1974 Wood Engineering Handbook (MOR)

[BadgerPE]

I am trying to determine what the existing capacity of an wood-framed stage built around 1980 is. I have the 1974 version of the Wood Engineering Handbook and have found the values given for eastern pine which is the assumed material based upon existing plans. The chart lists the MOR as 4,900-8,600 psi. Does anyone know what the typical FoS would be to reduce the MOR to allowable bending stress. My first thought is 3, but I'm looking for some verification of that. It should be noted that this stage is most likely going to be replaced due to serviceability concerns anyways, but I would like to get a good approximation of what the original strength is.

 

[msquared48]

Why don't you check either the 1976 or 1979 UBC values?

Mike McCann, PE, SE (WA)

 

[dhengr]

BadgerPE:
Most of the values in those handbooks, such as your MOR, are based on the average of many clear (essentially perfect) small samples of that particular wood species. They are of interest in evaluating and understanding the relative strengths, mechanical properties, etc. of the different wood species. They are not of much value in determining design values, because of the variability of each piece of lumber and the way grading rules play such a large part in determining design values for different applications. You might look at the grading rules, and how they have been developed, and see if they given/show any relationship with actual design values set by grading and the above more theoretical (perfect sample) values shown in the handbook. Rather than using the tabulated MOR and some FoS to arrive at some estimated design value, I would make some judgement about the grading rules and actual grade of the lumber you are considering; then see if that lumber and grade is shown in the NDS species listings of various design values. Or, pick a similar species of lumber which is listed, read what the handbooks say, in particular, about the two species and adjust its tabulated design values by the ratio of the two different MOR’s, etc. Note also, that different theoretical mech. prop. values (perfect sample props.) affect different design values in different ways. i.e.... relative MOR values may not have much to do with design shear values or compression perpendicular to the grain values. This is really a subjective exercise, subject to considerable engineering experience and judgement, but not an unreasonable use of engineering judgement. There are also on-site grading services which will make some judgement of the grade and species in place, in the structure. You can also cut samples out of the existing lumber and have them lab tested.

 

[BadgerPE]

Good thought Mike. Wisconsin wasn't using the UBC then, but the UBC probably references the NDS like the Wisconsin Administrative Code did. Thanks!

I was just reading some similar thoughts dh....