Beams stacked...how to calculate if sufficient size

[jimtheengineer10]

See the attached picture.

A client is building a 2nd story where he will be able to have chairs/tables for pancake breakfasts. I am trying to determine what kind of load the existing beams can support. He installed a 2x10 with another one directly below it. He will be installing a 2x6 around the column so that will provide bearing underneath the stacked 2x10's. The tributary width is about 7.5'. I typically use the forte software for sizing beams/joists but I don't think this type of construction is an option with that software.

 

[SlideRuleEra]

The worse case is the stacked 2x10s will have non-composite action. Each 2x10 acts independently to share the load. In other words, the section modulus of the two, stacked 2x10s is twice the section modulus of one, 2x10. The moment of inertia of two, stacked 2x10s is twice the moment of inertia of one, 2x10.

See this link, it concerns steel, but the principle is the same with wood: Link

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[dhengr]

Jimtheengineer10:
The two stacked beams on either side of the column act as individual beam members, they are not tied together in such a way as to make them act as a stronger single member. They deflect together and take loads in proportion to their relative bending stiffness, EI[sub]top[/sub] and EI[sub]bot[/sub].

 

[Triangled]

yes with above, and, on first glance i don't see any compression face lateral support to lower member which significantly lowers its capacity

 

[cal91]

The loading will depend on the diameter of the pancakes. Also, what type of pancake batter are you using? I'd recommend Aunt Jemima's. Will there be sausages/eggs involved as well?

 

[cal91]

But seriously along with what Triangled stated, make sure that the bottom member is torsionally stable so it won't flip onto its side.

 

[BAretired]

If you require a larger bending strength than the sum of the upper and lower beams, you could investigate the possibility of fastening them together to resist horizontal shear, thereby developing the full height of both beams.

BA

 

[dvd]

That is a lot of load resting on a piece of 2x6 (yellow pine?).

 

[jimtheengineer10]

There will be 2x6 installed on the face of the 6x6 so there will be a 2x6 directly underneath the edges of the 2x6 sticking out.

Can anyone provide with me a little more detail on how to perform the calculation and determine if the beam can handle the load I mentioned above?

 

[KootK]

Basically, you follow SlideRuleEra's advice. For example, for the top beam, Load = Total Load x (EI_top / (EI_top + EI_bot)). Once you have the load to each 2x10, you'll check its capacity taking care to properly account for lateral torsional bracing etc per Triangled's comments. With regard to the stiffness ratios, ensure that the two 2x10's are spliced at the same locations. If not, you may need noodle on it a bit harder.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[allgoodnamestaken]

Deflection in the upper beam will induce load on the 2nd beam, based on the zero spacing I see.

 

[oldestguy]

There may be a risk of buckling. Take design of a plate girder as an example.

 

[Triangled]

are your 2x10s continuous over the column? i can't make out a joint. if so, needs compression face support over post region also. it's a weird situation and i can only guess that the client changed mind on floor elevation after the lower 2xs already installed. the lower one is problematic due to buckling modes. if you're not crystal clear on moments and stresses etcetera, i'd suggest you consider beefing up the lower member sufficiently to support the whole thing by itself and brace it appropriately.

 

[rb1957]

The upper beam will bear up against the lower one, right? Maybe a sine distributed load ?

To determine how much load is transferred to the lower beam, match the beam deflections. The upper beam has the point load applied and the reactions are along the beam (pressure to the lower beam) and point loads at the ends. The lower beam is loaded along the span (sine wave ?) by the upper beam, and reacts this load at the ends.

another day in paradise, or is paradise one day closer ?