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Code Update: Change to ACI 318-19 Sets Lower Shear Capacities for Many Concrete Structures

Daryl Burns on July 29, 2022 - in Articles, Column

In January 2020, the American Concrete Institute (ACI) published the second printing of ACI 318-19: “Building Code Requirements for Structural Concrete.”

ACI 318-19 code sets the minimum criteria for materials, design and detailing of structural concrete buildings and, where applicable, non-building structures. The code is organized into 10 parts:

1. General

2. Loads and Analysis

3. Members

4. Joints, Connections and Anchors

5. Resistance

6. Materials and Durability

7. Strength and Serviceability

8. Reinforcement

9. Construction

10. Evaluation

The specification is split into two separate coordinated columns with the ACI 318R-19 commentary aligned in the right column and the ACI 319-19 code in the left column. According to ACI, the documents are bound together solely for the user’s convenience.

A Highlighted Change and Its Industry Impact

Many precast products are produced following the requirements in corresponding ASTM standards that reference ACI 318 for the structural design of the precast concrete products.

ACI 318-19 is then utilized by civil and structural engineers to design precast concrete products, including beams, slabs, columns, wall panels, retaining walls, buried manholes, buried vaults, wastewater treatment tanks, custom elements converted from cast-in-place and more.

National Precast Concrete Association (NPCA) members and precast concrete producers-at-large have been negatively impacted by some of the changes between the ACI 318-14 code and the latest ACI 318-19 code.

One such section that has had a particularly negative impact on the precast industry is the change to the shear capacity of reinforced concrete beams and slabs.

ACI Committee 318 updated the code provisions for shear capacity based on research data on the effects of flexural reinforcement and section depth. They added a size effect factor (λs) that reduces the calculated shear capacity by approximately 30 to 40 percent, especially if the design section has a low reinforcement ratio (ρ) as is likely in double-tee flanges, other precast slab elements or cast-in-place slabs.

This modification results in thicker structural slabs that can cause issues for precast concrete manufacturers—the main issue being costly manufacturing form upgrades.

The added size factor begins to affect slabs greater than 10 inches thick, which impacts buried structures such as manhole flattop slabs and vault top slabs. Based on the resounding successful performance of these buried slabs—with a previous required thickness of 12 inches or less for the last several decades—the precast industry strongly believes this shear capacity reduction modification should not apply to buried precast structure flat slabs.

An Adapting Industry

As a result of the code change in ACI 318-19, many civil and structural engineers employed by precast concrete manufacturers have transitioned to following the shear capacity formulas found within the American Association of State Highway and Transportation Officials (AASHTO) “LRFD Bridge Design Specifications.” The AASHTO Bridge Design Specifications do not apply a size-effect factor, as it can be argued that the AASHTO LRFD method is more appropriate in designing buried precast concrete structures experiencing vehicular traffic live loads.

For more information on NPCA and how it serves the precast concrete industry, contact NPCA Director of Codes and Standards Daryl Burns, P.E., at [email protected] or 317-208-6431. 

 

About Daryl Burns

Daryl Burns, P.E., is NPCA’s director of Codes and Standards; email: [email protected]

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