Masonry Shear Wall - Participating Element
Masonry Shear Wall - Participating Element
(OP)
I have a condition where we have 3 north/south masonry shear walls, 2 on each side of a flexible diaphragm, and one in the middle. The one in the middle is short and has openings, therefore is much more flexible than the 2 on each edge. The problem is that due to flexible diaphragm behavior, the middle wall takes a full 1/2 of the tributary width of the diaphragm, but it is the weakest wall. TMS 402 section 1.17.3 states that masonry elements MUST participate in the LFRS unless they are isolated in their own plane. This is a low-seismic building, but has pretty high wind loads due to tall parapets. We are struggling to get the middle wall to work, even as a full-grouted shear wall with horizontal reinforcement. My questions are:
1) Do people generally try to show that the diaphragm meets the criteria of a rigid diaphragm so that the load can be distributed based on stiffness instead of tributary area, even if the diaphragm would be classified as flexible (bare metal roof deck)?
2) Are you aware of any code provisions that allow the wall to be classified as non-participating, even if it is attached to the diaphragm?
3) What manner of isolation is normally used for this condition? It is supporting some gravity loads, so I would think any manner of attaching the diaphragm to the wall would need to be low-friction to prevent transmitting any lateral forces into the wall.
The attached file shows the rough layout of this structure.
Thank you for your thoughts and suggestions.
1) Do people generally try to show that the diaphragm meets the criteria of a rigid diaphragm so that the load can be distributed based on stiffness instead of tributary area, even if the diaphragm would be classified as flexible (bare metal roof deck)?
2) Are you aware of any code provisions that allow the wall to be classified as non-participating, even if it is attached to the diaphragm?
3) What manner of isolation is normally used for this condition? It is supporting some gravity loads, so I would think any manner of attaching the diaphragm to the wall would need to be low-friction to prevent transmitting any lateral forces into the wall.
The attached file shows the rough layout of this structure.
Thank you for your thoughts and suggestions.






RE: Masonry Shear Wall - Participating Element
Mike McCann, PE, SE (WA)
RE: Masonry Shear Wall - Participating Element
RE: Masonry Shear Wall - Participating Element
1) One path you could take would be to capacity design the wall. Determine an over strength flexural capacity for a nominally reinforced middle wall, make certain that a shear failure can't occur prior to flexural hinging, and let the wall hinge at the base. for the sake of any brittle Finishes / windows, it might be prudent to ensure that flexural hinging doesn't happen under service level wind.
2) You can use ASCE provisions to classify the diaphragm as rigid depending on the ratio of diaphragm deflection to VLFRS drift.
3)You could design everything ignoring the middle wall until the end. Then, design the middle wall for the displacement that you expect for the diaphragm where the middle wall ties in. Use a conservative value of the diaphragm deflection and don't forget to include the drift in the two end walls.
I like option #2 the best.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Masonry Shear Wall - Participating Element
One example (which I have not personally used, but have been the reviewing engineer for a bridge authority) is: http://structuralbearingpads.vossengineering.com/i...?
Seemed to work well and be simple to install however, again, I was not directly involved in the specification or install.
RE: Masonry Shear Wall - Participating Element
Earlier this year, I did some renovation work in a large OWSJ industrial building. Down the middle, along a line of steel beams, the designer had an expansion joint installed. It was unlike anything that I'd seen before. Little 1/4" U-shaped bits of steel were welded over top of all the joists seats with maybe 1/4" clear on all sides. It formed a "tunnel" of sorts through which the joists could slip along their own longitudinal axes.
I didn't like the detail much because of the slack in the direction perpendicular to the joists. Short of installing a PTFE pad, however, I thought that the detail would do a pretty convincing job of allowing longitudinal slip. Not so. The joist seats and supporting beams were painted in the field. It was pretty easy to see that, after forty years in service, no movement had occurred at all. Maybe it just never saw any serious thermal strain or diaphragm loads, I'm not sure.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Masonry Shear Wall - Participating Element
RE: Masonry Shear Wall - Participating Element
RE: Masonry Shear Wall - Participating Element
I read the ASCE 7 as stating that the diaphragm can be idealized as rigid if it is concrete or concrete-filled metal deck with a span/depth ratio of 3 or less (with no horizontal irregularities) - section 12.3.1.2 of the 2010. But most of these buildings are bare metal roof decks, so I don't think this applies unless you add concrete to the structure.
The section following is a way to calculate an idealized flexible condition, but I don't read this as saying that if the inequality is not satisfied you get to qualify as a rigid diaphragm. The result of this being that if you cannot idealize the diaphragm as flexible or rigid, you must analyze it using the relative stiffness of both the vertical and horizontal components (semi-rigid condition - as stated in 12.3.1). Does anyone actually do this? Seems like an intensive effort for a small structure.
RE: Masonry Shear Wall - Participating Element
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.
RE: Masonry Shear Wall - Participating Element