measuring eccentricity on a ledger

[a361183]

I am reviewing a not quite typical ledger masonry wall connection. The masonry wall has an angle bolted to it, with anchors at 24" on center. The vertical angle leg sits flush with the face of the 10" masonry wall. The horizontal leg supports steel joists. In designing the masonry wall, I have always measured eccentricity from the center of the wall to the bearing point of the joists. The connection I am reviewing implies that the angle internally resolves the eccentricity from the center of bearing to the back of the angle/face of wall. Then the masonry wall only sees the eccentricity from the gravity loads applied at the back face of the angle. Naturally, this reduces the eccentricity on the wall from 7.5" to 5.5", a considerable amount. This original design was performed by a well known company, so before I call them out, I was hoping for some help finding code references.
I've looked through several and only find 'use "e" to apply the moment induced by gravity loads' and no code backup for the measurement of "e".
Thanks for guidance or new places to look.

 

[BAretired]

The eccentricity is from the center of bearing of joist to the center of wall. There may be a code reference somewhere, but it seems to me it is just plain statics.

BA

 

[KootK]

Normally, I'll consider the joist reaction to be triangular which induces even more eccentricity into the wall design.

With properly designed welded or bolted connections between the joists and the ledger angle, one could make the argument that, locally, the ledger angle is really just a moment connected extension of the joists which would resolve the eccentricity internally. That's not normally done however.

This document probably doesn't address your specific question but it certainly deals with common assumptions regarding joist bearing conditions: Link.

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.

 

[Lomarandil]

So their argument is that the load in the angle anchor bolts is only shear at the face of the wall.

However, for the angle to "internally resolve" the eccentricity, it would create forces in and out of the plane of the wall -- bearing at the bottom of the vertical leg, and tension in the anchors, most likely.

Whether this couple is negligible (as their method implicitly assumes) or not would depend on the rest of the structural system.

Sorry, I don't have any code references for you either, but I agree with what you and BA are saying.

 

[a361183]

Thanks very much for your assistance. I did look at what KootK posted. Interestingly, that may yet pose another rabbit hole. The loads for this connection are very light, on the order of several kips per factored load location. I wonder if the force couple is negligible?

 

[bookowski]

You should get the same result either way:
- if you do the load times the wall to bearing, P x e, or otherwise
- P x e1 + M2 where e1 is center to face of wall and M2 is the P x e2 where e2 is the eccentricity from back of angle to bearing point.

Sounds like they aren't resolving the moment from the angle, otherwise it's the same thing.

 

[BAretired]

I don't think we know precisely where the center of joist reaction occurs. It depends on the stiffness of the wall, angle legs and joist. For example, if the joist is very flexible and the angle leg is very stiff, the reaction could be at the tip of the angle.

If the joist shoe is welded to the angle such that the angle is simply an extension of the joist, then the wall will take a portion of the fixed end moment of the joist according to the stiffness of the members.

It is usual to weld the joist nominally to the angle and to consider the reaction at the midpoint of the outstanding leg, but KootK's 2/3 is more conservative. Also, if the joist shoe does not reach all the way to the face of the vertical leg, the eccentricity will be larger.

BA

 

[KootK]

if the joist is very flexible and the angle leg is very stiff, the reaction could be at the tip of the angle.

I've noticed an interesting pattern in the junior engineers that I've worked with. Given a seated connection to design, they'll often come up with a ridiculously thick 3/4" thick angle in attempts to be conservative. Or they'll stiffen the angle unnecessarily. I always preach that my preference for modest loads is thin, un-stiffened seats. When the reaction moves out to the tip of the angle, the parts of the connection that are critical (bolts) get worked 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.

 

[a361183]

Thanks again!