Reinforced Masonry 1

Morokanu,

Basically the code is saying that typical vertical wall reinforcement should be ignored for compressive loading. .2% steel does little in compression. Masonry columns are more like concrete columns, thus the more generous capacity reduction factor. Seems logical to me.

hokie66,

I wouldn't worry about .2% reo contribution to axial load but the capacity reduction factor for unreinforced masonry is only 0.45 which means the capacity of the wall (that's it, I'm stuck with a wall) is 40% less (.45/.75).

Some more: take a closer look at the formulae. Block contribution is bigger than the grout contribution and it is logical to be the other way around. The block relies on mortar bed for transfer which is a weaker ring in the chain than the grout.
At the end of the day, by using a 25MPa block and 32MPa grout the wall ends up with a capacity in terms of compressive stress of less than 3MPa. I think the wall can do better.

morokanu,

I think you will find that most people will not provide specific restraint reinforcement along the wall. The clause says that it must be restrained but it does not say how. If you have all cores filled with grout then I would think you could claim that the grout is providing the restraint (same as a concrete column-only the concrete stops the bars from buckling inward).

I believe the reason why they say that it is to be restrained in both directions is for bars at the end cores of walls and for walls where only the reinforced cores are grout filled. For the latter there is still a chance of shell breakout in the length of the wall.

A single bar in the middle of the wall cannot be restrained, and in fact it is very difficult for then to build a wall with lateral restraint ties at each bar (the steel fixer has to follow the brick layer at each course). Best to design as unrestrained or to design concrete columns with a masonry'shell' where required.

If you are in Australia, you can cantact the Australian Concrete Masonry Association (CMA) for a local interpretation.

In most countries, if the vertical steel is restrained at the bottom and held in place at the top prior to grouting, it is considered to be restrained.

I have not checked the Australian code lately, but I would imagine it states that the grout should be about the same strength as the concrete in the masonry units, with an upper limit of 10% or 20% over the block strength. - This is to insure that the load is evenly distributed over the masonry area and is not carried by the material that has greater strength and less deformation.

In the U.S., the provisions (ACI 530?) for low lift grouting may have been revised (or are in the process of revision) and the limit may be as high as 12' 8" if I recall correctly. You may want to see if this provision has recently been incorporated in the area you are designing in.

Dick

Is the use of the term "restrained" construed to mean the same thing as "securing" the reinforcement during grouting, with say, positioners(or what folks in the trade refer to as spiders)? If it does, then I can see why this is very important....as it provides necessary clearances for effective grouting and adequate coverage...and maintains position of bars within in-plane and out of plane placement tolerances.

Morokanu and others,

The restraint provisions in the Australian code are very specific: has to be by 6 mm bars at spacings not exceeding least dimension of member or 400 mm, whichever is less.

Reinforced masonry provisions for compression generally not applicable for walls because of constructability issues, e.g. incomplete grouting, which is probably the norm rather than the exception.

Codes in other countries would be different, but the Australian Committee obviously has decided that grouted masonry walls are not in general to be considered as worthy of being considered as reinforced masonry when in compression.

The mortar actually has a small influence on the strength of a wall in compression.

When you compare compressive strengths of block, mortar and grout you are comparing apples with oranges. This is because of the different testing procedures for the individual materials and the failure modes. You cannot take the simplistic view of looking for the weak link based on lab tests or specified strengths. As an example, you can make a 4500 psi f'm ungrouted block face shell mortared prism using 1900 psi mortar because the block strength is actually the controlling factor in the strength. The prism assembly fails in shear through the block and mortar and not by compressing the mortar.

As an example, for mortar, a laboratory compressive test in which the mortar is cast in a mold and is not placed in the same manner as in the field, does not have the field compression by applied loads during construction and does not have the same sample aspect ratio. When applied in the field (3/8" by 1 1/4") mortar does not fail in the same manner as a laboratory sample because of the aspect ratio of the material. Quite simply, masonry units, mortar and grout are masonry materials that act as a assembly similar to a concrete cylinder.

Dick

A correction on the tests referred to earlier. I just reviewed the photos and material details.

The prisms were a set of ungrouted 2 block high FULL mortar bedded prism tested at 21 days. The average prism strength was 4610 psi based on the net mortared area.

Obviously prism tests are much lower than individual block strength because of the 2:1 h/t and the relatively slender configuration of the webs.

Details:
The block were 8x8x16 52% solid two core usints (conventional dimensions) with a net block strength of 8100 psi.

The mortar had a compressive strength of 2300 psi.

These were not ordinary block, but were a demonstration set to justify the feasibility. When you publish design tables (1980 vintage) units with prism strengths of 1750 psi and 3000 psi, you have to show an engineer that 4500 psi is possible.

the biggest problem was finding a testing machine to handle a 16" high sample that had a thick enough platten. It is not like routine cylinder testing with small areas and different strengths.

hokie66 is correct, generally vertical reinforcement will be ignored for vertical loads and the capacity reduction factor will be 0.45.

From the Commentary Clause C8.4;

"When the reinforcement is not adequately restrained in this manner, the masonry should be considered, for compression resistance, as grouted unreinforced masonry....
If the reinforcement is effectively restrained, the capacity reduction factor (? ) ... will be 0.75. Otherwise it has to be taken as 0.45."