We structural engineers here at Simpson Strong-Tie have a love/hate relationship with anchorage to concrete. Ever since the introduction of the strength design provisions in the 2000 IBC and ACI 318 Appendix D, anchorage to concrete has been a challenge for designers, building officials and manufacturers. SEAONC’s recent testing and the resulting code changes offer some relief to wood-frame designers for sill-plate anchor design at the edge of concrete, but many challenges remain.
With the increasing demand for high-density housing and urban infill projects, designers are now faced with anchoring multi-story wood-framed shear walls to relatively thin elevated concrete slabs (typically referred to as podium slabs). Overturning tension anchorage forces at the ends of shear walls in these projects can routinely be in the 40 kip range and even get as high as 60 kips or more.
A 1” diameter high-strength anchor in the middle of a 12” thick, 5,000 psi uncracked concrete slab will achieve roughly 35 kips in tension at allowable stress design (ASD) levels for wind and low seismic loading; much less than that at the edge of slab. In higher seismic risk areas, the code requires an additional 0.75 reduction in design strength. Appendix D also has ductility requirements that can be met in a few different ways. [For more details, see ductility discussion below for different code requirements.]
The anchor capacity in the previous example would be reduced to approximately 25 kips at ASD level for seismic, but it may not be suitable for use in Seismic Design Category C or higher because the steel strength exceeds the brittle concrete breakout strength.
So what are designers doing and what are building departments accepting? Well, lots of things. Some designs include large-plate washers, added reinforcement, or punching shear calculations used for columns. While some of these options may increase the capacity of the anchor, they don’t necessarily fit within the design provisions of ACI 318 Appendix D. This is a challenge we want to continue to address in the blog. But in the meantime, I’d love to hear about your issues with anchorage to concrete podium slabs.
What are you doing to address the challenge? Let me know by posting a comment.
SIDEBAR: Ductility Discussion
ACI 318 Appendix D Section D3.3 has seismic ductility requirements that require the designer to protect against a brittle concrete failure in moderate to high seismic areas. Reducing brittle failures and promoting ductile failures, such as steel yielding is fundamental to seismic design. However, the code language of ACI 318 can be confusing.
ACI 318-08 vs. ACI 318-11
ACI 318-08 Section D.3.3.4 requires that “anchors shall be designed to be governed by the steel strength of a ductile steel element.” Designers often question what steel and concrete strengths to compare when evaluating D3.3.4 – nominal strength, design strength, design strength with additional seismic reduction factors?
In lieu of meeting the ductile anchor requirement of D3.3.4, ACI318-08 has other options. Section D3.3.5 allows the design force to be limited by a ductile yielding attachment. Alternately, section D3.3.6 allows for a brittle anchor design provided you take a large reduction in design strength of the anchor.
ACI 318-11 has revised the ductility requirements to clarify that you compare the nominal strength of the concrete and anchor.
ACI 318-11 Section D.184.108.40.206 also considers four options for determining the required anchor or attachment strength to protect against brittle concrete failure. The first option requires the nominal concrete-governed strength to be 20% greater than the nominal steel strength, or: 1.2 x Nsa < Ncb. The second and third options require the anchor to be designed for the maximum load that can be delivered to the anchor. The fourth option is to design the anchor for E x Ω0 design loads. (It should be noted that the 2012 IBC modifies ACI 318-11 to delete these revisions and essentially reverts back to ACI 318-08.)
What are your thoughts? Visit the blog and leave a comment!