wood rafter thrust problem 5

[structuralnerd]

I have a wood framed building with a very tall and steep pitched roof. Unfortunately, the client wants the ceiling vaulted, so we cannot use trusses to frame the roof, only rafters with one intermediate brace toward the top of the roof. This results in quite a bit of thrust force at the bottom of these rafters. To take this thrust force out, I was thinking about resolving it into the sill. By analysis, this results in a really big sill plate. I was thinking of the sill plate as a simply supported beam. Is this the right way to go about this thrust problem? Would I need a pretty substantial connection at the end of this sill plate? Any other suggestions? Thanks in advance.

 

[jechols]

I would use a ridge beam.

 

[CowboyBill74]

Structuralnerd -

I have run into this problem before and the way that I handled the excess thrust from the vaulted ceiling rafters is to use LVL's placed flat so that the strong axis is in the plane of the thrust force. Not knowing how far of a span your LVL's will have to go you may need 2 or 3 to with stand the thrust force.

 

[structuralnerd]

jechols-
Sorry to be so naive on this subject, but I have limited experience in wood design. Can you please explain to me how the ridge beam resists the thrust force? I'm assuming a ridge beam is like a header at the ridge of your roof. Thanks.

 

[JRESE]

Sometimes you just have to tell the client that what he wants is not reasonable.

 

[csd72]

You said you could have a brace toward the top.

If you model the roof as an A-frame and put one support on rollers then if you limit the lateral deflection to about 1/2" larger rafters but no significant wall thrust.

Alternatively use a load bearing ridge beam as suggested by jechols, this will remove the thrust.

 

[structuralnerd]

Is this ridge beam like a header? Do I design it like a simply supported beam, therefore also making sure the supports are strong enough? Thanks for the suggestions csd72.

 

[ntpe]

Why not use scissors trusses? You will still have a lateral reaction but it will be manageable.

 

[Lion06]

structuralnerd-
Using a ridge beam eliminates the thrust by making your slope rafters simply supported.

 

[ChipB]

I second (third) ridge beam suggestion. If the span is long, you might need to go to a steel ridge beam wrapped in wood.

 

[structuralnerd]

Just to clarify, this ridge beam must sit on wall studs to support it, correct?

 

[ChipB]

Yes, it sits on the stud walls. I have in the past, created a frame in the wall, using LVLS, with cross ties between windows, when the windows were directly below the ridge beam.

 

[plasgears]

I would suggest cable tension members to complete the trusses. Some marginal wood trusses may need this help as extra insurance.

I have seen two major wood truss failures in five years, one in a state building in Lansing, MI.

 

[hokie66]

plasgears,

Structuralnerd is not using trusses. His client doesn't want a bottom chord. Your cable would work only for gravity loads, not for uplift.

 

[UcfSE]

It seems you should still have thrust from an unbalanced live load, even if they are simply-supported.

 

[hokie66]

In resolving your thrust forces, you need to consider lateral loading conditions, not just vertical. How is the horizontal reaction of the top of the wall resisted by the roof structure? And how is the horizontal reaction of the rafter (due to lateral loading) resolved? If there is a system of bracing or diaphragms to resist these forces, the thrust forces due to vertical loads will probably follow the same load path.

 

[Hemifun]

I still don't understand how using a ridge beam eliminates the horizontal force component at the wall. Is it assumed that the connection at the wall is a roller support which doesn't seem realistic??

 

[csd72]

Hemifun,

When you look at it you would think that there would be significant lateral loads due to the slope of the rafter.

But, the rafter only needs vertical reactions to be stable and it only needs a minute amount of lateral movement in the walls to alleviate any horizontal components.

Otherwise single pitch roofs would not work!

csd

 

[UcfSE]

Why does the rafter have only vertical reactions at each support?

 

[bylar]

Structualnerd.
Looks to me that you need a "real" structural engineer!