Ridge Beam - lateral and uplift load path?

[jgeng]

Can someone walk me through there opinion of the lateral and uplift wind load path of a 1 story masonry building with a ridge beam and rafters hung off beam? the roof has a very low pitch. considering the wind coming from the gable end would the wood roof diaphgragm bring the loads to the walls or would it be going through the ridge beam to the walls perpindicular to the beam at the end of the roof? is there any special detailing needed to insure both halves of the roof diaphragm act as one? How about uplift force, does it go through the ridge beam or only through the rafters to the wall? It seems to me typical light frame ridge beam connections are usually not designed to have these full uplift and lateral capacities?

 

[JAE]

There's probably very little stiffness at the top of the gable end-wall. Force follows stiffness so the lateral forces on a gable wall would go through the diaphragm and into the two orthogonal "shear walls".

For the diaphragm to act as one -through the ridge, there would need to be some type of chord connection at the tops of the gable end walls.

 

[msquared48]

If the rafters are sufficiently connected to the ridge beam to develop the uplift as they should be, the ridge beam will transmit that same uplift to it's supporting columns and to the foundation.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ToadJones]

JAE-
This is interesting.
Let me premise by saying diaphragm and shear walls are not something I design regularly....but are you saying:

For wind on a gable end as described, the actual diaphragm shear at the center where the ridge is would be zero, correct?

At the same location the chord force would be maximum, correct?

Is this why you are saying you'd only need chord connections at the top of the gable ends?

 

[msquared48]

Lateral loads transverse to the gable end are transmitted through the roof diaphrabm to any shear walls/frames/ whatever in the sidewalls parallel to the load. The same is true for the lateral forces that are normal to the sidewalls - those forces are transmitted through the roof diaphragm to any gable end shearwalls/frames/whatever.

As or any chord forces, I would expect that force would be greater at the roof/diaphragm connection in the longer direction of the structure. It still exists at the gable end and should be designed for though asw JAE suggested.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ToadJones]

Mike-
I was only talking about wind parallel to the ridge.

 

[msquared48]

Just checking...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[JAE]

TJ - yes I think I agree with what you say.

The diaphragm for a flat roof, with a center "ridge" beam running down parallel to the direction of the lateral force, would serve as a flat, horizontal beam spanning from side wall to side wall.

The shear at the midspan of the diaphragm is zero (the point where the ridge beam is). The shear at the side walls is maximum. This is premised on the fact that the two endwalls are receiving the wind and need to deliver it to something that can brace them laterally. Only the sidewalls will do so the load path goes through the diaprhagm and into the sidewall shear walls.

The moment in the diaphragm is maximum at the midspan of the diaphragm and a chord is required along the two endwalls to resist the tension and compression forces resulting from this moment.

For the pitched roof that jgeng describes, we have a kinked diaphragm but many engineers deal with it in similar fashion to that of a flat diaphragm. As the pitch increases, I would suppose there is some point where the chord forces aren't behaving as though the roof is flat.

 

[BAretired]

The masonry gable walls span vertically from ground to roof diaphragm. The diaphragm acts as a folded plate distributing the gable wall wind load to the side walls. There is no special detailing to ensure both halves of the roof act as one other than adequate attachment for the forces involved.

Wind pressure on the roof acts normal to the surface. It can be toward the interior or exterior. Decking must be attached to rafters and rafters must be attached to ridge beam for the worst case.

BA

 

[Lion06]

The shear is only zero at midspan for a basic wind case, not one of the 173 torsional or eccentric wind cases.

 

[ToadJones]

EIT-
I have to laugh when I read that.
I was just using the wind torsion cases the other day. Every time it comes up it is like a science project writing combo's.

 

[ToadJones]

EIT-
I was wondering if you had the name "StructuralPE" reserved for when the time comes?

 

[Lion06]

I was debating whether to use 978 or the 173. It is ridiculous!

I don't have the name reserved (I don't think they'll let me), but hopefully no one jumps on it between now and December (when I should find out I passed - I'm hoping I do first time around).

 

[ToadJones]

Just make a second name and wait to use it

Based on your input here, I'm sure you passed.
Which test did you take?

 

[ToadJones]

EIT-
I set up a spread once to write combo's for a building with an overhead crane.
By time I ran all the combos for wind and all the crane loadings, I kid you not, I have about 350 load combinations.
I of course, did not use all of them, but if you wanted to take a blind approached, thats how it worked out.

 

[Lion06]

Too late - someone already has StructuralPE

 

[JAE]

The shear is only zero at midspan for a basic wind case, not one of the 173 torsional or eccentric wind cases.

OK - yes.

But for this discussion the concept was for a straight-up simple gable roof. The torsional wind case wouldn't matter.

Why don't you use StructuralEITPE.....it's the natural progression right?

 

[ToadJones]

Or
"The Man Formerly Known as StructuralEIT"

 

[JAE]

Or perhaps StructuralEITSE
Or SEITSE
Or StructuralSE

...just trying to help!

 

[ToadJones]

Yes, I didnt think that maybe you took SE