Bond Beam on CMU wall 2

[kosrae]

Hi Everyone,

I am working on reinforced concrete masonry structure using 8" load bearing blocks,this is my first time to handle this type of structure. Details of walls indicate a "bond beam" on every 3 layers of concrete blocks. I have gone to a different site for CMU manufacturers and found differing configurations.

Could somebody enlighten me on this so called "bond beam"? Or can i just used 'cast in place concrete' instead? I propose this because of its inavailability on the island.

I would appreciate any response.

 

[msquared48]

You can use cast-in-place concrete instead, but it would defeat the purpose of using masonry in the first place. If you can get ahold of any, which sounds like it could be a problem too, you might consider K-Webb that is installed horizontally at every other course between the blocks in the mortar plane. This was used a lot in past years, but no longer. You still run into it in older buildings. The last time I saw it used in any design was in the 80's.

Normally, bond beams are placed horizontally every 48" vertically in an 8" wall with continuous (2) #3's, #4's, or #5's horizontally at the bottom of each bond beam depending on the desiger. They are also placed at the top of the wall and at the bottom where the CMU meets the concrete foundation.

I believe that #2 bars can be used in lieu of K-Web every other course too, but then...availability. I have not seen #3 bars used in this fashion, but maybe someone else has. Seems like the mortar joint would not be large enough.

Where are you? What Island?

Mike McCann
MMC Engineering

 

[concretemasonry]

Where are you and what type of structure? - Usually the availability is controlled by the local consruction and code requirements. Several thousand block size and shape units in different configurations are possible, so the units available are those in normal demand.

The use of closely spaced bond beams is very unusual in virtually any location in the world. A bond beam is NORMALLY not a separate structural element, but is used to make the wall perform as an integral structural element.

Site cast bond beams were used years ago as the top course in some masonry construction, but it became apparent that a bond beam using the proper shapes was more economical and structurally more effective.

Look to the local producers to see what is available. You will find much better information that by going on line to manufacturers that do not make units for your location.

A block producer can make thousands of different shapes depending on the local needs. I worked for a block producer that typically had over 1000 different units in inventory (not including higher strength units).

For most masonry construction the vertical spacing is the critical item (which can easily be specified) and the horizontal steel is used to provide continuity. A standard specifying a bond beam every 3 courses seems very arbitrary, even if you are building a "safe cell" for projectile protection, but blast resistance is possible.

What is the applicable type structure?

 

[kosrae]

Thanks you Guys for your informations and i've learned lot from it.

Well, i am in Kosrae, Micronesia one of the island in the 'Pacific ring' and i am working on a 2 storey school building designed by an engg firm from hawaii. It is a reinforced concrete masonry wall with ACI 530-02 as their reference.

I wonder why this design firm used this kind of structural framing in fact there's no available producer for those load bearing blocks in this island.

 

[kosrae]

My real concern now is the production of loadbearing blocks, few months ago i've heard that the a lone block producer in the island was able to produce blocks meeting our requirements of fm=1800psi.Sample was tested in Guam and the compressive strength exceeded the required. What bothers me is that, it is still necessary to make sure that it conforms to other requirements under ASTM C90 such as, net cross sectional area, face shells thickness, web thickness, absorption and density classifications? Can i still use the stuff ignoring the other requirements of ASTM C90?

 

[hokie66]

You will have to decide on whether you will stick to the design documents or try to have another method accepted. You could get the blocks shipped from Australia, probably Cairns or Townsville, or maybe from Guam.

Would cast in place concrete be simpler for you? If so, why not propose that as an alternate?

In Australia, it is common practice to use horizontal bars in every third or fourth course, and to grout fill the whole wall, especially in tropical cyclone prone areas. Achieves a robust wall in both directions.

 

[kosrae]

Thanks hokie66.

I cannot redesign the structure the fact that i am on the contractor's side. Even our inspector does not know whether to waive the other requirements or not.

I can get the stuff from guam but if it safe to use the locally made it would be better. I just want to know the disadvantages of ignoring those other requirements on structural sense and quality assurance.

 

[hokie66]

I don't think you should ignore any requirements. You either need to meet the requirements or have them waived by the design engineers. Does you local supplier claim to meet the specification requirements?

 

[Zambo]

I have seen some (primitive) bond beam units for horizontal beams/rebar made out of standard blocks. Cut/knock out the external and internal webs and lay them on the base of the block. Then the rebar can be layed horizontally in the blocks. Vertical stiffeners/rebar are of course easy.

 

[hokie66]

Zambo,

There is nothing primitive about using knock out blocks to place horizontal reinforcement. Our blocks in Australia are made so the webs are easily knocked out with a hammer. If H-blocks are used, they have a slot in the web for the horizontal bars.

kosrae,

I would try to use the locally produced blocks if possible. That is what they are for. Try to find out what standard the blocks are produced to. If not the US standards, they may be to another recognized standard, which you can ask the designer to accept as an approved equal. To a large extent, a block is a block. Concrete masonry specifications are full of boilerplate, and you should be able to build with the available materials. What type building are you constructing?

 

[concretemasonry]

kosrae -

First, there is a difference between the stregth of a single block and f'm, which is the measure of a two block assembly comsisting of block and mortar. If you are using grout to increase the compressive strength, be leering of using grout that is significantly stronger than the strength of the concrete in the block. Many engineers fall into that trap.

Second, when you are grouting solid, the details of some of the block elements is not applicable. If you look closely, the normal element minimum thicknesses do not apply for when the block is grouted.

Dick

 

[hokie66]

kosrae,

Sorry for my question about what type of building you have. I see you already said it was a two storey school building.

Personally, I would just tell the guy in Hawaii that I was going to use the local blocks, put in the reinforcing he specified, and grout solid the whole wall. Tell him to come and have a look at the blocks if he wants.

Sometimes in remote locations, you just have to force the issue.

 

[kosrae]

Thanks guys.

Hokie66, our local supplier does not claim they meet our requirements, they are not engineers and even not know what is ASTM C90 as reference.They just give the result of test from Guam and even the laboratory in Guam did not say either the blocks conform or not conform to the specs.
The contracting officer terminated the services of said designer from hawaii.

Zambo, i believe i have seen the one your refering to. I saw it from the site of one manufacturer in Guam the webs are cut maybe 2-3 inch from top and they term it ' bond beam'.

Dick, If f'm is the measure of a two block assembly consisting of block and mortar, so if our required f'm is 1800psi and mortar is specified to be 2500psi, what then is the compressive strenght of individual blocks that i have to use.Result from laboratory indicate only compressive strenght and not f'm. Also, our units are full grout.

Thanks

 

[concretemasonry]

Mortar really has a minor effect on the strength of a wall.

Mortar strength tests arenot in a configuration that is similar to or reflects the behavior of a wall. That is the reason for using prisms (the masonry companion to the concrete cylinder) to measure the conpressive strength on the materials used in a wall. I have tested block prisms with an f'm of 4500 psi using 1900 psi mortar.

Grout is not used to increase the wall strength, escpecially is you have any flexure. High strength grout will give a false f'm when the critical section is the outer fibers (the block) if you look at the stress disribution on a wall in flexure.

You can use either a prism test to determine the f'm or go to a table and get a conservative value based on the unit block strength and type of mortar. Here you will see that the block strength is by far the controlling factor.

 

[RVSWA]

From working with a few Hawaiian architects, I believe that on the Hawaiian islands, they use CMU block exclusively for foundations. We were told no-one on the island even owns wall forms. Take it for what it's worth, I can't offer much more. Perhaps they assumed they would be just as readily available on your island and thus a good construction material. That's all I've got, what's the weather like? Seattle is in it's prime time right now, but in another 6 months I might want to visit...

 

[kosrae]

Thanks, guys...