Review for Loads Imposed Responsibility
Review for Loads Imposed Responsibility
(OP)
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?
- 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?






RE: Review for Loads Imposed Responsibility
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.
RE: Review for Loads Imposed Responsibility
RE: Review for Loads Imposed Responsibility
If I'm feeling extra paranoid about something, I'll put a note below my submittal stamp to the tune of "anchorage to comcrete per appD or eqivalent by so and so and not me."
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.
RE: Review for Loads Imposed Responsibility
But I can wholeheartedly back you on your reluctance to use expansion anchors in such a critical application and pushed that hard. Expansion anchors in particular are very sensitive to installation conditions (especially if loaded in tension) and substitution between manufacturers.
You'd have to be careful to not absorb liability by getting involved, but I'd be wary at the very least.
In a different role where I was responsible for the anchors, I'd be very particular about the contractor drilling the holes properly (right size bit, located to avoid rebar so you don't get slop from a rebar cutting bit, cleaned thoroughly) and would be very cautious allowing substitute products.
RE: Review for Loads Imposed Responsibility
What type of anchors would you suggest?
RE: Review for Loads Imposed Responsibility
1. There's probably a big ol' factor of safety on those calcs/values, if that helps you sleep any better.
2. Why not just have them add the extra two anchors? But then, on the other hand, it gets you involved with the design of the anchor system and then comes the shared liability.
3. You could always ask for examples where the anchors were installed into similar conditions for similar heights etc, just to know that they have actually done this before.
Good luck!
Please remember: we're not all guys!
RE: Review for Loads Imposed Responsibility
- I have no problems with expansion anchors in shear.
I'm OK with expansion anchors in tension given the following:
- the system is redundant (no progressive failure if one anchor fails)
- the anchors are not loaded to 100% (accounting for all of the proper reduction factors -- sometimes designers miss reductions for different drill bit types or wet use)
- Anchors can be located to have a good probability of not needing to cut through rebar
- The contractor has experience using expansion anchors (nearly all do)
- The contractor is reminded of the importance of proper installation procedures
I don't know if 2 more anchors is added to 4 original anchors or 20, but that might be a viable solution.
Regarding the built in factor of safety -- the margin varies by manufacturer, but I was part of a project where expansion anchors in tension failed while picking concrete slabs (a well defined load). Installation issues were the eventual root cause, despite a pretty sophisticated contractor and crew being involved.
RE: Review for Loads Imposed Responsibility
http://www.dunlopmastclimbers.com/accessories/
When we spoke on the phone the add'l anchors were discussed but I didn't note it on the submittal or in writing for fear of implicitly blessing the anchor design. The current design is noted to avoid hitting rebar and that the actual layout will be re-checked by the designer.
I haven't reviewed one of these before and I'm surprised that they aren't more conservative based on the relative risk compared to the cost of putting a collar around the columns or some other more robust system.
Kootk - Is it your first trip to NY? This is in Brooklyn so it's not likely to be on your radar. If you want to walk by stuff interesting stuff/ in construction you could check out the 57th st jobs that are very tall/slender and very nearby is 53 W53rd St (nouvel moma tower) which is in construction and pretty cool looking and then the via 57th st pyramid job by big. If the weather is ok walk the brooklyn bridge, cooler than most of the new stuff that goes up in ny and great views all around.
RE: Review for Loads Imposed Responsibility
RE: Review for Loads Imposed Responsibility
This is my first real trip to NY, Great suggestions regarding the bridge and 57th street. Thanks. My guide Friday will be a friend/bridge construction nut that works for Kiewit NY so those suggestions will be perfect.
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.
RE: Review for Loads Imposed Responsibility
You have the submittal with calcs (of course sealed by someone else with a valid PE in NY or NJ or wherever), but you recognize that there are a number of assumptions and things that must go right so that the capacity in reality exceeds the capacity in the calcs.
Even if you've stepped clear of the liability, why not call the parties together: you, contractor, supplier, owner's rep, Hilti rep? Even for a conference call where you have the opportunity to say, "the calcs (done by the other guy who has responsibility for it) work, but they're dependent on x, y, and z. Let's pay extra attention to this." No drilling rebar. Good clean holes. Proper torque. Plenty of sharp drill bits on hand. What to do if one one of the holes is messed up for whatever reason.
Maybe you can have the hilti rep do some testing on the anchors somewhere on the structure before this gets installed. That might help answer Kootk's thought about he strain on the interior anchors.
This isn't just to make a page of minutes to cover yourself, but really to get everyone thinking that there's no redundancy here and it has to be right.
RE: Review for Loads Imposed Responsibility
I deal with equipment anchoring a LOT. But I do it from the other side- I'm the guy spec'ing equipment pads and anchors into your carefully designed, artfully constructed, perfectly maintained slabs/walls/columns.
I assume that a failure of an anchoring scheme is my fault 100% of the time- it is my responsibility to calculate such that the system will work without failure based on certain assumptions which I glean from the structural drawings (concrete compressive strength, rebar locations, etc) but it is also my responsibility to verify that those conditions called for in the drawings were actually put in place by the contractor. It is very rare that we anchor a piece of equipment without a representative core sample at minimum. Has the contractor done any verification of the as-built condition of their mounting area? If not, to me, this shows a lack of due diligence on their part.
With regard to wedge anchors vs. chemical anchors.. I have used both extensively. When properly spec'ed and installed their capacities are similar.
However.. I greatly prefer chemical anchors. Even with the requirement of cleaning holes before installation, in my experience chemical anchors are much more likely to be correctly installed.
Also, wedge anchors loaded in simultaneous tension and shear are at a disadvantage- a shear failure of the system which allows slip between parts can also directly result in a tension failure of the system, and a catastrophic result. Chemical anchors are less susceptible to this interaction.
RE: Review for Loads Imposed Responsibility
Many times I've been forced by reviewing engineers to modify my designs because I wasn't "conservative enough" or because they had to justify their review costs (I've occasionally left deliberate, obvious "errors" that don't change the resulting design in a design submittal for them to "catch"). Thus, I end up with over-designed reinforcement for very simple precast structures. The latest example was a valve pit which needed to be designed for an H20 vehicle load when the actual structure was embedded on a 45° slope and the only way to get a truck there was to crash one down the hill.
Plus, even when I've been conservative, a reviewing engineer might request "a few extra bars" and totally neglect that I had redundancy anyway. Conversely, if it's justifiable and saves a good amount of money, and I feel comfortable, then designing something to 99% of design capacity should be acceptable if the EOR has delegated the design responsibility to the manufacturer. Other then making sure I've not had a major goof in my design or failed to meet the project specifications then let me do my job.
On the flip side, and in regards to this topic, I fully recognize when you have non-redundant systems, highly dependent on installation QA/QC, with a high price tag in both life safety and money should anything fail. In those cases I would always try to identify any "cheap insurance" options. Additional anchor bolts, lifting inserts, a handful of rebar, etc. is not something I shy away from. Unless those anchor bolts cost $500 each, I'd throw in a few more without hesitation. So, I see nothing wrong with the overseeing engineer wanting a little extra redundancy in the anchor bolts if it's entirely reasonable; just by delegating design authority to a manufacturer or other engineer doesn't make the EOR immune and they should always have ultimate control over the project. I think you handled this appropriately.
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com
RE: Review for Loads Imposed Responsibility
This logic is exactly why a great deal of the equipment I deal with has 1.5 or 2 times as many anchors as required. Anchors are cheap, lives are expensive.
My peace of mind is, to me, worth another 2 hours of contractor labor and another $50 worth of anchors. Yours should be to you.
RE: Review for Loads Imposed Responsibility
I've been a fan of Hilti's Kwik-Hus anchor...seems to work for typical applications where expansion anchors would be used and the installation is much less sensitive. They also install faster and are more easily removed - the contractors I work with love them.
https://www.us.hilti.com/medias/sys_master/documen...
RE: Review for Loads Imposed Responsibility
jgKRI,
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.
Perhaps we're overdue for an extensive post-installed anchor bolt discussion, or perhaps there have been some already but, if so, I missed them.
Obviously it's an important part of what we do on a regular basis and I'd enjoy reading more about people's experience with the various products. Thanks.
RE: Review for Loads Imposed Responsibility
My thoughts:
1. Some Engineers simply seem to not get heartburn at max capacity, installation-sensitive applications, etc. Contractors obviously like these guys, and in general I'd be reluctant to interfere. As this designer said, he does it often, and he passed your phone-convo competency test. That's a good sign and not something I take for granted.
2. The graphic shows this thing flush-mounted to some tile-type facade. Even with concrete behind, that offset is much worse than flush plates against your concrete superstructure. (It of courses begs the question, how do they install the facade at the bracket locations.)
3. It's not your liability to check their anchors or ensure installation quality. You can emphasize its importance all you want but in the end the super and foreman you talk to are unlikely to communicate it at the right time to the laborer installing the anchors in such a way that changes how he has installed it a thousand times before. If anything, alert the special inspector to this, or ask for continuous inspection of these anchors.
4. Hilti (is contractor really buying these?) has been pretty good at accounting for installation quality in recent years, so that helps.
5. The lateral tiebacks may give some redundancy aginst anchor failure in tension.
Let us know how it ends up.
RE: Review for Loads Imposed Responsibility
The photo that kootk posted is correct, essentially the same setup except that on mine the anchor plate is flush to the concrete (that photo appears to show it over some type of facade). This is over an adjacent 1 story building and ours is 22 stories, so this will be 20 stories tall, >200ft. On our situation they are using two rows of 4 anchors (8 total) so if they add more anchors it would have to be by extending those rows and hopefully enlarging the gusset plate connection.
I have not asked them to change the design for essentially the reasons calvinandhobbes listed - I spoke to the guy and he seems competent, his calcs were thorough (I checked them and also ran the anchors in profis for a double check), and at the end of the day it's his design and it works on paper.
As far as qc - they are asking for pull testing of 25% of the anchors, the conc is spec'd at 7ksi and they've assumed 6ksi but almost all the breaks are coming back at 9+, I sent a separate email to the CM asking them to watch over this with special attention (although as c&h said the likelihood of that happening is small). I may ask them to get a hilti rep on site as well.
I'm all for designing to 0.99 but this one seems different. I'm not usually a wimp on site but I wouldn't go up on this thing. I hadn't thought about the tiebacks but you're right, if nothing else they'd probably give this thing a chance at hanging on and not totally going south - would hate to test out that hypothesis though.
Here's their detail for the hanger. They also provide an elevation of the plate showing anchor layout and another detail showing bolt layout in the angles.
RE: Review for Loads Imposed Responsibility
RE: Review for Loads Imposed Responsibility
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.
RE: Review for Loads Imposed Responsibility
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.
RE: Review for Loads Imposed Responsibility
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
www.americanconcrete.com
RE: Review for Loads Imposed Responsibility
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
RE: Review for Loads Imposed Responsibility
RE: Review for Loads Imposed Responsibility
Thanks for the insight.