Reinforced Masonry Wall Questions

[PT999]

For a reinforced masonry wall, when lapping reinforcing bars, do the vertical bars have to be tied with tie wire or can they be inserted into the grouted cells (without tie wire) as the work progresses? Are there insert devices that work with hollow masonry units, that make this easier than tie wire.

Are lapping length requirements for bars the same as for reinforced concrete? (Working Stress Design)

For a load bearing multi-level building, how long do you need to allow for curing of the masonry wall before you advance to the next floor level. I am recommending 7 days. If you tested mortar samples, what percent of full strength would you want to get before applying load?

 

[jheidt2543]

Lapped reinforcing bars in masonry walls DO NOT have to be tied, they can be just inserted into the grout. In fact, it is condidered a lap even if the bars are in the adjacent grouted cell!

There are PVC spacers made to hold rebar centered in a masonry cell. Talk to your local rebar supplier, he/she should know where to get them. You might also try your concrete accessory suppler or search the web.

If you wait seven days for the wall to cure, what do you do with your masonry crew during that time? That seems WAY too conservative. I bet that by the time you get the next level of joist and deck set the wall is strong enough to accept the dead load - 2-3 days.

 

[RDK]

Often the act of inserting the rebar into the grout acts as vibration and assists in consolidating the grout. The masons first pour the grout into the cavity and then use the rod to stir the grout and ensure that it flows. For this reason it’s my preferred practice to insert the bars after the wall is full height. If the bars are in place then shake them or whack them with a hammer to consolidate the grout

Only the portion of the bar that needs to be imbedded in the layer below is sticking out. This eliminates the need to lift the masonry units 8 ft up and thread them through the core. (Don’t laugh; I’ve seen designers swear that this is standard practice.) Lap lengths are the same as for reinforced concrete.

Ties and spacers are not needed. The masonry unit itself acts as cover. The transfer of load to the lapped bar is through compression in the grout so the stress flux will flow through a gap between the bars, even to the adjacent core.

By the time you get to the joist level, I’d simply keep going. You never worry about the load on the bottom course when building an 8’ wall, why worry about placing joists and metal decking. By the time you get the joists in place, place the decking, place the mechanical and electrical inserts and pour the floor, the grout will have developed sufficient strength. Even if you are using pre-cast flooring, the time lag between placing the blocks and developing any significant loads will normally be sufficient. Besides as jheidt2543 said, what do you do with the brickies in the mean time? Remember it’s only the dead loads.

It only needs the full 28-day strength for full design loads and if you can go from placing bricks to full occupancy in that time you are a faster builder than anyone I’ve seen.




Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[Lutfi]

Vertical bars do not have to be tied together. Provide the required lap and you all set. ACI 530 even permits the bars to be in adjacent cells. Yes, lots of engineers are not aware of this fact.

Good luck

 

[UcfSE]

I am aware that bars in adjacent cells are considered lapped. However, I do not think that is good practice, nor do many of the inspectors who will be inspecting your project. In the field, most times the rebar will be tied as you go up unless the lift heights are fairly low, such as 4-6 feet. Now think about it, your masonry strength depends on the depth of your steel "d". If the bar is not tied, it will likely fall to one side and rest in a corner, and then you have lost your rebar depth.

I spent 8 years in the field working with masons and masonry, and never once did I work on a project on which the rebar was placed after the concrete was poured. This would cause problems with having the men trying to place all the rebar before the grouts sets up enough to make it difficult. Besides all that, in 8 years I never had an inspector who would approve a job before rebar was in the wall. Perhaps things work different in Canada or other parts of the nation and that's good but in Florida for instance I do not think that would go over well. Maybe contacting your inspector or building department would save a future headache.

Concrete manufacturing and CMU manufacturing plants do manufacture inserts to hold rebar in position. I've never seen one of PVC but I have seen plenty of 9-gage wire cross ties. I'm not sure if that was your question or if you were asking about alternatives to tieing, such as mechanical connectors. Of course those are available if you wish to pay the extra cost.

I do not know the lap lengths for working stress design of concrete, but usually you'll see a longer length for a lap splice in masonry instead of concrete.

As far as how long to wait before advancing, you could calculate what you need. There are curves and information available to help you estimate a strength before 28 days is reached, and you could use that to check your stresses to determine how long before it's ok to advance. If floors are being installed/cast as the walls go up, I would think that amount of time would be plenty. By the time the masons stop working at this pour and move to another wall, when they return to the first one it will likely be plenty strong enough to continue.

I try to remember that just because the code says something is acceptable does not relieve me of thinking about whether it truly is. Even though rebar isn't required to be tied, I personally do not think it good practice not to do so.

 

[akastud]

Just additional notes to the previous comments. The rule of thumb we use in our office is that bar laps in masonry are always 48 bar diameters. It is acceptable to place bars in the next cell, but to be consevative in both concrete and masonry we make sure that the lap length is established in a shear cone that extends 45 degrees from the end of the bar. For example if we are using #5 bars in adjacent cells, we need a total lap length of 48 * 5/8" + 8" = 38" so that 30" is within the shear cone of the bar you are lapping. Also there are tolerances in the Amerine book as to where the bar centered in the wall can be placed. Typically here, the rebar is sitting in the cell when it is inspected, and as the grout is poured, the rebar is stood up in the center of the cell.