wheel loads on heavy timber decking

[BecomingCurmudgeon]

I am trying to analyze a existing 3x8 heavy timber deck floor being used as a passenger garage. I found resources for checking the bending in the deck but nothing for shear.

The decking is rough 3x8 Doug Fir planks laid next to each other with a small 1/4" gap. Not tongue and groove. The deck supports are spaced at 2 ft on center. There does not appear to be a mechanism for load transfer between planks.

The Owner has been parking cars on it for decades. I would like to do a shear check to answer the Owner's question about a Tesla's wheel load. When the wheel is just off of the support. there should be punching shear and horizontal shear in the deck. The same loads can occur if the Owner is using a jack to change the tire.

I can demonstrate that bending is OK by calculation using normal procedures. I am puzzled by the shear/punching checks. I can't find any shear values for decking in the AITC manual or a sample calculation for decking.

Anyone done something similar?

 

[OldDawgNewTricks]

I can't find any shear values for decking in the AITC manual

The AITC manual says "Heavy timber decking essentially forms a beam and must be analyzed as such. The basic criteria for bending, deflection, and shear must be satisfied. However, due to the typical flat-wise orientation of the decking pieces, shear rarely controls the design. The equations in Table 10.3-1 may be used to compute the allowable uniform load for the standard patterns. Additional checks might be necessary for point loads and short spans with heavy loads where shear becomes significant."

Refer to NDS for the required additional shear checks.

 

[Ron247]

I do not have an AITC manual, but do they have a formula for shear parallel to grain similar to (3*V)/(2*b*d) from NDS?

It sounds like you are looking for shear perpendicular to the grain. I do not know of published values on that.

 

[Tomfh]

Just check the shear capacity. A 3x8 Douglas fir should be fine for a car wheel

 

[BecomingCurmudgeon]

I am having trouble finding an allowable shear value. I am using Doug Fir Larch Commercial Dex values for the bending check. Bending, bearing, and deflection are fine. Table 4E in the NDS provides nothing for shear. A Doug-Fir-Larch value from Table 4D gives you an Fv = 170 psi. The shear stress with an IBC wheel load just off of the supporting beam would be 3 x 3,000 lbs / 2 / 7.25" / 2.5" = 248 psi. The IBC wheel load is applied to a 4.5" x 4.5" area or just one plank.

Something isn't right with the analysis because the car wheels do not punch thru.

What am I missing?

 

[XR250]

I am having trouble finding an allowable shear value. I am using Doug Fir Larch Commercial Dex values for the bending check. Bending, bearing, and deflection are fine. Table 4E in the NDS provides nothing for shear. A Doug-Fir-Larch value from Table 4D gives you an Fv = 170 psi. The shear stress with an IBC wheel load just off of the supporting beam would be 3 x 3,000 lbs / 2 / 7.25" / 2.5" = 248 psi. The IBC wheel load is applied to a 4.5" x 4.5" area or just one plank.

Something isn't right with the analysis because the car wheels do not punch thru.

What am I missing?

Because the wheel load is not actually 3,000 lbs and the design values are conservative. Also, draw a free body diagram of the clear span of the plank as the length and your point load 1/2 a tire's width from the end. The shear won't be 3,000 lbs there. It will be slightly less. You also can ignore any load within 2 1/2" of the support.

 

[BecomingCurmudgeon]

I was able to draw the FBD and show that when the point load is applied at 2.5", the shear drops to 2,688 lbs resulting in fv = 222 psi. Then if I use a Table 4A Doug-Fir shear value of 180 psi. And add in a 1.15 repetitive use factor, a 1.15 flat use factor, a 0.97 wet use factor to get Fv' = 231 psi and a DCR of 0.96.

Not sure if it is kosher to use both Tables 4A and 4E at the same time. One for shear and the other for bending. Seems like picking design strengths just to make the check work.

 

[Tomfh]

Use the actual wheel loads.

 

[OldDawgNewTricks]

Flat Use Factor and Repetitive Member Factor are not applicable to shear.

Something isn't right with the analysis because the car wheels do not punch thru.
What am I missing?

The Reference Design Value of 180 psi is not the failure stress. It has a factor of safety built in. So it looks like you would be using up some of your factor of safety if the design load of 3,000 lb was actually applied. Note that this is not likely the actual wheel load, but also accounts for the point load from a jack.

 

[XR250]

Don't think you should use repetitive member factor as the planks are not connected together.

 

[Ron247]

I agree with OldDawg, flat use and repetitive member are not allowed in NDS for shear, not sure about AITC.

 

[Greenalleycat]

I doubt the Tesla wheel loads are significant. Just tell your client not to jack their car on a timber floor as that sounds like a recipe for disaster

 

[Tomfh]

I doubt the Tesla wheel loads are significant. Just tell your client not to jack their car on a timber floor as that sounds like a recipe for disaster

Yeah, jacking is harsh. I saw a funny one where someone jacked their Land Cruiser and it punched through the waffle pod slab.

 

[Craig_H]

Forestry road bridges in my area have timber decks. They are typically two layers: one transverse spreading load to the bridge girders, and a second layer longitudinal which provides some additional load spreading and also functions as the wear surface. Most in my area would be as you described, 3" thick rough sawn timbers. These get beat on hard by loaded logging trucks running chains all winter, so they are certainly able to handle high shear stress situations (axle loads will be higher than for an EV).

While your deck may calc out, it might not hurt for the owner to add an additional layer for better load spread. It would likely solve your issues by spreading load to multiple planks in the lower layer, and solve the jacking worries.

 

[BecomingCurmudgeon]

I corrected for the non-permissible use of the load duration factor and flat use factor. I then ran a few wheel loads from actual vehicles. They checked out fine. Which should be the case since cars have been parked on the deck since 1980.

My efforts have proved that you should not try to jack up your car on the wood planks.