Concrete Beam Design (ACI 318-19 update)

Concrete Beam Design: ACI 318-19 raises limits on the specified strength of reinforcement in shear wall and special moment frame systems. The new standard allows Grade 80 reinforcement for some special seismic systems and no longer allows Grade 40 rebar to be used in seismic applications.

Concrete Beam Design (ACI 318-19 update)

The American Concrete Institute (ACI) 318-19 is the current standard for the design of concrete structures. It includes a number of changes from the previous version, ACI 318-14. Some of the significant changes for concrete beam design include:

• The maximum grade of reinforcement steel is now 80 ksi, up from 60 ksi in ACI 318-14.
• The minimum reinforcement ratio for beams is now 0.01 times the gross area of the beam, up from 0.008 times the gross area in ACI 318-14.
• The detailed method for shear design has been removed. The simplified method is now the only method that is allowed.
• The equations for calculating the effective moment of inertia in deflection checks have been updated.

In addition to these changes, there are a number of other minor changes to the concrete beam design provisions in ACI 318-19. These changes are intended to improve the accuracy and safety of the design process.

Here is a summary of the changes in ACI 318-19 related to concrete beam design:

• Maximum grade of reinforcement steel: The maximum grade of reinforcement steel is now 80 ksi, up from 60 ksi in ACI 318-14. This change is intended to allow the use of higher-strength steel, which can result in more economical designs.
• Minimum reinforcement ratio: The minimum reinforcement ratio for beams is now 0.01 times the gross area of the beam, up from 0.008 times the gross area in ACI 318-14. This change is intended to improve the ductility of beams.
• Detailed method for shear design: The detailed method for shear design has been removed. The simplified method is now the only method that is allowed. The simplified method is less conservative than the detailed method, but it is also less accurate.
• Effective moment of inertia: The equations for calculating the effective moment of inertia in deflection checks have been updated. The updated equations are more accurate than the equations in ACI 318-14.

These are just some of the changes in ACI 318-19 related to concrete beam design. For more information, please refer to the code itself. Concrete Beam Design

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. Concrete Beam Design

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. Concrete Beam Design

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.