What is the difference in weight/load bearing strength between one 2x4 vs. two (or three) fastened together?

Question

I have always been told that two or three statndard building 2x4's are exponentially higher in strength than its stand alone comparison- (the total is greater than sum or it parts) and an answer is usefull not only for building knowelege but as an analogy for teamwork- this is my currenmt interest in this factiod, so I can use it in a presentation that is stressing strenght in numbers. Thank you in advance! (ps. spent 1/2 hour on the web and cannot find)

Answer 1

In reference to a hot tub deck, I did a search a while back and also asked around the local lumber contractors basically this same question.

Basically, what I found was that two studs nailed together were simply twice as strong as one. (They would hold twice the weight with the same deflection.)

However, I found several references that were very interesting concerning the use of three studs. Two studs "sideways" and a center stud - arranged much like an I beam. This arrangement with only three studs was said to be anywhere from 13-14 times as rigid as a single 2x4 and 5-7 times more rigid than twin 2x4s !

There's a good analogy there - it's not just team work that counts, it's how the team works together that counts. A "team" of two 2x4s is twice as rigid as one - but if you add one more 2x4 and join them together, it's up to 7 times more rigid than the joined pair and 14 times more rigid than the single 2x4 !

Answer 2

In building design you would need to know the species of lumber to be used for you need to know the Max bending,Vert+horiz shear to calculate to(E) modulus of elasticity. Every type of board has a different strength. These you would get in the National Wood Products Assoc. manual or Architectural Graphic Standards. I can give you an example of numbers in strength used in the International Residential Bld Code.
Your Exterior bearing wall is 20' and you want to put a picture window in it. You need a beam across the top so the wall and roof weight are not on the window.
2-2x8 can span 6'-10" w/ 1 jack stud under each end
3-2x8 " " 8'-4" " "
Now here's where it changes:
4-2x8 can span 7'-0" w/one jack stud under each end.
That is because the jack studs holding up the beam on each side are also holding up a heavier beam and more roof load.
If I used 4-2x10 I could span 14'-1" . I know what your getting at and hope you can use this but the bigger the lumber the more you can carry but then two minds might be better than one

Answer 3

The question you are asking is a little more complicated than you might think. If you fasten three 2x4's together and orient them vertically, you have basically created a glulam...which is significantly stronger than a single 2x4. The reason for this is that the section modulus would be much higher...like 11 times greater. If you fasten them together and orient them horizontally, they will be stronger than 3 individual 2x4's, but not by much. They would perform better due to redundancy and increased lateral bracing.

Answer 4

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Answer 5

The engineering term for the additive effect described in the answer above is superposition - you can analyze a complex structure by analyzing the individual contributions of each of its component parts.

Answer 6

They just add. You can see this by looking at the deflection caused by a load: you double the spring constant, so can carry twice the load with the same deflection.