Reinforcing on Concrete Blocks Walls? 2

[Oson]

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.

 

[Wasdifferent]

The rebar acts as the extreme fiber on the tension side. The neutral axis is now in-between the rebar and the outside face of the wall.

 

[Lutfi]

If you go back to static and strength of material, you have to use modular ratio between composite materials. Then you need to apply this modular ratio to the sectional properties.

Regards,

 

[blake989]

This is how I design CMU based on how I support the wall, support at corners or perpendicular interior walls. For in-plane horizontal loads I would provide the vertical bars. However, you may be refering to out-of plane horizontal loading (wind or seismic loads). If this is the case, you may consider providing lateral reinforcement i.e. lathes or wall ties in the bed joints for the flexural load. If this is not a shear wall or load bearing wall, the spacing of the vertical bars may be reduced to an amount to control cracking. This may vary dependent upon your specific support locations, you may be trying to support at roof and foundation?

 

[UcfSE]

blake989:

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.

 

[BFPartners]

The wall design is based on the bar in the middle of the grouted core since the positioning will be unknown. Think conservative.

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.

 

[seattlemike]

Rigid frames are expensive, and very flexible relative to CMU wall. so, your rigid frame would have to be even more stiff (and expensive) to prevent damage to your unreinforced CMU wall. You might want to further explain the scope of your project.
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!

 

[blake989]

Whew, easy UcfSE. You are right, increasing the number of grouted cells does increase the mass of the wall. And you are right again, what I meant to say was ",the NUMBER of vertical bars may be reduced to an amount to control cracking." And I did incorrectly use the term wall tie, I should have said masonry anchors, you know the flat steel pieces used in the bed joints to tie the walls in question to pilasters and cmu cross walls to keep the wall from falling out of the building.

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.

 

[UcfSE]

That's pretty interesting. I don't just design for vertical rebar. I meet what the client wants. Most of the architectural firms I work with in my area don't want pilasters, or cast-in-place concrete, and there aren't a great deal of cross walls to use for support. I do design masonry to span horizontally when it's beneficial and I can actually make it happen with support conditions and not have an architect or contractor screaming about it. I can see where you could use joint reinforcing in theory but I disagree that it's a good idea to do so. Many times the joint reinforcing is left out or not detailed and installed well enough for me to feel safe using it for the primary reinforcement. That's why I use horizontal bond beams when I need to span the reinforcement horizontally instead of vertically. In my point of view, I would say it works on paper but I don't think it's a good idea for practice. That's my opinion though. Anywho I read my post again and I came across harsher than I intended. I do apologize for that.

 

[Wasdifferent]

UcfSE Makes a good point when he says "Many times the joint reinforcing is left out or...". A good 'rule of thumb' is: If it's made out of steel, and you didn't see it PROPERLY INSTALLED IN PLACE DURING THE POUR WITH YOUR OWN EYES, don't count on it making any contribution what-so-ever to the strength of the structure. Don't even count on it being in the same county as the foundation.
-Mike

 

[Oson]

Many thanks to all of you every post has been very interesting and I have learned from everyone of them.
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.

 

[Grizzman]

Your response sounds like all the more reason to reinforce every cell.(Poor Quality Control) Plus, you are in Jamaica. They get some pretty substantial hurricaines down there.