Pallet Rack Design: Engineering & Design of Concrete Anchor Bolts
In our last post, Concrete Anchor Bolts: Why These Little Components Are A Big Deal, we discussed the importance of anchoring your pallet racks for safety, the basic types of concrete anchor bolts, bolt sizes, holding power, and the number of anchor bolts needed. But that’s just the beginning. A concrete anchor bolt isn’t just any old bolt; there are complex code requirements that affect the design and engineering of concrete anchors, including designs for “cracked concrete.”
ANSI/RMI MH16.1 on Concrete Anchor Bolts
The ANSI/RMI MH16.1 — Specification for the Design, Testing and Utilization of Industrial Steel Storage Racks states that anchor bolts for pallet rack should be designed in accordance with section 7.3, which states:
The anchor bolt design shall be in accordance with the provisions of ACI 318 Appendix D*. The redundancy factor in the load combinations in Section 2.1 and 2.2 shall be 1.0.
We’re not going to reiterate all the calculations set forth in Sections 2.1 and 2.2 of the ANSI/RMI Specification, but note that 2.1 gives the load combinations for the ASD design method (Allowable Stress Design) and 2.2 gives the load factors and combinations for the LRFD design method (Load and Resistance Factor Design).
*Since the latest edition, ACI 318–14, the entire 50-page Appendix D on concrete anchors is now located in Chapter 17 of ACI 318–14 and is entitled “Anchoring to Concrete”.
IBC on Concrete Anchor Bolts
The International Building Code (IBC) also gets into the act by referencing ACI 318 Chapter 17, “Anchoring to Concrete”. In 2003, the IBC introduced state-of-the-art requirements for testing, evaluation, and design of anchors in concrete, including anchors post-installed in hardened concrete. The new design method referenced by the IBC for post-installed mechanical anchors established the design rules for anchoring to concrete.
Additionally, Appendix D requires post-installed anchors to be tested and qualified according to ACI 355.2 and includes testing for the performance of anchors in cracked concrete conditions.
While the IBC referenced ACI 318 for Strength Design anchorage, it also provided a table of ASD values for some cast-in-place headed anchors that did not resist earthquake effects. The option to use ASD anchors for some cases remained through the 2012 codes, but since 2015, all references to the ASD anchor values have been eliminated, and only strength design is now recognized as code compliant for post-installed anchors.
The term “cracked concrete” relates to the likelihood of the concrete floor developing cracks in the anchorage area at or below the expected service load level.
Cracking in concrete occurs for various reasons: concrete may crack during a seismic or high wind event or due to temperature fluctuations, soil pressure, shrinkage, etc. In general, it is tension in the concrete that causes cracking.
Only post-installed mechanical anchors that have met the requirements for use in cracked concrete in accordance with ACI 355.2 are permitted for use in applications where crack development is expected, or in moderate to high seismic risk areas.
Anchor performance can be critical to the safety of your storage racks, so you be sure to use anchors that are recognized under the current building code for cracked and/or uncracked concrete conditions.
Many design professionals choose to always presume “cracked concrete” and only specify those anchors that have passed the appropriate tests for cracked concrete.