Review for Loads Imposed Responsibility 4

[bookowski]

General and specific question about reviewing a loads imposed submittal for attachment to a building that you've designed:

- Does the responsibility of the designer for the equipment/system that is loading your structure include the anchorage/anchors? What if the failure is a pullout from your slab/building?

- Specific situation that is making me question this: I have a submittal for a dunlop mastclimber, essentially a small hoist system for installing facade. In order to install this over a neighboring building the designer is using outriggers which support a platform that the hoist is supported on. This will be a 20 story system sitting on the platform. For outriggers they are using double angle hangers that get expansion bolted into our concrete columns and shearwalls. Their calcs are good, including pattern live loads and wind etc. and all applicable spacing and edge distance factors - but their anchors are calc'ing out to in the 0.95 to 1.01 range which they are calling ok. I'm not so brave and if it was me supporting a 20 story hoist I would not trust expansion anchors to that level of confidence. I spoke to the designer and he seems very competent but I suggested using collars around the columns and through bolts through the wall, he said that they do these all over the county and this is what they always do. He said that he can add 2 more anchors if it makes me feel better but it's not necessary. I've reviewed the base building (cols and walls) and there is no issue with the loads imposed. Do I have any responsibility for the capacity of the anchors?

 

[CELinOttawa]

I've occasionally left deliberate, obvious "errors" that don't change the resulting design in a design submittal for them to "catch"

We've started doing something very similar with submissions to the AHJ when on a high sensitivity structure. If they don't have some areas of particular concern to mark up, they tend to ask questions which just make the job take longer to permit. We can also prepare a good chunk of our replies in advance of receiving the questions. *sigh*

Thing is, while it may work, it leaves me feeling professionally dirty.

 

[SwinnyGG]

Would you care to expound upon that? It sounds like you have a lot of experience with the matter. I saw your next sentence also, but I'm just try to get you to discuss it a bit more, if you're so inclined.

My preference for chemical anchors is based on a couple of factors. I suspect that if a large scale what's-the-best-concrete-anchor discussion were to break out, many people who have an equal (or greater) amount of anchor experience would disagree- so I say the following with the full understanding that what works for me may not work in the same way for other guys who don't use the same contractors as I do, don't deal with the same equipment or loading conditions, or whatever.

My preference for chemical vs. mechanical anchors really comes down to one thing- failure modes. Either type of anchor can fail because of bad installation; mechanical anchors don't like misshapen or misaligned holes, chemical anchors don't like dirty holes, etc. But mechanical anchors are, in my opinion, much more susceptible to failures due to loads not accounted for during the design phase.

In my line of work, by far, the biggest culprit is vibration. I find quite often that while the structural engineer who designed the building for customer X was very skilled and knowledgeable, and did an excellent job of accounting for vibration in the structure design, the customer X process engineer who sends me an RFQ has little or no idea what type of vibration loading the equipment he is buying from me will be exposed to through the connection with the floor, if he is even aware that the floor vibrates at all. You wouldn't believe how often I have to explain things like "well, the reason that PLC needs to be replaced is because you moved it from the isolated stand we supplied and bolted it to the floor right next to your stamping press, and the ICs inside have been vibrated into dust as a result, and no, you're not getting a free PLC under warranty"

When you're dealing with an application where the vibration transmitted through your interface is high or unknown, chemical anchors are an easy choice.

The second reason is just a personal preference of mine- but I don't care much for wedge type anchors because the shear and tensile failure modes are not independent. This is basically the same thing I said above.

With a wedge type anchor, if the system fails in shear and there is movement at the interface, the anchors get pulled out of alignment with their holes and can then be susceptible to a very significant reduction in tensile capacity. A chemical anchor does not exhibit this behavior when installed correctly. Shear failure of the system does not pull the individual anchors out of alignment with their bores, and the anchors do not lose any tensile capacity until there is enough displacement that they are bent or they completely fail in shear.

I will also add this: there are developments in chemical anchor systems over the last few years that have basically eliminated the issues with hole cleanliness and correct chemical anchor install.

Disclaimer: I do not work for Hilti. I am, however, a conduit through which a fair amount of money flows from my customer into Hilti's coffers.

Hilti has chemical anchor systems which require ZERO post-drill hole cleaning, are compatible with water saturated concrete, and can even be installed when there is standing water in the anchor holes. I am normally very skeptical when a sales rep calls me and says he's about to change my life, but the local Hilti guy did when he introduced me to Hilti TE-CD drills. Amazing stuff.

 

[TehMightyEngineer]

Thing is, while it may work, it leaves me feeling professionally dirty.

The only thing that I feel is dirty is that we have felt a need to do such a thing to get a reasonable submittal review. I've seen multiple times where the reviewer was clearly reaching to find something worth requesting a resubmittal or marking it "approved as noted". Even so far as having one critique my spelling. Of course I would never try to hide anything and if a reviewer is simply stopping at the first error they find then it shouldn't be on me that they're not performing their job appropriately. I've also learned that the more information you provide the more likely you'll get questions; submitting just the output of a canned program is always easier to get through a review than the same output plus 20 pages of hand calculations.

It sounds like the OP has the most beneficial and preferred method of reviewing IMO; verifying that the design is accurate, in line with the specification, and performed by a competent individual. Thankfully for every poor reviewer I've faced there have been a greater number of quality reviewers. It's especially beneficial if you can get someone with a "teamwork" attitude who is willing to work towards a common goal where you help save them time/money and they help save you time/money.

Of course, I'm not suggesting that a review of a design isn't appropriate. I value having someone else looking over my work to catch stupid mistakes and would much rather have to change a drawing then have to re-manufacture a piece.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[dik]

Ron should comment. A few comments:

I think that once you have reviewed the calculations and commented that they are marginal, then you have opened the door to liability. It's like opening Pandora's box. You should accept the design provided by a skilled professional.

If seriously deficient, and life threatening, another matter.

By commenting about the adequacy, in my opinion, you have accepted responsibility. If no change, and a failure occurs then you 'knew' about the problem. If additional reinforcing is added, the Owner may be open to a legitimate 'Extra to Contract'.

You could have informed him that the design is strictly his and outside your scope of work and that he should verify that all loading combinations should be accommodated. The way things presently stand, you may have assumed liability.

Dik

 

[MotorCity]

Most of the time, I try to pawn the anchor design off on the equipment manufacturer - basically throw it to the wall and see if it sticks. The reason is that, I have no idea what the dynamic forces of the equipment are, the center of gravity, etc. and when I ask the manufacturer, they hardly ever are able to provide such information so I am left guessing and making conservative assumptions. The flip side is that the equipment guys are only concerned with their equipment and are not at all versed in anchorage design according to Appendix D. Heck, there are structural engineers that have a hard time following Appendix D. Usually, my arm gets twisted into designing the anchorage but I cover myself by explicitly stating the assumed design loads with a few CYA notes.

 

[Archie264]

jgKRI,

Thanks for the insight.