Wood related question, Built-up beam floor support

[woodman1967]

I had a discussion with a manufactured housing company on how they deal with their built-up beam calcs. Their typical home has a rectangular footprint and has a built up beam down the center supporting floor joists...pretty standard. This beam is continuous over several spans, again, pretty standard.

The discussion we had was about how the loadings for the jackposts were calculated. Most building officials here now require the jackpost loadings during the permitting process. The way this company calcs the jackpost loading is to break the beam into each individual span. So, if the beam is 32' long with 4 spans of 8' they would calc it out as 4 separate 8' simply supported beams. Then they would add together the reactions to get the loading at a particular jackpost.

The other companies I've worked with treat the beam as a continuous multi-span beam to calculate the jackpost loadings. How far off from the actual loadings would the first company be, they are determined that their calcs are fine. I really don't know but was told that the loading on one span can affect the loading on another span, and even supports...for example a long span next to a short span can introduce uplift loads at the opposite end of the short span, these loads would not be evident in the series of single span beams.

Does this make sense? Just looking for some advice here, thanks for your time.

 

[jayrod12]

Generally speaking, a continuous beam of more than 3 or more spans (minimum 4 supports) will have 10% more than the simply supported case. Realistically, I'd be ok with their methods. Rarely does those numbers work out such that you're at 99% of the capacity of the post, or the footing.

 

[KootK]

I'd definitely keep an eye on this for beams with fewer numbers of spans. Consider:

1) For two equal spans with uniform loading, the simplified procedure underestimates the post load by 0.625/0.5 = 1.25. So 25%.

2) For unequal spans, things will only get worse compared to #1, potetnially much worse.

I've got a nifty mathcad sheet that I made to study this back when I was green. It's ancient V15.0 though. I'll see if I can resolve that and post it here.