Thursday, January 26, 2012

The curious case of code concludes

Building code was developed to help ensure that buildings are safe.  Code has developed over time, its evolution reflects the knowledge of observed results.   

In my last two entries on this blog, I wrote about the ways in which existing Code is not practical for triangular block.  I do not intend this as a criticism of Code, or as a complaint against those who have composed our current Code.  I simply mean to point out that current Code does not account for triangular block used in innovative masonry.  Triangular block does, in fact (and indeed) meet most building Code.
I want to stress that Code is intended as a guide which is meant to help the architect, contractor, builder and working mason to design and make safer buildings.  Much leeway is provided for in Code to allow for architects, designers and builders to express their creative and innovative approaches in providing solutions to the challenges involved in making good buildings.
This flexibility in the code is even specifically addressed by the National Concrete MasonryAssociation (NCMA) in their commentary on code.  The creativity and imagination of the marketplace in designing and developing novel unitconfiguration far exceeds the ability of prescriptive national standards to keep pace. Case in point, ASTM C90 addresses the minimum physical properties for loadbearing concretemasonry units. The requirements of ASTM C90 and the companion testing standard ASTM C140 are by necessity generic in nature – applying to the majority, but not all,
configurations of units. As special or proprietary units are introduced to the market, they may not be able to be evaluated consistently or accurately under ASTM C140, or may not have all of the relevant features for strict compliance with ASTM C90.”

In concluding my brief discussions on Code, I want to look at control joints.  Control joints are included in masonry construction to allow for the expansion and contraction of a masonry wall, and to allow for movement without the wall suffering from cracks.  Control joints mean that the wall is essentially “pre-cracked” and will thus allow for movement without breaking. 

Code addresses control joints in the following (rather open-ended) manner:  “Restrained or differential movement in building elements and building materials can result in cracking. Some common causes of movement are: loads created by wind, soil pressure, seismic forces, or other external sources; settlement of foundations; or volume changes in materials. For example, volume changes in concrete masonry units can be caused by moisture gain and loss, thermal expansion and contraction, and carbonation. To limit and control cracking due to these and other causes, proper design, detailing, construction, and materials are necessary.  Specification C 90 provides a maximum limitation on the total linear drying shrinkage potential of the units, but it is not within the scope of this specification to address other design, detailing, construction, or material recommendations. This type of information and related guidelines for crack control are available from other organizations.”  [Taken from Appendix X2, NCMA commentary]

Multiple industry publications are available on the subject of crack control in concrete
masonry construction, including:

TEK 10-1A, Crack Control in Concrete Masonry Walls

TEK 10-2C, Control Joints for Concrete Masonry Walls – Empirical Method

TEK 10-3, Control Joints for Concrete Masonry Walls – Alternative Engineered Method

TEK 10-4, Crack Control for Concrete Brick and Other Concrete Masonry Veneers

While each of these documents specifies distances between control joints, none of this applies to triangular block.  This is because triangular block break differently than rectangular block.  The key to understanding this is conjugate shearing, as I discussed here and here.  Triangular block are inherently disposed to conjugate shearing, whereas rectangular block are not.  When cracks develop on rectangular block wall, the result is a weaker, less attractive –some may call it ugly- fault in the structure.  Conjugate shearing, on the other hand, does not create any visible crack at all.  Code does not account for conjugate shearing, simply because rectangular block are incapable of conjugate shearing.  Shown below is a prototype dome, which is essentially composed entirely of control joints, via conjugate shearing between triangular block.

Although I could continue, this concludes the curious case of Code regarding triangular block.

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