This is the inaugural Q&A column that I was asked to prepare by the Publisher and Editor of Masonry Design. During my masonry career with new designs, forensic evaluations, code preparation and product development, I’ve made and seen problems. Most types of masonry interest me, be it modern or historic. If you have a masonry design or restoration question, send it along and I’ll answer what I can with a little help from my friends. While I am a structural engineer, this column will address masonry issues of interest to architects as well.
One service this column can offer is to provide examples based upon experience. These discussions on masonry topics are needed since many architects and engineers are self-taught or derive their masonry experience on the job; few have had a masonry specific course. However, if we are licensed, we are all expected to have the necessary experience. Within this forum, we can “bond” by sharing ideas and experiences with the goal of advancing the field of masonry design.
Why bother reading this column or offering comment? Maybe you’ve been designing masonry buildings or structures for years and those designs have worked successfully. Why try new methods? Well, sophisticated owners and clients demand efficient designs that are economical to build. The old masonry designs may work, but are they keeping pace with the competition from non-masonry systems? Now would be a good time to commit to updating your knowledge to include the latest techniques and avoid being too conservative or over-designing.
While under designing can affect the health, safety and welfare of the public users, over designing can be problematic because it adds unnecessary expense to a project and could lead to costly delays. Perceived over-design hurts the project and reflects poorly on all architects and engineers to our mason contractors and clients. Mason contractors can sense weakness in our designs. The outfall could affect confidence and undermine project cooperation. If as professionals we are to bring value to the design, we must provide efficient, economical masonry designs using up-to-date techniques.
To help, the masonry industry offers a wealth of knowledge through numerous organizations (The Masonry Society, International Masonry Institute, National Concrete Masonry Association, Brick Industry Association, Mason Contractors Association of America, Masonry Institute of America and numerous regional and local organizations) as well as structural masonry coalitions. Their experts are available to give assistance with architectural detailing, engineering design and more. Take advantage of the expertise if it is available in your region. Architects will particularly find the assistance useful in programming their projects for masonry.
Often, these coalitions and experts are evaluating projects on behalf of contractors and owners after they have gone out for bid only to find that a masonry option was not considered, properly detailed, or fully evaluated. The architect and engineer are left to explain why a more economical system was not considered. This could be an embarrassing lesson to learn!
So, let’s talk about some of the issues and common problems and make masonry designing better. By bonding we get stronger!
Q: When I design with structural steel, I can use the Allowable Stress Design (ASD) method or the Load and Resistance Factor Design (LRFD), I generally get the same solution. However when I design masonry with the ASD method, I often get a different solution than using the Strength Method. Why is this?
A:Thanks for the question. First, a little history.
The engineering design techniques developed for most structural materials including masonry, structural steel, concrete, and wood have evolved from ASD (or Working Stress Design) to a limit states version such as Strength Design (SD) or LFRD. Masonry design once used only empirical methods, but in the early 1900s ASD was developed to rationally design structures. Now, the strength design methods are recognized as the improved method for representing the performance of structures.
Usually, there is a development process within code committees whereby ASD and LRFD harmonization is considered. The American Institute of Steel Construction (AISC) developed harmonization for structural steel such that when the live load to dead load ratio is approximately three, the two methods provide essentially identical results. In addition when the LL/DL ratio is less than 3, ASD is more efficient; when the LL/DL ratio is greater than 3, LRFD is more efficient.
The American Concrete Institute (ACI) took a different approach for structural concrete. Early codes were based upon working stress theory. In 1956, they released their first version of Ultimate Strength Design (USD) as an appendix in its design standard, ACI 318. In the 1963 edition, both methods were given equal status. ACI chose not to harmonize ASD with USD and dropped ASD altogether (ACI 318-14).
Starting in 2005, the National Design Specification® (NDS®) for Wood Construction provided adjustment factors and other values that modified their working stress design methods to enable the use of LRFD with the strength design load combinations of ASCE 7. Basically, they have converted historical test results to LRFD.
The American Iron and Steel Institute sponsored research at the University of Missouri-Rolla, Washington University, and the University of Minnesota on cold-formed structural steel. The initial results were first published in the 1986 Edition of the North American Specification for the Design of Cold-Formed Steel Structural Members. The current edition incorporates ASD, LRFD and Limit States Design.
The masonry industry has taken its own path to implementing Strength Design. While many masonry design and detailing requirements derived from empirical issues, the working stress method was adopted from concrete by changing material properties. Little research was needed. However, from 1984 to 1994, international researchers formed the Technical Coordinating Committee for Masonry Research (TCCMAR) with funding from masonry industry and government sources, including the National Science Foundation. They performed testing with the goal of establishing limit-state design; strength design is one of the limit states. Most of the research revolved around seismic behavior and over the years the results have made their way into the masonry standard, TMS 402 (formerly known as the MSJC or ACI 530).
While the TMS 402 masonry standard committee is attempting to harmonize ASD with Strength Design (SD), limitations inherent in the TCCMAR testing have resulted in code variations. One example is the topic of maximum bar size; ASD has historically limited bars to #11 while SD now limits bars to #9. Why the difference? Because TCCMAR researchers limited their testing to #9 bars; so that’s made its way into the standard for SD. The current committee is working to harmonize this issue and more.
For a more thorough discussion of masonry harmonization, readers should see the article, Harmonization of Allowable Stress Design and Strength Design of Masonry, by Edwin Huston, May 2013, Structure magazine (http://www.structuremag.org/?p=246).
Future editions of TMS 402 are expected to eliminate the Empirical Design Method for new construction and relegate it to an evaluation method for older buildings. Someday ASD for new masonry construction may be will eliminated similar to what the concrete industry did. The natural progression would be for Strength Design to be expanded to fully cover Limit States Design for masonry.
By understanding how we came to having Strength Design in masonry, you can see that making the two methods (ASD and Strength) give comparable results is a challenge that might never fully be achieved. The current recommendation is that designers use the Strength Design method as the primary masonry design procedure and check whether ASD still provides some efficiency not yet addressed in the Strength method. While this may seem duplicitous, design software can be used to compare ASD versus Strength results with little extra effort.
Q: We have a school under construction. The walls are cavity wall construction with CMU and brick veneer. For the walls on one area of the building, the brick veneer was constructed with a mixture of 1/4-inch and 1/2-inch joints. We don’t like the appearance and our architectural masonry specifications require 3/8-inch bed and head joints. Are the constructed joints acceptable? Should we reject the work?
A: Variability in construction can be a concern and it’s rare that we see significant joint variability. However, the answer to your question likely depends upon your own project specifications.
I have been called to projects to render an opinion on the aesthetic quality of completed veneer work. My first response is usually that my opinion does not matter. What governs is what is in the contract, specifically the project specifications.
Generally, all construction materials and systems have tolerances that determine quality; and tolerances are a specification requirement. From my experience, most architectural masonry specifications do not specifically address tolerances. However, they often reference TMS 602, Specification for Masonry Structureswhich does list tolerances. Having TMS 602 as your go-to document for tolerances may seem adequate, except that its tolerances are solely intended to address structural issues as stated in the 3.3F Site Tolerances, TMS 602 commentary“The tolerances given are based on structuralperformance, not aesthetics”. Therefore by using TMS 602 as your source for tolerances, you’d be specifying tolerances for your veneers that are not intended to address aesthetics. The resulting work could be contractually acceptable and not represent the visual appearance you desire.
So, whether the joint sizes in your project are contractually acceptable is based upon your specifications.
- If you listed specific tolerances, they would apply or,
- If you only reference TMS 602, its tolerances would apply for both structural and veneer joints.
Let’s look at the tolerances in TMS 602:
Mortar joint thickness for bed joints between masonry courses ±1/8 in.
Mortar joint thickness for head joints – 1/4 in. + 3/8 in.
Using these tolerances with your example, your specified 3/8-inch bed joints could vary between 1/4 inch and 1/2 inch. Your specified 3/8-inch head joints could vary between 1/8 inch and 3/4 inch. Therefore, the 1/4 to 1/2 inch constructed joints for your project meet the TMS criteria and would be contractually acceptable. You may not like what you see, but the mason met the specifications.
What if your specifications did not provide either your own tolerances or reference TMS 602? Again, we need to look more thoroughly at your specifications for what is required contractually. Did you require a sample panel? One of the benefits of requiring a sample panel is to establish aesthetic quality (Figure 1). However, the specifications need to clearly state that the sample panel is to be the basis for acceptance for mortar joint tolerances and any other aesthetic issues (color, bond, tooling, etc.). Once the sample panel is accepted, the architect should document its construction fully and list the features of the panel that are acceptable along with joint variability.
The overall message here is that architects address aesthetic concerns in the project specifications and on the sample panel so that the mason contractor knows what is expected.
