Anchorage for wind design - do you use ultimate or service loads?

[SeizeTheMoment]

As I understand ACI and HILTI calculations uses LRFD, but I was not 100% sure if this only applies to seismic design. For anchorage design where wind governs, is service loads allowed? i.e. 0.6xUltimate?

Thanks in advance.

 

[Harbringer80]

If you are talking about what loads you use for concrete anchor design for wind or what you would enter into something like Profis, it is an ultimate level load (LRFD).

 

[ANE91]

Ok…Let’s start with some basic assumptions. You’re anchoring into concrete; you’re dealing with wind and not seismic; it’s a commercial building in the US—say RCII—with a target useful service life of 50 years. Right?

The anchor needs to resist all the loads that we expect it to see. We expect it to see service-level wind loads (10+ year MRI) and, with some small probability, ultimate (or strength) -level wind loads (700 year MRI).

We do service-level analysis for serviceability criteria, like member deflections and building drift and such. Anchorages into concrete are important; we want the building to stay put, so movement at the anchor is generally not tolerated. If your anchor fails at ultimate loads, then it could mean that the whole building comes down, absent redundancy. We know for a fact that connections are failure-critical elements. For these reasons and more, it should be clear that the anchorage needs to be designed for ultimate loads.

To answer your original question: no, an engineer is not allowed to under-design an anchor.

 

[dold]

@ANE91 Good explanation but hopefully that's not how you talk to colleagues too.

 

[SeizeTheMoment]

Thanks all. It is a louver gate anchorage. Unfortunately the sub already erected everything before sending calcs and I wanted to make sure I didn't miss any exceptions before telling them to tear a bunch of concrete apart.

 

[EZBuilding]

Generally, I agree that LRFD should be utilized when performing ACI based anchorage design, but that is not the only available approach to engineers when designing connections to concrete. There are a lot of anchorage products which provide allowable shear/tension capacities. It is my understanding that these are usually tested values divided by a factor of safety (I recall typically seeing 3 to 4). It is then typical practice to utilize service level reactions to compare to the products allowable capacity (while meeting the manufacturers recommendations i.e edge distance, spacing, etc). It would generally be UNconservative when using these product data sheets to utilize the ultimate capacity.

10 year MRI return wind speeds can be utilized for serviceability checks - which is not the same as a service level reaction.

If you design a cladding wall in ASD, you wouldn't be expected to go to LRFD just for your connection checks.

 

[canwesteng]

You wouldn't use service level loads, you would use ASD loads. I don't think ASCE 7 even gives service level loads for wind anymore, or at least doesn't give firm guidance. That is likely the confusion here, because for dead+live ASD is the same as SLS, but this isn't the case for wind or seismic.

 

[Aesur]

Thanks all. It is a louver gate anchorage. Unfortunately the sub already erected everything before sending calcs and I wanted to make sure I didn't miss any exceptions before telling them to tear a bunch of concrete apart.

keep in mind, you could always have them do a load test. We have had a few instances where something didn't calc out on paper, but load testing showed way more than enough capacity even when considering safety factors.

 

[human909]

keep in mind, you could always have them do a load test.

Not a bad idea. Though on the flip side I've seen epoxied anchors fail at significantly lower loads than their required load.... They are particularly dependent on suitable placement, hole cleaning etc....

 

[SeizeTheMoment]

keep in mind, you could always have them do a load test. We have had a few instances where something didn't calc out on paper, but load testing showed way more than enough capacity even when considering safety factors.

Hi, I've never actually asked the contractors to do a load test. What kind of testing will they do and how does it work? Is it tension and shear testing of the concrete and anchor that's already built? Thanks

 

[driftLimiter]

Hi, I've never actually asked the contractors to do a load test. What kind of testing will they do and how does it work? Is it tension and shear testing of the concrete and anchor that's already built? Thanks

I've just done tension load testing and basically said if it's working for tension it's working for shear. This was a case of epoxy anchors so the limit states for shear are not so much related to the bond strength etc.

 

[Aesur]

Hi, I've never actually asked the contractors to do a load test. What kind of testing will they do and how does it work? Is it tension and shear testing of the concrete and anchor that's already built? Thanks

Chapter 17 of the IBC has load testing requirements. Most that we have done so far consisted of loading a roof or other members to test gravity loading.