Masonry Wall Net Area vs. Equivalent Thickness

[maolson3]

It appears there is a discrepancy in calculating the net area of a masonry wall for axial stress (P/An) between the "Reinforced Masonry Engineering Handbook" (Amrhein, 1998) and the "Masonry Structures: Behavior and Design" text (Drysdale, 1999).

Looking at the RMEH on P.300, Table B-3a gives the "Equivalent Solid Thickness" for a 12" conc. wall grouted at 48"oc as being 6.5". In the footnotes, we see that fa = P/(EST)b, thus P/A. A in this case would be 6.5*12 = 78 in^2/ft.

Looking at the Drysdale text on P.873, Table B4, we see that An for Grouted Cores @ 48"oc gives An = 54.1 in^2/ft

Why do the two values differ so greatly? Which one should be used in the calculation of fa?

 

[concretemasonry]

Take a look at ACI 530 (Code, Specifications AND especially the commentary). That is modern consensus result of the Jim Amrhein and Drysdale books that were written in the mid 1990's according to the different codes and standards that varied regionally. Neither were in favor of just using only grout to achieve the desired results, since it far cheaper and more reliable to use high strength units.

I think if you look closely the 6.5 equivalent thickness is about the ungrouted or net area of the 12" masonry units.

In addition to ACI 530, The best place to get additional information is on the National concrete Masonry Association (ncma.org). On the upper right corner there is a place to click on.- Just click ANY state or supplier to get to the 110 TEK notes and go to the index and the structural section for the individual TEK notes you are interested in.

When the first ACI 530 was written, I remember Jim Amrhein and the NCMA staff engineers sorting things out. Drysdale was also involved in many meetings I attended.

Masonry is really designed by the wall panel and not the unit or materials (block, mortar, grout) since the individual materials are tested differently for different purposes.

 

[msquared48]

Concretemasonry:

"I think if you look closely the 6.5 equivalent thickness is about the ungrouted or net area of the 12" masonry units."

In my 1978 edition if Amrhein, the term is "equivalent Solid Thickness" and is defined as ..."the calculated thickness of the wall if there were not hollow cores, and is obtained by dividing the volume of solid material by the face area if the wall." It was intended for the determination of area only for the structural design of the wall, as in specifically for shear and bearing.

My edition also shows the 6.5" figure for the same conditions in Table B-3a on page 278.

Mike McCann
MMC Engineering

 

[concretemasonry]

The 1978 (over 30 years ago) publication by Jim was largely a regional view (Southern California) using the local terms and calculating methods. The knowledge has been worked into the ACI 530 documents, which reflect the current testing procedures, definitions and nomenclature. Many of the units used were not the commonly seen 2 core units most people see today, but there are some producers that nowo make specialized units to comply with the current codes and standards and the methods to determine the real properties.

I am not aware of any codes that use the old methods and procedures, although the terms and methodology have been updated and adapted to the knowledge the old documents provides.

Just as reality check, the minimum ASTM 12x8x16 units give an approximate equivalent thickness for the unit itself of about 5.8 +or-. The determination of the volume of the units defy calculation (only accurately determined by water submersion) because of the radii, core tapers and various other items (some of which are structural attributes and others are functional/production based. Currently, the term "equivalent thickness" in testing (ASTM C140) is the volume of the unit divides by the actual face area (not including mortar). there may some othe standards that still use the older concepts and determination properties. NCMA TEKs 14-1B (2007 - Properties of Masonry Walls) and TEK 14.7B (2009 - Allowable Stress Design of Concrete Masonry) are several examples of information that is available in Chapter 14 of the TEK notes to use with the common standards.

In general, all design allowables are based on the prism strength (f'm) of a composite 2 high block/mortar hollow prism. Jim Amrheim was very sharp and understood the big picture and the difference is terms locally. In the 1980's, I showed him a series of test reports for 5000 psi 8" hollow prisms and the photographs of the testing and failed samples. It only took him a few seconds to spot the errors in the testing (which others noted later) that involved changes in the ASTM testing procedures that were ultimately corrected to eliminate the older testing and volume/linear dimensions.

How you look at the wall properties is determined by current standards and not older books and papers.

I am partial to ACI 530, since I was involved in the document and the fellow voting members. I have seem some international projects (10 -13 to20 story partially reinforced loadbearing buildings) that were completed with no job site prisms, a minimum of mortar tests and no clean-outs. the engineers said they used what they learned in Southern California in the later 1960's, early 1970's complimented by technology, good detailing, planning and technology. They said they used U.S. standards, but they used them better.

Dick

 

[maolson3]

I believe the RMEH is actually requiring you to multiply the Equivalent Solid Thickness by the height of your block. Thus 6.5*7.625 = 49.56, much closer to 54.1. I did a quick hand calc of this area and came very close to 54.1, so I believe that to be an accurate value. Thanks for the help everyone! It is much appreciated.

 

[msquared48]

In thinking about it, considering that the masonry block and grout shearing stresses are unequal, technically, you'd have to use the lower allowable shearing stress when calculating the overall value for the wall - similar concept to not adding the shearing stress for GSB to that of plywood for a shear wall when one is applied to each side of the wall.

Mike McCann
MMC Engineering