Wood bending stress 2

[jlholt911969]

I have a barn floor using white oak joists. I am trying to get a capacity for the floor, but in the NDS the bending stress for white oak is less than that of Spruce Pine Fir which is what we have around here. The problem is that I know that white oak is denser and stiffer for the equivalent size SPF joist, and yet the NDS shows white oak bending stress as lower than SPF for the same visual grade. I have looked online at various wood websites, and the values given for bending stresses of White Oak I found were as high as 15ksi and the highest I found for Spruce, White Pine, and Doug Fir are 10ksi, 8ksi, and 12ksi respectively. However, the NDS shows bending stresses for these as nearly the same between White Oak, SPF, and Doug Fir. I am using these numbers only as a relative comparison between the different materials, and not as actual allowable stresses. What am I missing? Why is the NDS numbers so close together, but the numbers found elsewhere online so widely varied?

 

[XR250]

That seems off. Based on my experience, I would think White Oak is a lot stronger.

 

[ChorasDen]

Interesting question, but it comes down to what's the difference between workable design stress and actual member capacity for an individual piece. Keep in mind that visually graded lumber has a lot of variability from piece to piece. The nature of visual grading, while consistent and efficient, allows different strength piece of wood to all be batched into the same group for design purposes. You'll obviously get lower strength wood next to higher strength wood within the same visual grade 'basket'. You'll see this if you refer to Table F1 in the NDS, the coefficient of variation for visually grade lumber is 25% (exceptionally high in comparison to other structural materials).

Next we need to consider how are allowable design stresses in NDS determined? Large samples of products in their respective class (visual grade 1, VG2, etc) are tested to failure, and the failure point is then plotted to establish a standard normal bell shaped curve of allowable design stresses. Keep in mind that wood has a Load Duration Factor that indicates that wood can take larger loads for short time periods, and because these kinds of tests don't normally run for >10 years to test bending strength, the numbers will be elevated due to the effects of load duration.

Okay, so now we have a standard normal curve that has been increased due to the short load duration for testing, how do we get design properties listed in NDS from this? The NDS takes the lower 5 percentile of the standard normal curve, and bases the allowable design stresses from this number. Because the COV for visual grade lumber is so large (25%), our standard normal curve is very wide, and thus, the 5 percentile will be a relatively low number. Once the 5 percentile is taken, and then the allowable load reduced for load duration, the allowable design stress will be significantly lower than what you might expect from testing a 'standard' piece of wood for fiber bending on testing apparatus.

Why is this all done? To ensure that any standard piece of wood has a very high likelihood of being stronger than the design strength would indicate. Floor systems with multiple members increase the redundancy, by combining stiffer/stronger members with less stiff/strong members to balance out the structural effects.

 

[ChorasDen]

To add, this 5 percentile adjustment only applies to strength based allowable stresses, it does not apply to modulus of elasticity for wood members, as deflection is not normally considered a life safety concern. If you wish to see the effects of reducing MOE to the 5 percentile, refer to NDS Section F.2.

 

[jlholt911969]

So ChorasDen, considering what you said and since we really don't know anything about the floor joists except the species, I am thinking we should probably stack a 115% of the desired floor load and physically load test the floor rather than try and determine the capacity through design methods. We would be monitoring the deflection rather than the stresses to determine if the floor would be able to handle the load needed to meet code for its new use. The owner wants to turn his hay barn into a wedding venue, and has to have a floor that can handle 100psf. I have done a ton of these, and this one is actually the only one that I have had this problem with because all the others had to be reinforced with additional joists. This one already has more structure supporting the floor than I typically find in this situation, and the homeowner is convinced that it can handle weight because, "He has stored a lot more than 100psf worth of weight on the floor for a lot longer periods of time than he would be using it for." Would load testing in such a situation be acceptable within the code in this case? I have only started to look at this option and so far, my limited search has not come up with anything specific as to whether it can be done, and how it should be done.
Thanks for the reply, I like knowing the story behind where we get all these numbers and charts we use everyday.

 

[XR250]

You may want to see if you can find older versions of the NDS. They may have higher design values that would be more appropriate for the era when this was constructed.

 

[jlholt911969]

XR250, I will try that, but the barn was built about 100years ago. It is in amazing shape considering its age, and the owner has been doing a lot of maintenance and repair work on it since he has been using the area under the main floor as a game room/man cave.

Thanks for the help

 

[ChorasDen]

thinking we should probably stack a 115% of the desired floor load and physically load test the floor rather than try and determine the capacity through design methods.

This seems considerably more time consuming and complex. Are you indicating that the floor fails when designed with the properties listed in the current NDS? Did you include size factor and repetitive member factor effects in your calcs?

I'll be honest and say I've not done a lot of thinking regarding proof loading wood floor systems. Where did you determine the 115% from? In order to test in a reasonable amount of time, I would consider loading with 140% of the design load for a single day. 140% comes from NDS Figure B1, and I just rounded up for a 1-day load test. Depending on the size of the floor, that may be a lot of sandbags that need spread to achieve uniform 140psf, or do you intend to factor in live load reduction due to floor area?

When laying out sandbags, is the floor elevated? How would you ensure safety while loading/unloading the floor.

 

[ChorasDen]

the barn was built about 100years ago. It is in amazing shape considering its age

Is the lumber rough sawn, or dimension lumber? Rough sawn from that era may have different properties.

Pay special attention at bearing when looking at these older projects. 100 years ago was before hangers and large use of mechanical connectors, and so joists were often notched at end bearing to fit a certain floor height profile. This may dramatically affect the members performance in shear at bearing, especially if you intend to increase the design load.

 

[EdStainless]

Many years ago (~40) I was doing drafting work for a structural and he took a job on an old building with a similar floor.
He had them cut strips out of 20 or 25 planks that he marked and then had those tested.
I helped with the statistics.
I recall that we 'adjusted' the NDS values based on our actual tests.
I had thought that we based it on the lower 10th strengths, I don't know if this was practice then or his modification.
We also did some load vs deflection testing. I think to look for 'soft spots' in the floor.
I recall that we concluded that they could use a small bulldozer for the demolition.
That floor was stout.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[EcoGen]

@jlholt911969 I also have the same question as ChorasDen regarding the 115%. What I remember off the top of my head is 150% but not sure where that comes from either.

Also, how did you determine the exact wood species? Did you send in a sample for testing?

 

[jlholt911969]

ChorasDen at this point the 115% is arbitrary to some extent since when we test crane loads, we overload them 110% for certification. I figured 15% would be better for the floor, but I also figured if someone else has more experience they would suggest a more appropriate weight. The building is about 30x60 with access through the ends of the barn. At this point load testing is a very preliminary idea, and so I have not come up with a testing or safety procedure because I am still not sure if we have to get to that point quite yet.

 

[ChorasDen]

Got it. I would still suggest loading with more than 115% because of the effects of load duration. If you are trying to verify that the floor can support 100psf at a 10 year load duration (floor live load is considered 10 year), then if you load with 115% of the new design floor live load, you would need to leave the load in place for at least 2 months to equilibrate it to a 100psf floor live load.

 

[phamENG]

For load testing, take a look at IBC 1708: In-Situ Load Tests. I've read white papers performed on roof truss systems that used inflatable pools. You pump water in to the desired level, and have an emergency drain plug you can pull if things go sideways.

 

[jlholt911969]

The wood species was given to us by the owner since his family built it when he was a kid and that's what they used. The wood is rough sawn 2"x8", and has the grain structure that I am used to seeing with oak, but yes, we would have to get it tested if we wanted to get a true determination of the species. He has taken some samples, but I told him to hold off until we determine a more specific plan of action.

Thanks guys, I really appreciate all the good advice

 

[phamENG]

You may also want to consider working with a wood consultant. A coupon can be removed from a sampling of the joists and sent off to a specialty lab. They can determine the baseline strength and give you guidance on allowable stresses for your analysis.

 

[kissymoose]

You can get the lumber mechanically graded.
There was an episode of grand designs (a show where English people build their dream houses), and a couple ended up using local larch for their house and had it mechanically graded.

 

[XR250]

and so joists were often notched at end bearing to fit a certain floor height profile. This may dramatically affect the members performance in shear at bearing, especially if you intend to increase the design load.

This is a good point. The only floor system I have seen collapse was in a Frat house during a dance party. The joists all failed at the large notches at the ends.