Thanks for the reply. It goes to show that codes are not always applicably to every case. For instance, assuming no snow loads and only dead load, if the pitch of the roof is 60/12 (V/H) or steeper that you still need that ridge beam the same size as a flat roof. It gives no allowance for the sheathing and the rafters as one unit, likely to put significant or total load at the eaves. That's why I say you don't need a ridge beam for 12/12 or similar pitch.

 

[XR250]

I would still use a ridge beam on a 12:12 pitch unless you want to go thru all the messy calcs and detailing to try to make it work without it and hope it gets built correctly in the process.

 

[oldestguy]

I have seen and have worked on some of Frank Lloyd Wright's buildings and can imagine the consternation of his staff or engineers when he calls for removing a column in the middle of a room (column holds up part of a flat roof). Funny thing but it worked due to nearby features that were not designed for it, but assisted, but the ceiling sag didn't seem to bother Mr. Wright. Somehow leaking roofs and other problems were not of his concern. This will be an interesting job to see how the beam is installed to take up the load and we don't get to see how things behaved without it. Mr. Wright may be looking up from the fiery furnace.

 

[hokie66]

Not only did Wright not bother with leaking roofs. He didn't worry about tall people. Florida Southern College in Lakeland has the largest single site collection of his buildings, and I can't use the covered walkways without hitting my head.

 

[msquared48]

You will probably run into something like a GL 5.5 X 24
24FV4.

Mike McCann, PE, SE (WA)

 

[oldestguy]

Last year I sold a log house that may have had similar roof, pitch. When building, we lacked sufficient length of ridge "board" (3" x 10") to run the full distances of the ridges. A simple lap joint was made, with no attempt to carry any bending moment. Sorry, but my photos retained now don't have a shot of ridge on inside. But will try to load two photos. The joint of rafter to ridge "board" was simply toe nailing with two 8" timber screws. A few ties at 2/3 height were used with no attempt at a strong joint, but to provide a little benefit for the rafters.

 

[oldestguy]

Here is outside.