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?

 

[KootK]

I think that you're in the clear here, as much as you can ever be. The anchors can't be properly specified without considering the concrete failure modes. Of course, slab moment and shear matter too so it's a bit blurry as to where their responsibility ends and yours begins. If you have records on file that show that that the designer did the anchorage checks, I'd be comfortable with that.

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.

 

[bookowski]

Thanks. That's what I figured but the thing makes me nervous so wanted some confirmation.

 

[KootK]

I'm in NY next weekend. I'll check on it for you during my city tour

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.

 

[Lomarandil]

I am not knowledgeable enough to speak to the liability question..

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.

 

[XR250]

@Lomarandil

What type of anchors would you suggest?

 

[SLTA]

Three thoughts.

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!

 

[Lomarandil]

XR250, I'd have to have a better idea of the detail to make a good suggestion.. but a few thoughts below:

- 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.

 

[bookowski]

There's a picture on the mastclimber website that shows basically the same setup, scroll all the way down 'cantilever bracket'. The current design shows 8 anchors for the top connection (the diagonal hanger).

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.

 

[wannabeSE]

Am I missing something or misinterpreting the situation. As I read it, the contractor's engineer has provided calculation that have the expansion anchors 1% overstressed, but otherwise the calculations are fine and the structure has plenty of capacity for the loads. But, you want to have the contractor's design revised because you don't like expansion anchor used for this type of construction loading.

 

[KootK]

Jeebus. Now that I see what you're dealing with, I can see why you're a bit squeamish. With the 12 anchor situation shown below, I only consider the inner six effective in resisting tension. Has that been accounted for in your situation? Not that I'm changing my mind about it being your wheelhouse mind you.

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.

 

[kipfoot]

This is one of those cases where communication is key.

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.

 

[SwinnyGG]

Finally a structures thread where my input wouldn't be totally completely useless..

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.

 

[TehMightyEngineer]

I've seen both sides of this type of situation. For this it seems like the relatively small cost of a few anchors can go a long way to ensuring against a major failure.

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

http://www.americanconcrete.com

 

[SwinnyGG]

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

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.

 

[CANPRO]

where would the extra anchors go? Unless you're modifying the mounting plate it looks like you're just adding more anchors into the original failure cone of the concrete (based on Kootk's picture). I guess it allows for a couple of anchors to be installed improperly without failure...but if they put a couple in wrong, they're likely all going to have some issues.

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...chnical_information_ASSET_DOC_LOC_1543414.pdf

 

[Archie264]

Very interesting thread.

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.

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.

 

[calvinandhobbes10]

Fascinating question and thread. Thanks for posting.

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.

 

[bookowski]

Thanks for all the opinions, looks like my squeamishness is warranted.

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.

 

[bookowski]

kootk - I also recommend walking the highline if you happen to be in that area, some cool buildings to see from there (HL23, 100 11th ave nouvel bldg, iac center etc) but just a generally nice walk through nice areas. I'm sure your tour guide will have his own ideas.