Next Level Looks at ACI 318, “Building Code Requirements for Structural Concrete

In follow up to our last post, Next Level Looks at ACI 318, “Building Code Requirements for Structural Concrete , we ‘re now going to dig a little deeper into this code and look at what used to be called Appendix D, the section that specifically addresses concrete anchorage and anchor bolt design. The Specification for the Design, Testing and Utilization of Industrial Steel Storage Racks (ANSI/RMI MH16.1) references ACI 318 Appendix D for anchor bolt design regarding securing pallet racks to the concrete floor. Section 1.4.7 of the specification states that all rack columns be anchored to the floor with anchor bolts, which are to be designed in accordance with Section 7.3, which in turn states that anchor bolt design shall be in accordance with the provisions of ACI 318 Appendix D. The International Building Code (IBC) also references ACI 318 Appendix D regarding its requirements for concrete anchorage.

A Brief History of Appendix D

The first formal concrete anchorage design provisions were set forth in Appendix D of the 2002 edition of ACI 318. Prior to this, designers relied on the manufacturers ‘ design tables which did not always represent all the characteristics associated with a particular anchor ‘s design capacity. The applications in this first code for anchorage design were limited to cast-in-place anchors and post-installed mechanical expansion anchors. In the 2011 edition of ACI 318, provisions were given for another type of post-installed anchors- adhesive (or bonded) anchors. Currently, screw anchors fall outside the scope of ACI 318 Anchoring to Concrete, but will be studied for inclusion during the next code cycle.

While “Appendix D technically no longer exists in the most recent incarnation of the American Concrete Institute ‘s code (ACI 318-14), the design provisions, Anchoring to Concrete, are still in the code, residing in Chapter 17 rather than an appendix.

“Appendix D in a Nutshell

Appendix D (or the chapter Anchoring to Concrete), outlines the general requirements for concrete anchors including the theory of design, seismic design requirements, and strength reduction factors. It also introduces the different anchor failure modes and includes both steel and concrete failure modes. In a nutshell, Appendix D is an all-encompassing document for every anchor situation. 

A Note on Cracked Concrete

The state of the concrete can significantly influence the anchor ‘s capacity, and design professionals must determine whether to consider the state of the concrete “cracked or “uncracked. Appendix D explicitly does not define which concrete applications should be categorized as cracked or uncracked, which leaves the designer to determine by analysis the likelihood of the concrete cracking in the area where the anchors are installed. In the absence of such analysis, many designers choose to assume “cracked concrete. There are however, two situations that require designing for cracked concrete:

1. If a structure is in Seismic Design Categories C, D, E, or F.

2. If analysis indicates cracking at service level loading in the anchor location.

Additional factors that have the potential to result in cracked concrete are:

  • Temperature changes
  • Soil pressure
  • Differential settlement
  • Restrained shrinkage

Because concrete anchors are constantly evolving in the structural engineering industry, the ACI 318 design provisions for concrete anchorage will continue to be improved and updated along with modifications in the IBC. As new provisions and changes are made, manufacturers will develop new anchor types to answer engineering requirements.

Photo Credit: By Dmitry G (Own work) [CC BY-SA 3 or GFDL], via Wikimedia Commons

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