Diaphragm Chord v Expansion Joint 1

[MikeStout]

This is a silly question. Sorry but I feel confused and would be grateful for a clarification.

We install expansion (control) joints at regular intervals to allow a wall to move and therefore prevent cracking. At the same time we put rigid elements such as a diaphragm chord or a bond beam to prevent movement. Is there a contradiction here and how in practice structural engineers resolve it?

 

[SteelPE]

I design expansion joists as a 100% split between two adjacent structures. Each structure acts independently from the other. Each structure has their own lateral force resisting system. I try to get expansion joints in odd shaped buildings (L or U shaped). I definitely use them to be in accordance with The Federal Construction Council Technical Report 65 for long structures (say 700' long x 300' wide).

I believe you you are talking about is a contraction joint. More specifically in a masonry building (where contraction joints should be spaced at 30'-0" o.c. +/-). In these types of joints I have been told that the joint must go all the way to the top of the wall. My old boss never believed in continuing the joint through the bond beam as he wanted to keep the diaphragm chord intact. I still detail this way. However, if I wanted to continue the joint through the bond beam I would run a continuous angle around the perimeter of the building to replace the bond beam as the chord. This is a better but more costly detail.

 

[MikeStout]

Thank you very much.

If understand you correctly, the first option is to leave the bond beam intact which means that we should anticipate some movement between the wall and the bond beam. Is that right?

I am not sure I understood the second option. Can you please rephrase?

 

[SteelPE]

Again, if we are talking about masonry, I believe running the bond beam bars through the contraction joint would lead to a little bit of cracking.

On the second option, we commonly use steel joists spaced at 5'-0" +/- o.c. We would run a continuous angle at the end of each joist for the full length of the building (on top of the joist top chord). The deck can be welded to one leg of the angle. So in effect, you are forming a deep beam with the deck being the web and the angle being the beam flanges.

 

[KootK]

Yeah, it is a bit of an inconsistency. But in many practical structures, some kind of compromise needs to be made between the wall which wants to be broken up over long stretches and the drag struts and chords that, by definition, need to be continuous over long stretches. Using angles is an improvement but even that is not without its compromise. Most chords are also drag struts for a different combination of lateral loads. So you can't usually install them with slotted holes or otherwise allow differential movement between the angles and the walls.

For very long structures, I favor SteelPE's approach of having complete expansion joints with independent lateral systems on either side of those joints. For the contraction joint case, I think that we're implicitly making the argument that we're mostly addressing an aesthetic issue. We want the the cracks to occur at controlled locations and, hopefully, bond beam continuity will not compromise that. At the same time, if we get a modest amount of cracking in the bond beams, that probably doesn't compromise their functioning either.

the bond beam intact which means that we should anticipate some movement between the wall and the bond beam. Is that right?

I don't believe that we should anticipate this and, if it were to come to pass, it would probably represent a real problem structurally. As SteelPE mentioned, what we anticipate for bond beams is a small amount of through depth cracking in them at the locations of the wall joints. For the case of steel angles as chords, I've heard the argument that small amounts of bolt hole slack might allow for the differential movement that you've described. That's a bit dubious in my opinion, as I've described above.

 

[XR250]

I was trained to continue the bond beam bars thru the joints.
I typically end up using the edge angle as the chord however. I typically don't get into buildings that are super long so it likely does not matter for my purposes.