Continuous Chords - Deck Angles

[preynolds]

I need a sanity check.

Situation presents itself in several construction types, particularly with load bearing precast wall panels and masonry load bearing walls.

For the joist bearing side, I am of the opinion that a continuous angle must be present to transfer diaphragm forces from the deck into the joist with the joist taking the force into the walls (provided joist rollover is sufficient). When examining older buildings, we often see no continuous element of that described.

For the deck bearing side, there will typically be a continuous angle for bearing, thus you have your continuous chord element.

Am I correct in the direction? I just don't see how one can use the deck itself as the boundary element on a diaphragm.

In the existing building condition, when you see an angle is not present, what do you do?

 

[JAE]

I would agree that an edge angle is preferred.

Without an angle present, the lateral force in the diaphragm will have to be pulled through the deck-joist fasteners, essentially dragging the joists laterally and using the top chord as a lever in which to drag the forces from the deck into the walls/frame below the joists.

This creates a challenge with analyzing whether this is safe or not as the relative stiffness of the deck and joists determine how the shear forces vary from the outside fasteners back inward along each joist top chord. Essentially I see some kind of shear lag in the fasteners with the highest shear in the fastener closest to the end of the joist, with diminishing forces applied to fasteners as you extend inward. How quickly those forces diminish, and what the maximum force at the end fastener is would be tough to quantify.

 

[jdgengineer]

Hey preynolds, a sketch may be helpful to convey your point.

 

[msquared48]

If the edge angle is not present, what is? Something must transfer the shear to the sidewall.

Seems like this could be a problem area for roofing leaks too.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[preynolds]

all of you are providing the check for my sanity. we are all on the same page.

the reason for my bringing the question up was a precast supplier asked why a deck angle was needed if joists are bearing on a W10 hung from plates off wall panels. From the way he made it sound, they aren't used to seeing the angle to which I replied, I have no idea why the angle would be missing in the past jobs. Since he brought it up, I thought it was a good time to revisit and make sure I was sane, or at least sane related to this question.