Masonry - "Stretcher" Block Properties

[shuff213]

I've come across existing construction with 12" masonry "stretcher" blocks...see the appended photo. Trying to perform calculations, but haven't been able to find typical section properties for these type of walls.

I could conservatively use section properties for typical ASTM C90 12" CMU ungrouted blocks, but my thoughts are that may be too conservative.

Any help would be appreciated....thanks.

 

[Shotzie]

Unless the blocks were actually grouted wouldn't the load transfer just occur through the face shells, meaning that your calculations wouldn't be impacted by the additional webs?

 

[MotorCity]

Agree w/ Shotzie. Unless you are relying on the webs for shear strength or shear transfer, they won't affect the flexural capacity of the wall. Any chance of getting field measurements? Otherwise I'd assume 1 1/2" thick face shells and go with it.

 

[concretemasonry]

shuff -

Those certainly look like some older CMU shapes that are far beyond the ASTM minimal requirements. I would suggest to look at the ACI 530 requirements. The NCMA TEK notes also provide a good reference. When we wrote the design codes that led to the current codes and standards(ASTM C90,etc. and ACI 530) the basis was from older panel tests.

To eliminate any confusion and provide you with better information you should ask for test results for a prism test (hollow, ungrouted 2 block high) that reflects the CMUs structural capacity (f'm).

The basic C/M design parameters are based on the performance of wall panels and not individual units as long as they meet ASTM requirements. In general,with any manufacturing equipment and processes since the 1930's, it is difficult to make any block that is not 30% over the minimums even if a manufacturer wanted to. Unlike most concrete, the cement content is not a factor, because the CMUs are highly compacted by a vibration mold on a 6 to 10 second cycle and transported through the short (6 to 24 hour)curing process. When in the concrete block industry, we made hollow block with 8000 psi net compressive strengths that tested 4800 in two block prisms to determine the f'm as the measure, and the mortar strength does not have an effect on the f'm that is the measure of a wall panel.

Since typically, block delivered are drawn from existing inventories test reports of previous dates are usually acceptable, but you can specify that samples be sampled on job site, but the building may be completed before you can get results.

Dick

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