Providing lateral stability for wood frame building

[asixth]

Hi guys

I have an educational facility where a ceiling has not been fixed to the underside of the roof trusses. As a result, I can not rely on diaphram action to transfer the lateral forces to the shear walls. What would be the best method (cheapest method considering material costs and constructibility) to provide lateral bracing to the building. What I have been doing is as follows:

1. Fixing a hot-rolled channel to the external wall top plate. The ceiling is suspended at 2700 (9') above the slab. The wall frames are 3000 (10') high so it gives me a 300 ceiling space (12") between the ceiling and the underside of the trusses. I have only provided channels on two adjacent external wall and have used 4"x2" ties fixed to the underside of the truss to strut forces from walls on the opposite side of the building to the channel which then transfers forces to the shear walls.

What I would be particularly interested in hearing are other methods. Is it feasible to fix a sheet of plywood to the underside of the trusses and use that to transfer the forces to the shear walls?

All comments will be appreciated.

 

[vandede427]

roof decking as a diaphragm: plywood or osb.

 

[BAretired]

I agree with vandede427. Roof decking is usually considered to be a diaphragm.

It is also possible to create a wind truss in the horizontal plane below the trusses, but unless there is some doubt as to the adequacy of the roof diaphragm, this seems unwarranted.

BA

 

[DaveAtkins]

Agreed. Normally in a wood framed building the ROOF sheathing is the diaphragm--you must get the lateral loads up to the roof somehow.

In unusual circumstances, I HAVE put plywood on the bottom of trusses to act as a diaphragm.

DaveAtkins

 

[asixth]

Thanks guys

Because of general construction practices in my part of the world, roof sheathing is not common. Generally the roofing membrane is light gage roof sheet which isn't considered a diaphram.

How is a horizontal wind truss created. Would it be a truss formed from say 90x45 (4x2") and screw fixed to the underside of the roof trusses?

 

[msquared48]

If Aussies don't use roof diaphragms, what do you usually use?

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

 

[DaveAtkins]

If you are talking about corrugated steel decking on wood purlins on roof trusses, then you DO have a diaphragm.

DaveAtkins

 

[BAretired]

There was a recent thread in which hokie outlined the way the Aussies do it. I don't remember the link, but as I recall, the typical roof framing in Oz is purlins at close spacing with very light gauge deck spanning between which is not deemed to contribute to diaphragm action.

Instead of diaphragm action, the Aussies rely on bracing in the horizontal plane. Maybe hokie can come to my rescue because I cannot remember just how he said they do it.

But remember, those of you who live and work in the northern hemisphere...the Aussies don't really understand the finer points of engineering, so you will have to make allowances for them.

BA

 

[msquared48]

Man...! Is Hokie going to be pi$$3@ or what!

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

 

[apsix]

Yes, but we understand the coarser points of engineering very well.

That's a lot better than having no understanding at all.

 

[asixth]

BA

I'm doing everything that I can to try and increase the quality of engineering in this country.

I think the thread that you are referring to is this one thread726-253176.

I went to a workshop towards the end of last year presented by a group of people who specialize in cold-formed steel design. These guys basically said that they are trying to get some people on board to participate in testing of light gage metal sheet to determine the diaphram capacity but haven't had any luck in doing so.

DaveAtkins,

The following is a link to the particular material that is being used for the roof sheet,

http://lysaght.com/files/dmfile/TrimdekJanuary2010.pdf

Can you please refer me to a product which has roof diaphram capacities provided with compliance certificates from testing.

I was speaking with a colleague today who had specified horizontal wind trusses fixed to the underside of roof trusses spanning approximately 20000 (65'). The truss manufacturer returned with a very thick roof truss fabricated from a high strength species of timber which was quite expensive.

I would be prepared to use plywood opposed to plasterboard (which is considered a diaphram if spanning less than 30') because plywood is covered within the timber code and has been structurally graded. The problem that we have in this country is that there is plywood coming in from overseas which has been stamped to say that it complies with the relative standards, sold as a higher stress grade and found out at a later stage it is an inferior product. This is a big problem in the formwork industry where peoples lives are put at risk because inferior products are being sold on the market.

 

[DaveAtkins]

Your proposed product is a little different from what I was thinking of.

Here in the US, there is a trade organization known as the National Frame Builders Association that publishes a design guide with diaphragm values for steel roof decking from companies such as Fabral. I think the guide is called the "Post Frame Design Handbook," or something like that.

DaveAtkins

 

[rowingengineer]

asixth,
Ignore these crazy Americans, this is the reason so many of there building fall down during cyclones (hurricanes to people that are crazy).

There have been studies on the shear capacity of the sheeting done in the past, but because we fix through the ribs and not the pans and we don’t provide the detailing to ensure the diaphragm action occurs, we don’t design with it. I won’t beat an old drum here but all I will say that there were a large amount of failures in the cold formed steel shed area again in the cyclone that crossed in Mackay, and this time it wasn’t even close to the design event.

I not mind the sound of your channel idea, if detailed correctly I will work, but if not, you will can get torsion in the channel.

For you situation you have a few choices:
1. Provide steel cross bracing to the underside of the truss’s with a few binders/trusses for the axial load.
2. your idea with a steel bracing frame
3. ply wood secondary
4. Some crazy ideas. Like external supports (raking columns) or cantilevering your walls, I could go on but I’m not American.


An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[asixth]

RE

I was just thinking today GI strap with 90x45 MGP binders screw fixed to the underside of the roof trusses to create a wind truss that spans the load to the shear walls. The biggest issues I see is providing enough shear capacity for the GI strap.

If I fix a 1200 wide plywood panel to the shear wall I have 8kN lateral resistance. This means I would need a minimum 2/30x0.8mm GI (1-1/4"x20gage) strap with 6 nails to each top plate to ensure the shear wall can be fully utilized.

This sounds like the cheapest solution and simplest to install on-site.

 

[PostFrameSE]

DaveAtkins mentioned the NFBA Post Frame Design Manual. He's right on. You already have a decent diaphragm if you ran screws through the flats and less of a diaphragm if you ran screws through the ribs. The important thing is that you screw the laps together with a stitch screw or similar.
Why bring more materials and labor into the equation when you already have load-resisting-sheeting on the building?

I was considering writing another post, but it seemed that since we're onto diaphragms on this post, I would append to it. ASCE 7 calls for a 10psf minimum load on the vertical projection. That generally results in a load that is 50%-100% higher than what the load would be based upon the wind coefficients, which I might add were determined from wind tunnel testing. Now.....let's resist these fictitiously high loads with diaphragms that have a safety factor of 2.5 on them!!!! Pile on the safety factors here. :-(

Would I be out of my mind to think that on a code-compliant building that I can resist my fictitious lateral load with a diaphragm that has a FS on it of say 1.75 or 2.0 instead of the 2.5 that is required for metal clad wood diaphragms?

Thanks.

 

[asixth]

In my particular case the wind co-efficients on the vertical projects result in a racking for of 0.96kPa which is double the 10psf minimum requirements.

We don't usually screw fix the flats for waterproofing reasons.

I am looking into post-frame construction which DaveAtkins mentioned (for my own education purposes more than anything).

Regardless of whether the pressure co-efficients are high, they are still there and must be accounted for.

 

[hokie66]

I didn't comment earlier because of the title. The reasons given by asixth and RE for not using metal roofing as a structural diaphragm are well founded. It is not decking, it is roofing. Whether a concealed fastener system (standing rib) or a crown fixed profile, there is little proven capacity in the plane of the sheet. Our bracing methods typically occur at the ceiling level in the form of horizontal trusses.

 

[PostFrameSE]

hokie66,

Perhaps we're not talking about the same thing. I myself have tested diaphragms according to the ASAE EP558 and there is SIGNIFICANT diaphragm strength and stiffness out of simple 29ga corrugated steel roofing when screwed through the flats into 2x4 SPF purlins setting on top of wood trusses. I've also done simulated wall diaphragm tests as well where we've screwed throught the steel into the wide face of 2x4 SPF girts. I'm talking "Pole barns" here as many call them, but those types of structures are very highly engineered buildings.........when done right .

Now.......if we're talking a standing seam roof here, then I would have to agree that there isn't any diaphragm to speak of, except what you get out of the wood decking that the steel oftentimes lays on top of.

 

[hokie66]

I wouldn't disagree that roofing screwed through the flats would give substantial diaphragm capacity. But in Australia, screwing through the flats is a no-no for roofing. Voids the manufacturer's warranty. We screw through the crowns with neoprene washers under the screw heads. For wall cladding, we do often screw through the flats, but still most engineers here won't count the cladding as a diaphragm.