Is masonry what it's cracked up to be?

Designs for loadbearing masonry walls must heed the true
August 11, 2010

Architects, structural engineers, facility managers and contractors working with masonry structures would do well to consider more than building codes when developing new designs. In fact, real-world structural failures and the track record of selected materials must be examined carefully.

Although previous building codes provided for masonry strength variability, as do current codes for other materials, the Building Code Requirements for Masonry Structures has no such provision. That code-the 1999 version of ACI 530-99/ASCE 5-99/TMS 402-99, published by The Masonry Society, Boulder, Colo.-permits average strength to equal specified strength, in which case half the masonry used in a project could be below specified strength. It is often said that the factor of safety compensates for the omission.

Ordinary workmanship, rain, freeze-thaw cycles and fatigue substantially reduce the cracking and ultimate compressive strength of masonry. If the effects of those variables are accumulative, the compressive strength of brick masonry at first crack may be a third of the static strength obtained under laboratory conditions on well-cured, dry specimens built with ideal craftsmanship.

Although more research is necessary to determine strength reduction factors with precision, reasonable approximations based on available research indicate that for real-world brick masonry the current design factor of safety in flexural compression is inadequate. Luck prevents most failures.

Design and performance variables

In developing designs for loadbearing masonry structures, the building team should consider key variables that affect material and system performance. Incorporating these issues into up-front building planning can bolster structural assumptions, factors of safety and ultimately the value of the buildings.

The most important variables to consider-and those often neglected-include:

  • Rain: A saturating rain can reduce the compressive strength of masonry materials by 25 percent or more.

For further research and consideration of the effects of rain, the British Masonry Society's journal, Masonry International, offers two useful references: Dieter Kasten's "The Compressive Strength of Masonry Units Determined According to Different European Standards" (in the summer 1990 edition), and Pietruszczak and Pande's "Strength of Saturated Masonry" in the April 1994 issue. Also, an important addition to any designer's library is Ira O. Baker's 1909 classic, A Treatise on Masonry Construction (John Wiley & Sons).

  • Freezing: One-winter freezing and thawing in some parts of the United States can reduce the compressive strength of brick masonry by about 7 percent. It would take longer for that to occur in milder climates.

These effects were recorded as early as 1930 by Palmer and Hall in "Some Results of Freezing and Thawing Tests Made with Clay Brick," published in the West Conshohocken, Pa.-based American Society for Testing and Materials (ASTM) Proceedings.

  • Fatigue: As few as 40 cycles of compressive load can reduce the static compressive strength of brick masonry 30 percent or more. (For initial calculations of the ultimate compressive strength of masonry materials, see "Rules of thumb," page 58.)

The most recent study of compressive loading was reported in the proceedings of the 1999 North American Masonry Conference, sponsored by The Masonry Society. Another useful reference can be found in the proceedings of the 7th International Brick Masonry Conference, sponsored by the Brick Development Research Institute in Melbourne, Australia in 1985.

  • Workmanship: The effect of workmanship on strength of masonry has been well recognized for at least 70 years. Building owners, general contractors and construction managers have an obligation to monitor and inspect ongoing masonry work to assure the highest level of craftsmanship (see "Select and inspect," on page 58 for guidelines).

In 1929, Stang, Parsons and McBurney reported on the ratio of compressive strength of clay brick masonry built without inspection to that built with inspection. Several investigators have since verified their work. It is clear from the work of those researchers that ordinary workmanship reduces the compressive strength of masonry about 30 percent.

Useful references

Other good references for the building team in this regard include studies conducted under the auspices of the Reston, Va.-based Brick Industry Association, such as Historical Survey and Analysis of the Compressive Strength of Brick Masonry (Monk, 1967) and Recommended Practice for Engineered Brick Masonry (Gross, Dikkers and Grogan, 1969).

The effects of site conditions and labor are further examined in "Investigation of Effects of Workmanship and Curing Conditions on the Strength of Brickwork" (James, 1975) and "Site Control of Structural Brickwork in Australia" (Anderson, 1975), from the proceedings of the third International Brick Masonry Conference, held in Bonn, Germany. The effects of test proceedings on the compressive strength of masonry structures were also reported at the Canadian Masonry Symposium in 1980.

  • Cracking: Over the past three decades, researchers have reported that brick walls under compression begin to crack at about 65 percent of ultimate strength.

Of course, cracking does not cause immediate structural failure; however, cracks increase water permeance, which accelerates deterioration of masonry and corrosion of embedded metals-both of which can eventually lead to structural failure. Cracking is a serviceability failure and incipient structural failure.

For more detail on these performance issues, see this author's report on the compressive strength of brick masonry in the 1984 issue of Masonry International.

  • Luck: In spite of the reductions in the cracking and compressive strength of installed masonry materials, most buildings benefit from a combination of overdesign and underloading.

A recent study demonstrated the first factor. Compressive strength tests were made for quality control on 70 samples of three masonry prisms each from 10 projects in seven states. An analysis of the data indicated that the strength of masonry is typically 36 percent greater than required.

Moreover, the forces to which masonry is subjected very rarely reach design levels. For example, wind loads are typically based on a 50-year mean recurrence interval. Thus, stress seldom reaches allowable limits.

Lucky 13?

So what should a building designer or owner conclude? Currently available data show that-because of rain, weathering and fatigue-there is an estimated 60 percent probability of cracking at allowable stress in flexural compression when the strength of inspected brick masonry equals the required strength. On that basis, there is one chance in 13 of structural collapse, if stressed to design levels.

It gets worse, however: With rain, weathering and fatigue there is an estimated 70 percent probability of cracking at allowable stress in flexural compression when the strength of ordinary brick masonry equals the required strength. On that basis there is about one chance in two that strength will be less than allowable stress. In that case, the in-service factor of safety is nearly one.

Fortunately, strength is typically much greater than required and stress rarely reaches the allowable limit, but life safety should not be based on luck. Design and construction professionals should look beyond the Building Code Requirements for Masonry Structures when determining compressive strength and factors of safety for loadbearing masonry assemblies. In the meantime, one can only hope that the document will be revised in the near future to provide for masonry's strength variability.

Clayford T. Grimm is a consulting architectural engineer based in Austin, Texas.

         
 

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