Mixing It Up with Concrete Specification

Around Christmas, the Engineering Department does a white elephant gift exchange. We have no idea who framed this picture and wrapped it up the first time.

Lab Guys Concrete Pour

Several of our lab technicians (plus a product manager) are posing for the camera, and obviously trying to flex while sucking their bellies in during a concrete pour to test our SSTB(R) anchors. The tradition has it that if you end up with this picture, you hang it on your wall and re-gift it at next year’s gift exchange – so there it is, on the wall in Engineer Dustin’s office. The trick has become wrapping it so that nobody recognizes that it is the picture frame.

Speaking of concrete, between our test labs in Addison, Ill., Stockton and Pleasanton, Calif., we test a lot of concrete. We will certainly be doing a lot more testing to continue to support our new Repair, Protection and Strengthening Systems for Concrete and Masonry product line. But I will ask the lab technicians to keep their shirts on.

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New Simpson Strong-Tie Anchor Designer software

Remember back to the days when you used allowable stress design for designing anchorage to concrete? Once you had your design loads, selecting an anchor was quick and easy. The 1997 UBC covered the anchorage to concrete in less than two pages, so the calculation was painless. Post-installed anchors were even easier, since allowable loads were tabulated and you just needed to apply a couple of edge distance and spacing reductions.

Since the introduction of strength design provisions and the adoption of ACI 318 Appendix D, first in the 2000 IBC, designing code-compliant anchorage to concrete has become much more complex. At least once (and probably not more) armed with a pencil,  calculator, and an eraser, most of us have set out to design a ‘simple’ anchorage to concrete connection using the  Appendix D provisions. Several pages of calculations later (and hopefully with a solution to the problem), most of us, I imagine, came to realize that designing anchorages to concrete by hand required much more time and effort than we anticipated or could allocate time for. As a result, many of us probably created an Excel template to speed up the design process using built-in functions and some Visual Basic programming.

I'm never going for looks on my spreadsheets.
I’m never going for looks on my spreadsheets.

The question is: are you still using the template?

For me, the answer is an emphatic “NO”, mainly because the spreadsheet I created has limited capability given the complexity in adapting the design methodology to complex anchor layouts, changes to the design provisions with each new code edition, and the need to add/modify data each time a new post-installed anchor product is introduced.

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Are the Load Combinations Balanced?

In April’s post about the Omega Factor, one commenter asked of the 1.2 increase allowed by ASCE 12.4.3.3, “Why do they allow a stress increase for allowable combinations? Seems unconservative for steel now that they have essentially balanced the ASD capacity with LRFD.”

To be honest, I have never spent much time analyzing which design methodology was more or less conservative. If I was designing with wood I would use ASD, and if it was with concrete I would use LRFD. Steel was strictly ASD early on in my design career, but LRFD usage grew. The question about balance made me curious. Are the load combinations balanced?

2009 IBC Basic ASD Load Combinations
2009 IBC Basic ASD Load Combinations
2009 IBC LRFD Load Combinations
2009 IBC LRFD Load Combinations

Of course, just comparing the load combinations would be meaningless. We know the LRFD combinations result in higher design forces. But those higher forces are compared to higher design strengths. So we need to normalize things.

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The Anchorage to Concrete Challenge – How Do You Meet It?

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.

Concrete Breakout in a Shallow Slab

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.

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