Masonry Shear Wall Flanges
Masonry Shear Wall Flanges
(OP)
I'm doing low-rise masonry shear walls in seismic design category D. There are several piers in each line of resistance, and some of them have flanges due to intersecting walls. What's the industry doing out there to account for the flanges? Do you place joints to isolate intersecting walls from each other to simplify the analysis? The strain-compatibility analysis to determine the wall's flexural strength and level of ductility is intensive. The code requires flanges be considered, since they drive up the flexural strength, which is what the required shear strength is based on. Maybe I'm wrong, but I get the impression most engineers ignore them, which is unconservative.






RE: Masonry Shear Wall Flanges
You'll have to count the rebar in the effective flange width for tension but fortunately compression flange typically doesn't move neutral axis around much. Effective flange for tension is based on wall height, so fortunately shouldn't have a huge effect for low-rise assuming your walls aren't packed with reinforcement (this could be why you get the impression most engineers ignore them).
NEHRP has a guide that provides some guidance for this: Link (PDF)
Excerpt:
Most reinforced masonry codes and design guides provide rules for the design of simple, planar wall elements as in Figure 2-2(a), but in practice such walls can be part of complex structural elements and systems that affect their behavior, as illustrated in Figures 2-2(b) through 2-2(f). The designer can choose to design and detail such walls to have an integral cross section, with the wall segment aligned parallel to the lateral shear force acting as web and the perpendicular wall segments acting as tension or compression flanges as in Figure 2-2(g). Alternatively, the designer can choose to treat groups of intersecting walls as individual planar elements, provided that they are sufficiently separated so that shear cannot be transferred between them either through the masonry or through stiff horizontal diaphragms. Depending on the nature of the diaphragm, small gaps between wall segments as in Figure 2-2(h) may not be sufficient to decouple the walls, and some separation may be required as in Figure 2-2(i).
RE: Masonry Shear Wall Flanges
ASD applies a 50% increase to all in-plane shears due to seismic in special walls to try and mimic this, but there's no requirement to develop the flexural capacity of the wall.
Most engineers still do ASD, so there's another reason it may seem like the flanges are getting ignored.
Code section is 1.18.3.2.6.1 in ACI 530-11.
RE: Masonry Shear Wall Flanges
Even for ASD, you still have to make sure you do not exceed the balanced reinforcement ratio for a special shear wall. All those extra bars in a flange can add up and push you over the edge into a masonry crushing failure mode. Add all the extra shear ties you want, and a 50% increase to the shear load, but if you're trying to get a plastic hinge to form you can't exceed the balanced ratio. Otherwise the wall will fail in a non-ductile flexural mode, and it will behave more like an ordinary shear wall.
The unbalanced ratio is a nightmare to calculate. It's influenced by bar arrangement, axial load, and flanges, and it's an iterative calculation. I think most engineers are just saying "that's too complicated" and skipping the crucial part of the design that controls the ductility. The common practice I've seen is that engineers add "hold-down" reinforcement at the ends of the walls, but to me this just makes the wall less ductile by potentially over-reinforcing it for flexure. Beyond the first bar placed, every additional vertical bar in the wall makes it less ductile (even though it adds capacity) by decreasing the strain in the extreme tension steel, especially bars added near the ends.
There will also be a stiffness change to the piers with flanges. Flanges increase the moment of inertia very quickly. Any piers with flanges will draw more load to them.