T&G Floor decking shear 1

[jeffhed]

I have been asked to run calculations on T&G floor decking in an existing residential garage floor. There is no concrete topping, just 2x6 Fir T&G flooring. Why doesn't the NDS give shear values for decking? The supporting framing is very close together (6" o.c.) and the decking will work at that span for the 2000 lb point load in the IRC as well as the uniform loading. But for shear what do I check if I don't have an allowable shear stress value?

 

[JAE]

There's been some other posts here that talk about decking diaphragm shear.
Typically it has no shear value and you must add a plywood or other sheathing to create a diaphragm.

One recent post, I seem to remember, had some new values for decking shear but not sure.

Do a search and you might find something.

 

[jeffhed]

JAE,
I have found the same diaphragm shear posts. But I am looking for allowable shear for gravity loads. A 2000 lb point load on the 2x6 decking is a shear stress of 1.5*2000/(1.5*5.5) = 364 psi. The NDS tables for visually graded decking give Fb, (Fb)*(Cr), Fc, E and Emin. No column for Fv. How can I check the shear stress for the decking without an allowable stress? With only a 6" span it seems crazy that the decking couldn't support the point load, but how can I check it? 364 psi seems high. Looks to me like a thicker decking would be needed, but without a published value I only have judgement. I don't want to use just judgement if it involves some sort of remove and replace or retrofit.

 

[woodman88]

Shear (thru the fibers) of sawn lumber is not a problem. Shear (Fv parallel to grain) between the fibers and crushing is the problem. Check the Fv and Fc(perp) along with the other values.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[msquared48]

How do you get around the fire and gas/oil drippings issues here with no concrete topping?

When the car rolls in wet, where does the water go but beween the deck members and below?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[jeffhed]

woodman88,
You say shear through the fibers is not a problem. Does that mean it doesn't need to be checked? Or are you just saying I check crushing instead of shear?

msquared48,
I have brought up the fire rating issues as well as the gas/oil drippings to the contractor. There is living space right next to the garage with living space below, so the fire rating issue could kill the whole thing. As of right now, I am just checking the floor framing and decking to see if it could even support the garage loading. This project was built with no engineering, an unlicensed contractor, and no permit was pulled. So the new licensed contractor is going one step at a time, can the floor support the garage loading? Then we worry about fire separation, etc.

 

[woodman88]

jeffhed, Crushing occurs at the surface of the sawn lumber where the load is applied and for a distance into the sawn lumber of 1/8” plus/minus. Until you decrease the thickness of the sawn lumber to less than this 1/8” plus/minus you will have a crushing failure before you get any shearing through the sawn lumber occurring. If you want or need more information I would look at the APA plywood (which is basically 1/8” thick lumber sheets layered with the grain at 90 degrees of each other) information about this.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[jeffhed]

woodman88,
So is that why the NDS gives no value for Fv for the decking? Because if there is no crushing it would be impossible for shear failure to occur?

 

[woodman88]

Jeffhed, To be truthful I have never done a detail analysis or an in-depth research of this situation and base my answer only on my engineering judgment from what information I have read, but yes.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[jeffhed]

woodman88
What you are saying about the crushing before shear makes sense. I am looking through the APA plywood stuff now. It would also make sense then why NDS does not provide an allowable shear stress for decking. Thanks for the input. I will look some more. If I find something that says the opposite I will post it here.

 

[focuseng]

jeffhed: do you mean you are looking for Fv values for lumber used in the flatwise application? If so, I don't expect the Fv value to be any different than the typical direction. I believe only bending values are effected for flatwise application because of the locations of knots and such are graded based on their locations with respect to the wide or narrow face. This is why there are different published values. Also the visual grade for "decking grades" sometimes is concerned with other values than structural (read aesthetic). Shear though the grain appears to be what woodman is referring to.

______________
MAP

 

[jeffhed]

focuseng and others,
I have found this document from Forest Products Laboratory - USDA Forest Service that discusses shear perpendicular to the grain. It is short but supports what woodman88 said. Testing has shown that getting wood to fail in shear perpendicular to grain is difficult because shear perpendicular to grain is approximately 2.5 to 3 times stronger than shear parallel to grain. However, the horizontal and vertical stresses at any point are equal, so with this being the case, I still need to check shear parallel with the grain as well. So now with the 2000 lb point load on 2x decking my shear stresses are 364 psi, which doesn't work. I'm probably going to get some surprised looks, but I'll have to tell the client that this decking needs to be replaced, overlaid, or something to get the decking to support the garage loads. My calculations show I need at least 3x decking. Here is a link to that document in case anyone wants to read it. Thanks again for the input.

http://www.fpl.fs.fed.us/documnts/pdf2001/green01d.pdf

 

[woodman88]

In the shear calculation (for fv=3V/2bd) you should be using V=1000. So that for a 2x6 you have fv=3(1000)/2(5.5)1.5=182 psi.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[jeffhed]

woodman88,
The 1000 lbs would be with the point load located at the midspan. Shouldn't the decking be checked twice, once with the point load located for the worst case flexural loading (midspan V = 1000 lbs) and once with the point load located for the worst case shear (close to one end V = 2000 lbs)? That is how I was taught to do it.
However, I have since changed my mind from my previous post. I went out in the parking lot and started measuring tire widths. All the tires I measured were wider than 6" which is the floor joist spacing supporting this floor. With that bit of information Would my shear force ever be 2000 lbs. Would it ever be 1000 lbs? The tires would always be on at least one joist. I'll need to think about what that shear force will be over my beers this weekend.

 

[woodman88]

With joists at 6" oc your point load is really a uniform load over a 4" to 6" lenght of the decking. You will never have a V more than 1000 lb. Look at shear diagrams for an uniform loaded simple span, cantilever and three-point bearing.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[shobroco]

Jeffhed: I have never seen wood joists @ 6" c/c so I thought maybe it was a typo (16?). I read through this whole thread wondering what you were loading the floor with to get shear if it was 6". I'm glad you decided to look at tires because I am sure your joists are going to govern the design. If I trusted the joists, I'd drive my car on a sheet of 1/2" plywood laid on joists at 6" centres. If they are 1 1/2" thick you could drive on them with no decking: only a bicycle tire will go through a 4 1/2" space.

 

[jeffhed]

woodman88 and shobroco,
I was too close to the problem to see the answer. The original joists were spaced at 12" o.c. but could not support the point load. I have added floor joists in between that can support the point load. Then I checked the decking and I couldn't get it to work with the 2000 lb point load. Looking out my window while I was trying to think what was wrong I started looking at the cars parked outside. Then it dawned on me that most tires are likely at least 6" wide. So at this point it looks like I can make this garage floor work structurally speaking. As far as fire and oil drips, etc, may kill the entire thing.