Structural Engineering

Omni Block stretcher and corner/jamb block meet  ASTM C-90 Standard Specification of Loadbearing Concrete Masonry Units (CMU) and exceeds all minimum requirements as set forth in the ASTM C-140 Standard Test Methods for Sampling and Testing (see sample test results).

These two tests are nationally recognized as the fundamental tests that standard CMU must pass to be considered a structural block.

The over 100 variations (bond beam block, half block, solid bottom bond beam block, U-shaped block, etc.) all must pass the ASTM C-90 and C-140 tests in order to be considered “structural”. None of these CMU types require an ESR report, rather all are engineered using Allowable Stress Design (ASD) or Strength Design using The Masonry Society (TMS) standard tables.

Omni Block stretcher and corner / jamb block are in the same classification as standard CMU.

There is industry accepted component software provided by the National Concrete Masonry Association (NCMA) that allows rebar to be specified in alternating locations. This is especially invaluable when designing structures using Omni Block’s System 12.

Omni Block does not employ any structural engineers and therefore relies on the independent professional engineer to structurally engineer Omni Block. Omni Block is an insulated system and insulation is always “trumped” by structural requirements, but hopefully the two can coexist. Every cell that is grouted that could be insulated negatively affects Omni Block’s thermal properties, advantages and thermal performance.

National Concrete Masonry Association
Comment by Jason Thompson, V.P. Engineering

Between 2011 and 2013, masonry design standards (ASTM and TMS) went through a series of revisions to allow alternative unit configurations…effectively allowing units such as Omni Block provided that they still met minimum criteria as defined in ASTM C90. Detailed background discussion is provided here: https://ncma.org/resource/new-concrete-masonry-unit-configurations-under-astm/

All these changes were first captured in the 2015 edition of the International Building Code, and subsequent editions of the IBC. If an alternative CMU configuration is used (other than the conventional two cell, three web unit configuration) it can be structurally analyzed using the provisions of TMS 402-13 (which is referenced by the 2015 IBC). However, there are a couple nuances when using units such as Omni Block:

  • Because the unit configuration is different, the cross-sectional properties are different. Hence, the unique properties (such as cross-sectional area) need to be considered in design. This isn’t anything new for designers as the cross-sectional properties of an assembly change based on which cells are grout. Now they change slightly differently with different unit configurations.
  • When inserts are added to a unit/assembly, they displace grout – grout that normally bonds to all the inside surfaces of the unit to create a ‘composite mass’. When inserts are added, they prevent the grout from bonding to some surfaces. As a result, when inserts are added, there is an additional “web shear” design check (or see page 4 in the ASD design example below). It’s a simple check, but ensures that the webs in the unit are sufficient to transfer stresses throughout the cross-section of the assembly.

Effectively, you can use the entire cross-section of Omni Block to resist loads (no need to assume an 8 inch unit is only 6 inches thick) you just need to verify that the webs are robust enough to transfer the loads applied to the assembly.

Architectural Details

A comprehensive set of Omni Block architectural details are available to view and / or download that are commonly used in a building’s design.

Typical Vertical Steel

The illustration below depicts typical ideal vertical steel placement, if possible, when numerous variables such as, wall heights, wind loads, soil conditions, dead and live loads, Seismic Zone and others are factored into the structural calculations.

The Omni Block 8″ stretcher block grout cell d dimension is equal to 2.50″ in lieu of the typical d of 3.81″ for traditional 8″ CMU

Standard concrete masonry rebar and grout installation techniques are used when installing Omni Block.

Horizontal Steel

The detail below illustrates the ideal horizontal steel solution in order to maximize the amount of insulation contained in the wall. Depending upon the building and its unique conditions, whenever possible the use of ladder rod in lieu of a bond beam(s) is desired.

Allowable Stress Design Example

The following pages consist of an actual set of calculations using Allowable Stress Design and are included here to illustrate how structural engineers can calculate rebar and grout requirements in the individual block units for Omni Block walls.

The custom residence used as the example required 1 #4 every 48″ o.c. for the Omni Block walls and 1 #4 every 24″ for the standard CMU parapets.

Mr. Hart is an independent structural engineer and is not an employee of Omni Block, Inc.

Strength Design Example

The following pages consist of an actual set of calculations using Strength Design and are included here to illustrate how structural engineers can calculate rebar and grout requirements in the individual block units for Omni Block walls.