Technical Information | Selecting Strength

(Excerpted from Design of Reinforced Masonry Structures, published by CMACN)

Desert Block Co., Inc. supplies this information as an educational aid in understanding the benefits of concrete masonry construction and our products. It is the responsibility of the user to obtain engineering or other advisory services from licensed professionals as the basis for incorporating into any project any information, detail, or product offered herein.

When designing a building, the architect and engineer must determine a number of key design parameters that significantly affect the overall cost, quality, and performance of the structure. For masonry designs, three of these key design decisions are: 1) select strength design or working stress design; 2) the selection of the design strength(s) (f'm) to be used, and 3) when working stress design is selected the determination of the inspection level(s) to be required. Each decision directly affects the stress levels that can be applied to the masonry structural system. In turn, the stress levels affect wall thickness, reinforcement requirements, and consequently job cost. While the selection of these three design parameters is critical, the methodology for making that selection can be somewhat ambiguous. The following are recommended guidelines to be used in making f'm and inspection decisions.

Initially assume: working stress design, eight inch units, f'm =1500 psi, no special inspection. Eight inch units are the industry standard and as such are readily available in the widest variety of shapes and textures. Mason production with the units is excellent, resulting in a low cost wall. Fifteen hundred psi design strengths are obtainable with units meeting the minimum strength requirements of ASTM C-90. These units are less expensive than higher strength units and are often available from stock. When working stress design is used selection of no special inspection required results in a fifty percent reduction in stress allowables, but saves on inspection and quality control testing expenditures.

Add special inspection, consider strength design, before you increase the f'm.
Special inspection adds inspection and testing expenses, as well as additional coordination requirements to a project. However, should the preliminary design assumptions indicated above, result in reinforcement levels in excess of minimum requirements, special inspection and strength design can be a cost effective way to obtain the design benefits of higher stress values. This recommendation is dependent upon the availability of qualified inspectors and assumes that the job will be manned with a normal (or large) masonry crew size. Special inspection may become expensive if inspectors must travel long distance to the jobsite or the crew size is very small.

Increase f'm first from 1500 psi to 2000 psi. Then to 2500 psi or 3000 psi, if necessary. When a 1500 psi design strength assumption results in reinforcement congestion, transition to a higher f'm. Concrete masonry units capable of achieving design strengths to 3000 psi are manufactured regularly in industry light and medium weight concrete mixes. Units capable of producing design strengths beyond this 3000 psi limit are less commonly manufactured and require significant adjustments in the aggregate mix which to achieve the necessary unit strengths. This change in aggregate mix will provide design strengths above 3000 psi. It will most likely produce a block that is substantially heavier and consequently may be more expensive to lay than lighter units.

Special inspection can be used on selected building components, if desired.
Special inspection is a design option that can be applied to individual building components. Exercising this option provides the benefit of increased stress allowables on highly stressed structural members while preserving the economy of no special inspection on other areas of the building. Keep in mind that having both no special inspection and special inspection on the same job creates a coordination challenge between the contractor and the inspection agency. This problem can be minimized by keeping the inspection scheme simple and clear.

An increased f'm can be used on selected building components.
Design strength, like inspection levels, can be varied over a single job. However, because of the concrete mix adjustments required to accomplish the various design strengths, there may be color variations between strengths. Consequently, for clear sealed jobs the f'm should be transitioned where a possible color variation poses no problem. Again, due to coordination issues, keep it simple.

Apply special inspection to critical details.
Many designs have one or more details that are complex and/or critical to the structural integrity of the building. The structural engineer may choose to have these details special inspected.

Special inspection and 3000 psi not enough... Consider design change, larger units, and/or higher strengths. Should the combination of special inspection and 3000 psi design strength be inadequate to avoid steel congestion in eight inch units, the design professional must consider either modifying the design (e.g., reduce spans, change to pilaster design, etc.), increasing the units size (to 10 or 12 inch units), or increasing the design strength beyond 3000 psi. Evaluating these options requires the balancing of a number of complex issues including; labor productivity, unit availability, color implications, etc. The designer should seek the advice of local industry organizations, suppliers, and/or mason contractors to help determine an optimal solution.

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