Correct application of Omega for equipment anchorage to concrete per ASCE7-10 Chapter 13

[mrsutton]

We've been getting weird results when anchoring equipment to concrete with the application of Ω<sub>0 at the lateral force E. We used to follow the code literally but after getting enough large equipment (over 25k) that no reasonable number of post-installed anchors would work for) and some nonsensical results (net tension on the anchors, but none on the bracket itself)we decided to apply Omega to the net anchorage forces--which we believe was the intent of the code when overstrength was added at the last minute. The code as worded states that Omega should be applied at the lateral force, but under the previous code concrete anchorage that didn't preclude possible brittle failure was simply reduced to 40% (1/2.5 = 0.4). Our opinion was that keeping that basic DCR relationship was the intent of the code writers.

A local city has come down on us for our "unorthodox" interpretation of the code, and they have the literal wording of the code to back them up. SK Gosh's approach seems to agree with the wording of Chapter 13--but I'm not sure any of these code writers have actually anchored hundreds, or perhaps thousands of pieces of equipment, per their own reference back to 12.4.3 and run into some of the design consequences of the code.

Has anybody else run into issues with overstrength as it applies to seismic equipment anchorage? I'd like to hear any comments and/or opinions.</sub>

 

[KootK]

This thread should help: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[nutte]

I agree that your interpretation is "unorthodox." Despite what you think the intent is, the wording is clear.

No argument on the ridiculous results. There seems to be a disconnect between the code writers and practicing engineers, and then between those engineers and their clients, the owners tasked with paying contractors to install larger concrete supports and deeper, heavier anchors.

To reiterate a point from the linked thread, this link (

http://skghoshassociates.com/SKGAblog/viewpost.php?id=49)

shows Omega as a multiplier on the lateral load, not on the anchor force.

 

[amec2004]

I have a couple comments

1. For large heavy equipment, it's not wise to use post-installed anchor like HILTI, use cast-in-place anchor bolt instead

2. Omega factor needs to apply to anchor shear force if you choose ACI 318-11 D.3.3.5 Option c) as the option to comply with additional seismic requirements

3. You can use anchor reinforcement like horizontal hair pin to get the design pass

anchor bolt design per ACI 318-14 and ACI 318-11 crane beam design

http://asp.civilbay.com

 

[mrsutton]

Just to respond to everybody...

I asked SK Ghosh for guidance on the application of Omega for equipment anchorage. He basically said we should apply it per the letter of the code and that it was really ACI's, not ASCE's, call. So it appears we'll be reverting our spreadsheet back to the application of Omega at the lateral force regardless of the design results.

I'm currently working on the post-installed anchorage of a 45k tool on an existing 4" slab on grade (lab owner doesn't want sawcutting or facility downtime). This is going to be interesting.

 

[SAIL3]

getting any reasonable post-installed AB capacity in a 4" slab is going to be really difficult....effective embedment may only be 2 1/2"....what is the tension on the AB with the omega factor applied to the seismic load??