Standard Depth for Rebar in Flow Through Bond Beam Block

[rlmhawk]

Is there a standard (ASTM or other) that governs the depth of the vertical slots or knock out web depth used in nominal 8" deep flow through CMU bond beam block? This appears to me to be somewhat arbitrary, varying between block manufacturers and by region. If there is no standard, what depth, "d", do people typically design for when placing their reinforcement in a standard 8" bond beam? Do you use 5", 6", other? Obviously this gets deeper in taller beams, but given the 8" depth, where do you assume the center line of the rebar is at? In most instances I don't make my designs so refined that this makes a huge difference, but I am curious what others typically use. Similarly, what do you assume when placing rebar in lintel block?

 

[concretemasonry]

rimhawk -

You are correct about the variations locally and internationally. This is because ASTM does not have any requirements for detailed dimensions of masonry units. It does provide requirements for the overall width, length and height and the acceptable tolerances. Since it is extremely rare for a bond beam to be used as a unit resisting flexure, the units are intended to be used to be grouted both vertically and horizontally, such as in an intermediate bond beam. In many areas 16" high lintels (8" along the wall length) are available in 6",8",10" 12" and possibly 16" wide units. I have even seen 6" wide bond beams that were 8" high on one side and 3" high on the other side to integrate a slab into a reinforced 6" wall on high rise buildings.

If a bond beam is used for a short span, conventional solid bottom bond beams work well if 8" high coursing is used and shore, if necessary. There is no standard for the steel location since it can be up to the specifier to use the units locally available. Usually, 8" solid bottom bond beams have, at most 2" solid bottoms, so the "d" distance would be about 5".

For greater spans, there are a number of methods that can be used depending on the desired appearance if exposed. Normally, a 16" high unit (8" long in the wall) is used, but I have used a conventional solid bottom course plus courses of flow-through bond beams that eliminate the change of the appearance without any exposed steel. This works well for larger openings in buildings with taller walls and exposed architectural units (colored, split, ground face, textured, etc.).

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.