PEMB Shear Breakout 1

[NedHead]

Currently designing a pier foundation for a PEMB building, and am trying to wrap my head around best practices for shear transfer between the anchor bolts and the pier. The column I'm looking at has 97k (ult) of compression alongside its worst case shear of 37 k (ult). After looking through a bunch of threads on this topic I'd like some opinions on a few questions I have:

1. Shear breakout is controlling for any reasonable pier size, leading me towards shear breakout reinforcement per Widianto. Is there any advantage I can take for the heavy compressive load I have on this column alongside the reinforcement. Intuitively it feels like I should get some benefit.
2. Widianto outlines the 20ksi limit for nodes at seismic hooks, which is quite limiting. I'd like to take advantage of the typical ties alongside additional stirrups, and I don't overly trust schemes that call for alternating ties to actually be built that way. This previous eng-tips thread mentions a study that the hook alone (not wrapped around a transverse bar) is enough to fully develop the bar. It seems that the 20ksi is very limiting, and maybe this just points to specifying stirrups darn near anytime you need shear breakout reinforcement. Curious to hear what others typically do.
3. In normal building design I'm well aware of the issue of shear transfer through anchor rods alone, and that only 2 bolts can be relied on due to the oversize baseplate holes without welding washers to the plate. On PEMB buildings, I've yet to meet an engineer who specifies weld washers to the baseplates, saying it's on the PEMB designer to get the load into the anchor bolts. I also have yet to see any welded washers once the building is erected, but with quick numbers you clearly need more than just two of the anchor bolts for shear transfer. Am I missing something?

 

[driftLimiter]

1. Widianto is the best way to design anchor reinforcement that I have seen yet. Its tedious but effective. I don't think the compression is going to do much at a basic level. Perhaps some Mohrs circle effect on the stresses but I have not seen that addressed.
2. Yep stirrups everytime for piers. Since the Widianto is the only available 'guide' document for this I have always tried to follow it closely.
3. I have learned not to trust the PEMB designers specification for anchor rods. Recently I have had to spec Gr. 105 rods to get it to work when they are calling for Gr. 36. But I still use all the bolts, no welded plate washers. This is simply the approach my office has taken for PEMB anchor bolts and I'm sticking with it because like you say we've never seen plate washers, and we don't have the luxury of specifying larger diameter rods.

 

[straub46]

As an FYI for #3, the PEMB engineers assume all the anchor bolts work in shear. They also only provide a standard hole for the anchors (not oversized), hence why they don't spec or call out plate washers.

Source: former engineer for one of the largest PEMB manufacturers in the states.

 

[KootK]

Source: former engineer for one of the largest PEMB manufacturers in the states.

Thanks for that. Are you the former engineer or is it someone else in your network? I just like to know what we've got kicking around here for resources.

 

[straub46]

I am the former engineer. I reviewed and sealed drawings in 5 states. That time working there has vastly helped me now that I'm back in the consulting world.

 

[bones206]

I like to bundle all the stirrups and ties in pairs. It helps with capacity and helps emphasize the alternating hook ends. Especially helpful in shear since most of the load transfer is near the base plate and you can only fit so many ties at the top of a pier.