An Explaination of ACI 15.10.5.3 (specifically b)

[engreg]

The code reads as such....
(b) Anchor or anchor groups shall be designed for the
maximum tension that can be transmitted to the anchor or
group of anchors based on the development of a ductile
yield mechanism in the attachment in tension, flexure,
shear, or bearing, or a combination of those conditions,
considering both material overstrength and strain-hard-ening
effects for the attachment. The anchor design tensile
strength shall be calculated in accordance with 17.10.5.4.

I understand that a) allows for the use of sleeves to provide the ductility.
and d) designs around overstrength * load.

I don't comprehend what the code intends by b. If anyone understands it fully could you explain.... I am designing a 3 story moment-framed building in SDC C and trying to get the base plates smaller, without using sleeves.

Does it mean I can disregard overstrength if I ensure a ductile failure.... the anchor bolts in tension, perhaps.

 

[TLHS]

I don't have the code in front of me to figure out the context, but I think this is the clause that allows for fusing in the attachment (i.e. the thing that attaches the anchor bolts to the structure). This might be something like a plate that fails in large deflection bending so that the load to the anchors is limited. You then capacity design the anchors.

 

[JStephen]

Agreed, it's saying you design for yielding, but that yielding can be in the item attached rather than in the bolt itself.

 

[TRAK.Structural]

I've done the sleeves before and I've also just given in and taken the massive reduction by using overstrength, but I've never seen anyone try to detail the "ductile" mechanism that this provision talks about. Obviously it is permitted to go down that route; I'd be interested to see what you come up with if you pursue this.

 

[driftLimiter]

I interpret this as analogous to AISC's 'Expected Strength'
If you have a BRBF for example, its expected strength is well known and could be used in lieu of overstrength for the connection(anchorage).

For a 3 story moment frame, I think it will depend on the system. If its an OMF your essentially guessing at the plastic mechanism. If its a special mf, you have a pretty good idea of the lateral load that pushes the beam over to plastic behavior.

Compare that level of load (take out any phi factors) with the overstrength level of load.

 

[hoshang]

the title should be read as

An Explanation of ACI 17.10.5.3 (specifically b)​

 

[engreg]

Good input guys. Typo on my end in the title.

Someone in the office was mentioning an interpretation of (b) as if the footing becomes unstable under the moment and lifts off, then the need for the anchorage to fail ductilely becomes mute, has anyone heard of this interpretation? as the footings aren't required to be sized by overstrength values.

 

[driftLimiter]

The line of reasoning makes sense to me but I don't see that as ductile mechanism.

Additionally I think we need to question if the foundation is able to 'rock' up and down vs complete overturning failure.

 

[engreg]

Right,

But it's also a question of redundancy. If the footing "would" overturn with the strength values. The attachments of the anchors past that point would seem inconsequential.

 

[TLHS]

Foundation stuff isn't really what that section is on about. In Canada at least foundation rocking is specifically dealt with in other ways. If you're going to capacity design, it generally requires you to do it the other way (i.e. design the foundation to the capacity of the lateral system). Using the foundation as the fusing element is pretty risky unless you have a really specific design. There are a lot of ways you can end up with way more capacity than you're expecting in a foundation from the geotechnical and soil weight components. You don't want to fuse the foundation structural components because they're really hard to inspect or repair.

I've seen the attachment fusing thing done, but not in buildings. I've seen it done for non-structural component types of stuff with reasonably light seismic clips. It doesn't seem all that practical in comparison to just building ductility into the anchor rods with sleeves or with anchor chairs.

 

[driftLimiter]

Here is a post I made previously on this topic with some good insight from around the world.

https://www.eng-tips.com/threads/overstrength-connection-at-grade.499266/

I think @TLHS makes the case for not using rocking foundation quite well.

 

[HTURKAK]

I could not look to the previous posts and hope that my respond is not a copy and paste of one of them.

I don't comprehend what the code intends by b. If anyone understands it fully could you explain.... I am designing a 3 story moment-framed building in SDC C and

17.10.5.3 Anchors and their attachments shall satisfy (a),
(b), (c), or (d).
....
(b) Anchor or anchor groups shall be designed for the maximum tension that can be transmitted to the anchor or
group of anchors based on the development of a ductile yield mechanism in the attachment in tension , flexure ,
shear, or bearing, or a combination of those conditions, considering both material overstrength and strain-hardining

Does it mean I can disregard overstrength if I ensure a ductile failure.... the anchor bolts in tension, perhaps.

You are expected to ensure that , when you design with item (b), ductile yield mechanism in the ATTACHMENT can develop . Lİterally , the anchor shall not fail before attachment.

Regarding design as per (d) Elements designed with the overstrength factor. This does not mean that the anchor shall remain elastic
during an earthquake. But it is expected that these elements will suffer less damage.

I have experience with Eurocodes and i have copy and pasted relevant clause .

EC-1998-4 3.5.2.3 Anchors

(1)P Anchoring systems shall generally be designed to remain elastic in the seismic design
situation. However, they shall also be provided with sufficient ductility, so as to avoid brittle
failures. The connection of anchoring elements to the structure and to its foundation shall have
an overstrength factor of not less than 1,25 with respect to the resistance of the anchoring
elements.

My vote would be , design the anchor bolts to remain elastic in the seismic design and try to provide reasonable stretch length .