Reinforcing on Concrete Blocks Walls?
Reinforcing on Concrete Blocks Walls?
(OP)
Can anyone tell me, if there is really any structural advantage in placing 1 #4, vertical reinforcing steel rod in the center of every hole in a concrete block wall (6 or 8 inches), rather than making rigid frames in the wall, columns and beams.
Placing steel in the neutral axis of the wall will not help take the tension efforts generated by horizontal outside forces.
Placing steel in the neutral axis of the wall will not help take the tension efforts generated by horizontal outside forces.






RE: Reinforcing on Concrete Blocks Walls?
RE: Reinforcing on Concrete Blocks Walls?
Regards,
RE: Reinforcing on Concrete Blocks Walls?
RE: Reinforcing on Concrete Blocks Walls?
Umm, in-plane forces are handled by the cmu itself (and grout) unless the shear stress is too high. Then you add some horizontal bars in addition to the vertical bars or increase the area of the wall by grouting more cells. You put reinforcing at each end of a shearwall for the forces at the end of the shear wall. Again, typical wall rebar is not used and most of the time isn't even necessary for in-plane shear loading of a shearwall. The shear stress of the cmu handles it. Granted, you can reduce the spacing of the rebar to reduce the stress. The rebar does not contribute to the in-plane shear strength but rather the reduced spacing of grouted cells increases the cmu area, reducing the shear stress. Instead of reducing the spacing of vertical bars, you could keep it instead and just grout the wall solid. This will give you the same increased area without adding bars.
Joint reinforcing is exactly that: for the joint. It doesn't help the vertical rebar handle the out-of-plane load. It helps the joint and the mortar in the joint transmit the force to the rebar, and mostly helps to control cracking in the bed joint.
Vertical rebars are for controlling tensile forces, not simply crack control in most cases, especially in non-load bearing exterior walls that have wind or seismic forces and no axial loads to help reduce the tensile stress.
Wall ties? How do wall ties help reinforce the cmu? Wall ties hold veneer such as brick to the cmu, they don't reinforce the wall.
blake989, I disagree with the majority of your post for these reasons. Perhaps you live in a completely different area where things are done different but nevertheless I don't see rational engineering behind your statements. I have been wrong before however and will be the first to admit it when I am, and apologize.
RE: Reinforcing on Concrete Blocks Walls?
Most walls are designed spanning to the floor slab and roof decking in a simply supported design. Alternatively, the wall can be designed as a cantilever from the foundation below.
RE: Reinforcing on Concrete Blocks Walls?
Is it a basement wall, a crawlspace wall, a one-story house, etc, etc?
The following is also more flexible than a CMU wall, but I've seen people completely bypass their CMU walls and build wood shear walls adjacent to them.
Good luck!
RE: Reinforcing on Concrete Blocks Walls?
However, I interpreted Oson's post as a question of is there any other methods for designing CMU walls. When floor to floor heights are large, MOST engineers will support their CMU wall between pilasters or cross walls within the building. As I was taught, typical flexural design of CMU walls is either verical one-way flexural behavior (which UcfSE describes and obviously only designs for), or horizontal one-way flexural behavior. With horizontal one-way flexural behavior, "Horizontal reinforcement may be steel reinforcing bars embedded in bond beams and/or joint reinforcement. Numerous studies have shown that joint reinforcement can be used as the principal steel to provide the bending resistance for laterally loaded walls." (Masonry Structures Behavior and Design - Drysdale, Hamid, and Baker 1994)
If you read my reply Oson, I tried to be very specific that the design I described was based upon specific support conditions. I hope this will seem a little more rational.
RE: Reinforcing on Concrete Blocks Walls?
RE: Reinforcing on Concrete Blocks Walls?
-Mike
RE: Reinforcing on Concrete Blocks Walls?
I have built many CB walls leaving the hole empty, but inside a rigid frame.
Now I am working in Jamaica and I am required to build walls, filling every CB hole with concrete and placing a #4 rebar in the center, I find this a very expensive and almost useless practice.
I must add that concrete blocks they use here are very weak and that if the holes were not filled with concrete it would be very easy to break the wall from the outside. I can also say that the mortar used in the joints is so poor that the steel helps a lot to keep the blocks together, and probably is also helping to the compression efforts.
I tried to make an analysis of the reinforced wall and I find that I can not analyse it as a beam, nor as a slab, nor as a cantilever, and found out that the holes are not completely filled with pure concrete because it is mixed with the poor mortar used in the joint that falls inside the hole, the concrete can not be vibrated, the concrete around the steel have to be considered just covering concrete, and the bonding length require for the steel is not the one required by the codes, there is no horizontal steel to take the shear efforts.
Analysing a diagram of efforts that takes place in the crossection of the wall I find that the compression forces acting on the wall seems to be enough to resist Sismic (zone 1) or wind loads.
Most opf all I wanted to hear the opinion of qualified persons in this matter, and I have done it, thanks again.
RE: Reinforcing on Concrete Blocks Walls?