Wall stud analysis on a 3 story wood frame building

[VBI]

I am reviewing a 3 story wood building (platform framing) with 6' foot window headers. My reactions at the end of these headers are R3 (at the top) = 348#, R2 = 3520# (including the 348 above) and R1 (1st floor window) = 6692# (which includes the 3520 above).

R3 is low because it is only supporting the ceiling load while R2 and R1 carry floor loading.

2 2x4 Kings Studs carry a 2414# roof load while
2 2x4 Jack Studs carry R1, R2, and R3.

My information is that the capacity of the 2 jack studs would be 2200#x2=4400# which is LESS than my R1 reaction at the first floor header.

My question is that this appears to be underdesigned. Am I missing something? The load through the Jack studs should be seperated from the loads through the King stud. I did not design the building but am reviewing and believe that this may be "underdesigned" for these header reactions. I thank you in advance for any insight one may have regarding this analysis.

 

[COEngineeer]

jack studs are trimmers right? You just have to make sure that they put 2 studs above the header to continue the load path of the jack studs. That means you have 4 studs (2 king and 2 jack) working together to take the gravity load, then you will have 2 studs (king) to take the lateral.

In real life, there are plenty of time where they forget to put the blocking above the header. That is why some people design the king stud to take gravity load so blocking is not needed. I hope I am making sense.

Never, but never question engineer's judgement

 

[RVSWA]

I concur with COEngineeer regarding the continuous load path above the trimmers. In the field I see it constructed both ways (with and without); unless it is detailed or specified, I wouldn't assume a continuous load path through the trimmer. One challenge 2x4's have under this load is the out-of-plane buckling when one considers the wind load case. You didn't really say how you got your "information", a column calculation followed by a plate crushing check will give you a binary answer. Red-line the plans to require a continuous load path if you find the calculations prove out.
A testing lab rat once told me "One good test is worth a thousand expert opinions". I believe with a small twist, "One good calculation is worth a thousand expert opinions".

 

[VBI]

Yes, I guess I mean Trimmers when I say "jack studs." They are typically not blocked behind the rim board, though the trimmers do continue up. (The windows are lined up over eachother on the 3 floors). I thought the trimmers act independent of the king studs in supporting the gravity loads. I guess I am wrong? Thank you for your input.

 

[msquared48]

Personally, I think that they should be termed "Jack" and Jill" studs.

Mike McCann
MMC Engineering

 

[FSS]

I can't imagine how you can get 2x4 studs to work for the first floor of a 3-story building.

 

[VBI]

Is the problems with 2x4 construction on a 3-story building transferring the gravity loads or more of an issue with wind loads?

My original question was...
Can I combine the capacity of the 2 jack studs with the 2 king studs to carry the reaction of the first floor 6' header. The 4 studs would work but the 2 jack studs alone would be insufficient to support the 6,692# reaction load at this location.

 

[RVSWA]

In this case, wind load and gravity loads are not independent of one another, to say one is more of an issue than the other is academic. In your design, you need to consider all of the load combinations for that member. After that, you need to consider how the load gets there and where it is going. If you are a simply reviewing the plans and are unable to independently verifying this, then you should ask the EOR to provide you with calculations. You can always call them on the phone and save the time delay of writing a letter.

 

[VBI]

OK...but nobody weighed in on if the 2 jack studs and 2 king studs work together at the bottom to support the gravity reaction load of a 6' header on the first floor.

The header is supported directly by the 2 jack studs, but should the 2 kings studs be considered as well in determining the support capacity.

 

[COEngineeer]

If you have enough bearing then you can theoratically transfer the load through the nailing on to king studs. There are 2 ways (or more) to do this.

1. You design the king studs only taking wind load, thats it. Then you design trimmer to take the gravity. If you have 3 windows stacking, more than likely your header on the first floor will be longer (because you use more trimmers).

2. You design the trimmer only to take the load from 1 floor above (doesnt see the load from 2 floor above). Then you design the king studs to take all the gravity load from 2 floor above and higher plus wind. This is a little complicated because you ahve to do compression and bending interaction equation.

I think if your windows are not stacked perfectly, then use method 1. If you have windows same size directly one another, I do method 2. I hope I am making any sense.

Never, but never question engineer's judgement

 

[Robert216]

Why couldn't you design the jack studs to handle gravity and king studs to handle lateral? Yes, they will act together since they will be nailed together, but if each "system" is design to handle their respective load, the only failure mechanism could be shear perp. to grain in the nails/connected members that transfer the lateral load from the window, through the jacks and into the kings. Also, I have struggled with a requirement in the NDS that limits built-up columns with nailed attachment to 3 members. Here, I treat the jacks as built up columns and the king as a verical "beam" since it technically is not required to work in the gravity system. This is the only explanation that I could rationalize the fact that I have NEVER seen bolted jamb studs on a project. Any thoughts? Thanks, Rob

 

[VBI]

I did not design the building but am reviewing ...essentially I am trying to justify the engineering of the building. There are only two jack stud under the 6' header on the first floor which would not be adaquate to handle a 6692# gravity load so I must assume that the designer intended the 2 Jack studs and 2 King studs to work together to support this high first floor reaction.

You stated...
I have struggled with a requirement in the NDS that limits built-up columns with nailed attachment to 3 members.

I did not know this...so essentially the capacity of the studs is limited to 3 members nailed together 6,600# +/-

You stated...
I treat the jacks as built up columns and the king as a verical "beam" since it technically is not required to work in the gravity system

They must work together to resist the gravity/reaction at the 6' header to justify the deisgn.

THANK YOU ALL FOR YOUR THOUGHT OUT INPUT

 

[COEngineeer]

2-2x6 DF trimmer can take 6692 # assuming you have 10 ft floor to floor.

Never, but never question engineer's judgement

 

[VBI]

Yes...but the studs are 2x4's. 2 trimmer studs cannot take this load.

I do not design wood structures but am analyzing this 3 story wood structure and could see that it probably should have been constructed of 2x6 instead of the 2x4's.

 

[COEngineeer]

well.. check the walls first. If that works, then you should analyze the kings and jacks working together. But I agree, 2x4 is too small for 3 story wood structure.

Never, but never question engineer's judgement