The other checks like shear, moments and deflection are all impacted by the size of the beam (combined with other properties).
The bearing check usually doesn’t impact the size of the beam per say, even though a beam with a greater width will have a larger bearing area. More importantly it is usually checking that the supports are providing enough “bearing length” and hence bearing area. If the pressure at the support(s) is too high the wood fibers will crush and cause issues with settling of the structure, cracking drywall etc… If the bearing check fails it does not mean that the beam itself will fail catastrophically, and is usually not a matter of life safety, so it is less critical than the other checks.
The idea behind the Cb factor is that when a heavy point load is applied to a wood beam it will slightly crush the top most fibers and cause an indentation, which in turn will engage additional wood fibers to resist the load on both sides of the applied load. That is why the factor cannot be applied at the ends of beams (x < 3"). The other rule is that the bearing length must be less than 6" inches, so larger point loads cannot use this factor either (not entirely sure on the rationale behind that one).
The other issue I am a bit unclear on is the unbraced length (lu) especially in the case of checking negative moments in multi-span situations (unbraced bottom). I’ve checked a number of examples in Donald Breyer’s book “Design of Wood Structures”. Rather than considering the lu as the actual span he is calculating the lu as the distance between the points of zero moment. I could use a bit of clarification on this. Section 3.3.3.4 of the NDS (page 17) only talks about the distance between points of intermediate lateral support.
After giving this some more thought and digging through the NDS a bit more I think the reason that Breyer makes this assumption is that the language in the NDS for computing the Cv (volume factor) does say the distance between points of zero moments. He then seems to extends this idea to computing the CL by using the same logic to determine the unbraced length (on both sides of a support). See example 6.28 in chapter 6.16.
My only problem with this is that it would seem like it would be unconservative in many cases with multi-span beams where you are computing the CL for negative moments (at supports). However by using the full intermediate span length as the unbraced length perhaps it is too conservative. I wish the NDS would give more guidance on this matter, I can only guess at the intent and supposed correct algorithm at this point.
Let’s consider the example shown in the image below:
If we consider that there is no lateral bracing at the intermediate support at 84" (bottom of beam) then per Breyer’s method the unbraced length is between points of zero moment (x=67" to x=108"), so the unbraced length for the negative bending (neg. moment) is equal to 41". However I would argue that it is the full beam length, both spans, so 144".
If we do consider that the beam is laterally braced (bottom of the beam) at the intermediate support at x = 84" then Breyer considers the worse case of the two conditions 84 - 67 = 17" and 108 - 84 = 24" and he concludes that the unbraced length should be 24". I would look at both spans on each side of the support or max. negative moment and take the larger of the two 84" > 60", so the unbraced length should be 84".
Thoughts? Am I too conservative?
On a slightly different note I would use 41" length to compute my Cv for the negative bending (for both cases given above). This is per the NDS verbage (Sec. 5.3.6).
I have to say I find this thread absolutely fascinating, and it’s so interesting reading your explanations of how all the calculations work. I studied structural engineering at a very basic level almost 50 years ago so a few of the terms you use ring a very small bell. I realise your plugin is aimed at timber structures but do you think any of the principles would carry over to glass structures?
I am not familiar enough with glass structures or engineering to say much about them. Without a doubt they probably have their own engineering standards and equations (ASTM E1300). I’m mostly familiar with wood, steel, reinforced concrete and aluminum structures. The IBC does cover some general requirements like deflection.
I try to put as much detail and information in this thread as I can so that my development path is transparent. I also like to show how the “engineering” sausage is made. I think it helps people realize all that goes into developing tools like these.
Switching between the load cases, and watching the graphical representation move reminds me of the days spent designing post-tensioned concrete, and my mom’s 2 span cloths line in the back yard. Oh what memories.
Similar to RISA 2D and RISA 3D I think it would be nice to show the actual deflected shape of the beam superimposed on the real beam, as well as a simple tool to exaggerate this deflection. Here is an example of the “passed” shaded deflected beam (deflection scale 10X) .
I think this is a great idea, especially for those that are not used to looking at moment, shear, and deflection diagrams. It will be important to make the viewer/user aware of the magnified deflection scale.
