Tension Chord in Three-Sided Diaphragm

[omoreno80]

I am workng on an addition that has three walls being added to an existing structure. Do the layout of the roof framing plan I am not able to tie in the addition to the existing structure. I looked at the building as a three-sided diaphragm via a Vulcraft publication that I have. My question is, can I use a bar joist to develop the diaphragm tension/compression load for the open end? I have put a call into Vulcraft but I have not heard back yet. I am exploring this as an option.

 

[DaveAtkins]

You must be referring to lateral loads acting perpendicular to the open side of the diaphragm, since for loads parallel there isn't a chord.

The answer to your question is yes. If the chord force is significant, you should put a note on the drawings instructing the joist supplier to design for the force. But I have found the chord force due to D + 0.75L + 0.75W (or 0.75*0.7E) is usually less than the chord force due to D + L.

DaveAtkins

 

[Lion06]

How do you get a chord force in a diaphragm for D+L?

 

[DaveAtkins]

I can see how my response is confusing.

In the first paragraph of my response, I was referring to the diaphragm chord.

In the second paragraph of my response, I was referring to the top chord of the joist which is acting as the diaphragm chord.

DaveAtkins

 

[vcomguy]

I can see your point about the forces being less, but the D+L will put a compression force in the top joist chord and the lateral load would either add a tension force, reducing the D+L compression force or add a compression force, depending on which direction the lateral force is applied. Correct?

Would also think that the D only due to gravity should be checked with the two lateral load conditions.

 

[DaveAtkins]

Correct. But I still think you need to consider roof live load or snow load in combination with lateral load--check the applicable Code to be sure.

DaveAtkins