Anchor Reinforcement for Concrete Podium Slabs

Guest blogger Scott Fischer, R&D Engineer

Guest blogger Scott Fischer, R&D Engineer

This week’s post comes from Scott Fischer who is an R&D Engineer at our home office. Since joining Simpson Strong-Tie in 2006, Scott has worked on cast-in-place connectors to concrete. He helped develop the current testing criteria for cast-in-place concrete products and also performed the testing and code report requirements needed for these product lines. Prior to joining Simpson Strong-Tie, Scott worked for nine years as a consulting engineer. His experience includes the design and analysis of concrete structures, including post-tensioned slab design, concrete lateral systems and foundations. Scott is a licensed professional engineer in the state of California and received his bachelor’s degree in Architectural Engineering from Cal Poly San Luis Obispo.

How often do you get the opportunity to high five a co-worker in the office? Maybe it’s when you just worked through a really complex calculation, or finally figured out that tough detail. Whatever it might be, there are times when we should raise a hand and celebrate the hard work that we do. So when we recently relaunched the Simpson Strong-Tie Strong-Rod™ Systems website, which includes a link to our new Shallow Podium Anchorage Solutions, there were a few high fives going around the office. With that in mind, we want to share the latest developments and continue our anchorage-to-concrete blog discussions that began in May 2012, continued with a March 2014 post referencing the Structure magazine article on anchor testing, and more recently one discussing our release of anchor reinforcement solutions for Steel Strong-Wall® shearwall to grade beams.

Anchor Reinforcement Testing and Research Program

Simpson Strong-Tie has been studying cast-in-place tension anchorage and anchor reinforcement concepts extensively over the past several years. Designing an anchor solution in a thin concrete slab for a high anchor demand load while meeting the ductility provisions of ACI 318-11, D. is extremely challenging. A strong industry-need for a safe, logical, yet economical design solution, led to a cooperative research program between Structural Engineers Association of Northern California (SEAONC) members and Simpson Strong-Tie. Testing was initiated by a Special Project Initiative grant from SEAONC to members Andy Fennell, P.E. (principal at SCL) and Gary Mochizuki, S.E. (principal at Structural Solutions at the time, now with Simpson Strong-Tie). We completed the program with continued involvement of SEAONC members. This is the second time we have partnered with SEAONC on non-proprietary anchor bolt testing. The first partnership, on sill plate anchor bolts in shear, resulted in successful code change provisions (led by SEAONC) restoring the capacity of these connections to pre-ACI 318 Appendix D values.

This current research program has focused on non-proprietary anchor reinforcement detailing that increases the nominal breakout capacity of concrete slabs. The anchor design satisfies the seismic ductility requirements of ACI 318-11 Appendix D and also significantly increases design capacity for wind applications. The project goal was to provide a design solution for the industry with independently witnessed proof testing of the anchor reinforcement detailing and application of ACI 318-11 Appendix D design procedures. (Note: Appendix D is moved into a new Chapter 17 in ACI 318-14.)

Test picture of anchor reinforcement

Figure 1: Test picture of anchor reinforcement

Anchor reinforcement detail at mid-slab

Figure 2: Anchor reinforcement detail at mid-slab

Significant Test Findings and Design Concepts

Anchoring to relatively thin concrete slabs introduces many unique challenges, so testing was bound to reveal some unique findings. The goal was to increase concrete breakout capacities and also satisfy the ACI 318 anchor ductility requirements with anchor reinforcement detailing. Here are some of the significant findings:

  • Relatively thin concrete slabs do not allow the placement of anchor reinforcement to drag the load down into a larger mass of concrete as shown in RD.5.2.9. Modified anchor reinforcement was required (Figure 2). The required area of anchor reinforcement is based on D. where the required area of anchor reinforcement exceeds the anchor steel strength, or 1.2Nsa < (nAsfy x 0.707). The 0.707 is for the 45 degree slope of the bars. The proof testing showed the horizontal leg development outside the cone and continuity through the cone adequately developed the anchor reinforcement.
  • ACI 318-11, D.4.2.1 states that when anchor reinforcement is provided, calculation of concrete breakout strength is not required. You know we love load path discussions, so where does the load go once it gets into the anchor reinforcement? The tests indicated that when the anchor reinforcement is provided, the concrete breakout area increases. This limit state is an extended breakout area past the anchor reinforcement bends that will form when that reinforcement is properly quantified and configured. The extended breakout is similar to multiple anchors loading the slab at each bottom bend of the anchor reinforcement. We have applied this concept to the calculations to evaluate extended breakout past the anchor reinforcement bends.
Initial breakout surface from head of anchor

Figure 3: Initial breakout surface from head of anchor

Breakout surface from bends in anchor reinforcement

Figure 4: Breakout surface from bends in anchor reinforcement

  • Let’s follow the load path some more. Once the anchor is connected to the slab, what is the slab bending capacity? The testing showed that to achieve the anchorage capacity, the slab must have an adequate amount of flexural reinforcement with anchorage forces corresponding to ACI 318-11, Section D. applied to the slab. Guidance from this section says to apply the anchor tension loads obtained from either design load combinations that include E, with E increased by Omega, or 1.2 x Nominal steel strength of the anchor (Nsa). If the anchors are not oversized, designing for 1.2Nsa should be the most economical solution. For wind applications, the slab Designer should consider the project specified design loads.
  • A vertical concrete block shear forming at the anchor bearing plate is possible if the anchor embedment is shallow and the anchor reinforcement is working to resist the initial anchor bolt breakout area. Our testing showed that this block shear is separate from Appendix D Pullout and is dependent on embedment depth, perimeter of the bearing surface and concrete strength.
  • For anchors with shallow embedment that have a double nut and washer, the concrete breakout can begin from the top nut, thereby reducing the effective embedment depth. To address this, we eliminated the top nut from our specified anchor assembly kit to insure the breakout begins from the top of the fixed-in-place plate washer.
  • The testing and modeling also allowed us to re-examine the appropriate bearing area for the plate washer, Abrg. The flat top surface of a nut is typically circular due to the chamfer at the points, so the resulting bearing area of the plate washer is circular extending out the thickness of the plate from the flat-to-flat dimension of the nut. For near-edge conditions, the side-face blowout capacity can be the controlling limit state and where the plate washer bearing area becomes more important.
  • Edge testing with anchor reinforcement details showed that the breakout area will spread out and begin from the anchor reinforcement bends, like the mid-slab. In a mid-slab condition, the breakout slope follows the 1.5:1 or 35 degree slope used in Appendix D. However for the near edge, we found that 1.5 x effective embedment (hef) from anchor reinforcement bends only holds true parallel to the edge. Due to eccentricities, the breakout angle from the anchor reinforcement bends into the slab (perpendicular to the edge) is steeper and a steeper 45 degree slope should be used.
Edge test with anchor reinforcement

Figure 5: Edge test with anchor reinforcement

Edge anchor reinforcement configuration

Figure 6: Edge anchor reinforcement configuration

  • The testing showed that even though we had cracks intersecting the anchor as it was loaded, the capacities exceeded uncracked assumptions. This is likely due to the flexural reinforcement running through the breakout cone providing continuity across the cracks. We’re still studying the effect of the flexural reinforcement, so for now we recommend assuming cracked concrete and providing a minimum of 4 – #5 flexural bars each way at anchor locations that require anchor reinforcement. Your slab design may require more flexural bars or they may already be there to meet other slab design requirements.

What Solutions Are Now Being Offered and How Do I Get Them?

With our newly launched Strong-Rod Systems webpages, you now go to the Shallow Podium Anchor link to find anchorage solutions. On the website, you will find anchor reinforcement detail drawings and design load tables with slab design recommendations. We’ll also be adding sample calculations, a guideline for selecting your anchor solutions, 3-D anchor reinforcement graphics and guidelines for addressing your condition if your installation is outside the scope of the current solutions that we offer. The anchor reinforcement is non-proprietary and is fabricated by the rebar supplier, but the configuration and placement is described in the details. In addition, you will see detailed information about the Simpson Strong-Tie Shallow Anchor Rod and Anchor Bolt Locator that is specified as a kit in the load tables. Note the absence of the top nut for reasons described above.

Shallow Anchor Kit

Figure 7: Shallow Anchor Kit

How Do We Specify It and Use It?

So now you know what is on the website but how do you put all these pieces together and apply them to a specific design? As a design professional, you will drive the bus on applying these details and design tables onto your drawings. Similar to specifying Strong-Wall® shearwalls or Strong Frame® moment frames, it just takes a little upfront coordination on your drawings. Typically, you’ll start with a slab key plan that shows the anchor bolt locations. You will need to know the design uplift forces from the light-frame structure above the slab and some basic project variables like specified concrete strength, slab thickness(es) and whether the structure is in high seismic or wind-controlled areas. Now you can choose the necessary design tables from the website by clicking on the individual design tables tab. Your key variables will help you select your specific table based on slab thickness, concrete strength, near edge, etc., and of course wind or seismic.

Sample Shallow Anchor design table

Figure 8: Sample Shallow Anchor design table

Once you have selected your design table, just match your project demand loads or your project-specified anchor bolt with the tabulated ASD or LRFD capacity to select the appropriate shallow anchor callout and reference detail that you can identify on the key plan. The detail callout from the table will send you to sheet SA1 where you will find the anchor reinforcement details and Shallow Anchor Kit recommended for your condition. As mentioned previously, the anchor reinforcement would be fabricated and bent by the rebar guys, but would follow these details. You can download the details shown on sheet SA1, place them on your construction documents and then coordinate them with your plans or schedules similar to how you might provide a shear wall schedule. The footnotes that accompany the tables provide important slab design information and other design and installation recommendations. You’ll soon be able to download the sample design calculations, use them as a tool to help follow the design procedure of the recommended details and submit with your project.

What if My Situation Does Not Fit the Details on the Website?

Though a great number of installations will be covered by these details and tables, there will be conditions that currently cannot be addressed with anchor reinforcement solutions. Installs that may be outside of the current scope could include: a demand uplift that’s too large, a slab that’s too thin, lower concrete strength, corner installs, double wood frame shear walls or two close anchors in tension. To address these conditions, we suggest alternatives like slight adjustment or reconfiguration of the shear walls, thickening slab edges, adding downturn concrete beams or extending the anchorage from above down into a cast-in-place wall or further extended down into the footing at ground level.

The joint SEAONC-Simpson Strong-Tie testing project has shown that anchor reinforcement details can greatly increase the breakout strength of concrete to support cast-in-place anchor bolts in concrete slabs. The testing also showed that the design provisions in ACI 318-11 Appendix D can be rationally applied to these anchor reinforcement details. The testing and Appendix D calculation approach are the basis of the details, load tables, graphics and application guides that can be found on our new Strong-Rod Systems webpages. We’re updating these pages as we create more content, so check back frequently. We look forward to working with you on your anchorage installation challenges and hope that some of these solutions will help you with projects you are working on today. How about a high five?

What do you think about these new anchorage solutions? Let us know by posting a comment below.