Ways to improve stiffness of wood frame roof diaphragm. Smart ideas 1

[fastline12]

We are still working out some details on a personal building. This is a post frame building, trusses on 12ft centers, and recessed roof purlins on 3ft centers, in joist hangers.

The stiffness values coming from this setup is not ideal and is loading too much into our columns. I need to obtain a higher stiffness for better load transfer.

In looking at a few studies, it appears the stiffness may be lower for hangers. I am not sure if this is due to tension issues at the interface. I am looking for some ideas to correct this? obviously we can throw more and more lumber at this but right now, I really cannot pinpoint the stiffness issue. the Strength is not an issue, just need to get deflections down.

 

[msquared48]

Usually post-frame buildings use joinery with knee braces to develop moments at the joints, not hangers.

Can you post a diagram of your building?

Mike McCann, PE, SE (WA)

 

[fastline12]

Hopefully the pics work. This is a crude frame up at this point but basically the columns and trusses are 12ft oc, and the purlins span that distance, at 3ft oc. All applicable loads and deflections look good except for the diaphragm performance.

 

[fastline12]

another pic. Cannot figure out how to put both in my post.

 

[jwilki]

It seems to me you need to cross brace the roof back the end walls and then provide cross bracing in the end walls to provide lateral support to the structure. I don't see how you can have a meaningful moment transfer onto flexible columns. Alternatively add the required knee braces if it does not interfere with your headroom.

 

[fastline12]

Using standard practices to design the roof as a diaphragm, technically, "it meets code"... The shear performance is well within spec so if I can get the stiffness up, we won't need cross bracing.

I am just not yet sure where the extra 'play' is coming from.

 

[Wallache]

What type of material is the diaphragm made of?

Are those 25' tall wood columns?

 

[fastline12]

The diaphragm is 26ga corrugated steel. Yes, the end poles get to about 25ft long. They are glulams. I realize they are but a stick in the wind but Actually well supported by the roof and wall systems, and buckling by the gurts.

 

[KootK]

We'd be in a much better position to help if you were able to tell us:

1) What exact movement is it that concerns you? Middle of diaphragm? Top of columns? Lateral movement? Vertical movement?

2) What are your vertical lateral load resisting elements for the two axes of the building? Braced frames in the walls? Cantilevered columns?

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.

 

[FoxSE14]

Is this building under construction, or still stuck in the computer?

Are the end walls exhibiting high displacement under lateral forces? If they are not moving much (by code or by your SE's judgement), then you can investigate the diaphragm. If the end walls are moving significantly, then focus on the lateral design for the end walls first.

What kind of fixity do you have/expect at the base of the 25' tall end poles? Are foundations embedded to a depth sufficient to handle the moment on each pole?

The building may/may not "stiffen up" when the diaphragm is installed, and the wall sheathing is installed - depends on the use of proper load path & quality of detailing.

For the safety of you and others, strongly recommend hiring a structural engineer to review your personal building before you begin construction.

 

[fastline12]

I apologize, the pics provided are only to show the configuration of framing. the area owhere the purlins are in the pic is not an area of large concern and there are no immediate concerns of shear strength in the end walls but they will probably require additional bracing that is not pictured. What we are looking at is lateral loads applied to the center of the long walls. This force transmits from the poles, to the roof system, which needs to beam the load back over to the end walls in shear. If there is to much displacement in the roof system, lateral load to the poles increases. If the roof system stays stiff, the load is better transferred to the end walls.

Due to the pitch and size of the roof, vertical forces are negative (uplift), and we are way over built for that. No problems.

The building is not currently under construction.


The columns will be embedded, then encased in concrete, then a concrete floor will be poured which will help support lateral forces applied to the base of the columns.

The end walls are looking fine at this point in shear but as I mention, the roof system needs the appropriate stiffness to efficiently transfer the load to them.

The shear strength in the roof system works well within margins but without enough stiffness, this is putting to much lateral load at the top of the columns.

The roof system is 26ga corrugated steel roof paneling, applied directly to the framing.

 

[XR250]

Why not go with a 24ga roof deck? Can you add more fasteners to increase stiffness?
Also, just thinking out loud- Does the bottom chord of a scissors truss need bracing to remain stable under gravity loads? Inquiring minds want to know!

 

[Teguci]

For pole barns such as this, I detailed 2x cross bracing lacing the top chords of the trusses next to the end walls just as jwilki suggests above. I was never a big fan of using 26 ga corrugated steel as a solid diaphragm. I used it, but I always considered it fairly flexible and preferred providing bracing in the higher demand locations.