MultiSpan Built Up Beam with Overlaps 1

[4thorns]

Not sure how to approach this. It's not 3 simple spans nor does it appear to be a multispan built up beam (Wood 2x). I'm trying to determine the loads on the posts supporting the beam. I can determine the loads based on the 3 simple spans and if the beam is continuous end to end but not sure how to go about a beam with splices over the supports as shown. Can anyone point me in the right direction to accomplish this.

 

[woodman88]

What you have is a continuous beam with one ply needing to be able to resist the moment over the post by itself. Why not splice both over the same post?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[4thorns]

Thanks for the reply Garth. This type of construction is common in residential buildings. I've seen guys build a 30' long, 4 ply 2x12 floor beam with staggered splices right on the basement floor and then lift the whole thing into place. The splices are always (I hope) over the support posts.

My intent is to determine the difference in reactions (load transferred to footings) of these two scenarios. For example, at Post 2 (see attached).

Thanks again.

 

[msquared48]

Assuming the beasms are nailed together adequately, and since they both have to deflect the same at any point along the beam assembly, the beam section that is continuous over the column will take more load than the other due to local continuity.

You could look at one beam system, determine the reactions from that, then reverse the reactions and add the two beam systems together. Or you could computer model it equalizing the deflections...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[woodman88]

When you stagger the splices you have a single beam system and to get the correct reactions (assuming you are going to ignore the changes in reactions due to foundation settlement) is to analysis it as a continuous beam and check that the single (or double members in a four ply beam) member works for the moment and shear at the splice. If you splice it over the post than shear is not a problem.
I am not sure how close to a correct answer you would get doing a double analysis and adding them together.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[4thorns]

Thanks guys. I understand the concept of the continuous member taking the brunt of the moment and shear over the post. To give you more of an idea of exactly what I'm doing I've attached another drawing. It shows the 3 forces acting on support posts in the basement.(Dead loads only). The loads are determined using simple beams. Generally, as built, they will not all be simple beams. I can dictate where the splices are in the design phase and by doing so strategically place heavier loads or distribute them as even as possible. Just need to make sure I understand how to determine the loads in a staggered splice situation.

 

[4thorns]

OK, I feel like and idiot! It took a few for it to sink in. I was so fixed on the splices that I didn't realize that the overlap, if secured properly, turns the two pieces into 1 single beam. (I know Garth...You told me that it was a single beam! Just didn't register)If this epiphany is wrong please let me know but it makes perfect sense.

Thanks again.

 

[JAE]

One thing to remember here is that you aren't exactly accurate if you assume a pure continuous beam.

The splice at the support does affect the through-stiffness of the combined member so in effect, the splice at a support affects the distribution of moments and deflections along the span.

Think of it this way - say you have a beam made up of 10 2x members. Suppose ALL of them are continuous. You would definitely have a continuous beam and could analyze that as such.

Now assume ALL of the members are spliced at the support. Now you have nothing more than a simple span.

Now assume that you have SOME of the members spliced at the support. You now have something in between fully continuous and simple span.

The comments above are correct in that you have to assume only the through-members over the support in strength calculations.

But the number of splices, and positions of splices, create little weaker segments that affect the distribution of bending moments.

 

[4thorns]

Understood JAE. In this situation I would (as Garth said above) analyze the continuous members for moment and shear with the full load and the analyze the whole beam as continuous to find the loads transferred to the footings. As long as the continuous members can handle the moment and shear, and the overlap connections are adequate, wouldn't this give somewhat conservative loads on the columns/footings?

 

[msquared48]

It would be conservative for the negative moments and shears, but not for the positive moment. I would take the SS moment for that value to be conservative without running a full analysis.

Summing the two systems would work if the two beamlines are not stitched together. However, if they are stitched, then an analysis based on equal deflections would have to be run to get the precise reactions.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[woodman88]

You will never get “precise reactions” with non-simple span wood members. Wood is graded to the 5th percentile of the grade. The actual values of each piece varies within the grade and also along the length of each member. Than, if you add the difference in settlements of each bearing into it, the numbers change again. With wood you just want to get close to what would be the result would be. One way I have (conservatively) used is to use half of the end spans for the end reactions and two thirds of each adjacent span at the interior reactions.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[shobroco]

I much prefer splices at the inflection point of a continous beam rather than over the post; splices transfer no moment so it's the logical place for them. If your splices are over the posts you have to assume simple spans, you're not taking advantage of the continuous action. The negative moment is frequently higher than the positive moment so one continuous ply over a post does nothing for you. Obviously it depends on length of adjacent spans and loading but without analysis, rule of thumb put them at 1/4 point of the span and no more than 1 splice at 1 location.