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Re: Precast Column Base Connection[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Precast Column Base Connection
- From: "Jake Watson" <jake.watson1(--nospam--at)gmail.com>
- Date: Wed, 4 Oct 2006 13:27:07 -0600
I generally am a "high seismic" guy being in Salt Lake. As was pointed out to me in another email, the 200Ag number is quite easy to reach if you simply splice the vertical reinforcing. Precast concrete doesn't have the best track record in earthquakes and in my opinion it's because people avoided the "typical" cast-in-place details. If I had my way, all tilts would be done with splice sleeves in seismic zones.
That said, there are many ways to accomplish the ductility of cast-in-place and still take advantage of precast's efficiencies. Even if you are in a low seismic area, I feel good engineering practice looks towards the reasonably good track record of cast-in-place.
Remember that precast relies on distinct and often non-redundant and non-ductile connections. You need to be absolutely sure you understand all potential load cases before you bet the farm on a couple of anchor bolts (or studs). What are the potential consequences if your connection fails? I think precast could vastly improve its performance by using ductile connections.
Off the soap box....
Jake Watson, P.E.
Salt Lake City, UT
Sounds like we're all in agreement regarding the 200Ag tension tie requirement for the base connection. I don't really disagree with your "defensive tone". In my case these columns do not transfer moment to the foundations, only gravity axial load and possibly a little shear. FWIW we are the specialty engineer for these columns on this particular project.Feedback on past projects where we have used the splice sleeves has been mostly positive. Only negative comment has been in regards to the cost. We have stacked numerous tilt-up panels using these connections. I would think twice before using the splice sleeves for a column base connection. Seems too easy to have misalignment of the dowels in the footing, which would certainly cause erection issues.Mark E. Remmetter, P.E.Steinbicker & Associates, Inc.
From: Jim Getaz [mailto:jgetaz(--nospam--at)shockeyprecast.com]
Sent: Wednesday, October 04, 2006 8:05 AM
Subject: Re: Precast Column Base Connection
The precast community thinks that the 200Ag tension force requirement in ACI 318 is something that "all those cast-in-place guys on the committee force on precast" to keep precast a pimple on a gnat compared to the rest of the construction industry. Do you detect a defensive tone, here?
Most precast column to foundation joints are designed as pins, so require shear capacity only. If they are designed as carrying moment, it is easy to do an interaction diagram to determine how much steel is required for the moment, usually much less than minimum steel for a compression member and much less than 200 Ag. It is even easy to design a splice in a column as a pin – if the location is chosen properly, it does not change the moments in the rest of the column much.
If the connection needs to carry substantial moment, NMB Splice Sleeves and Erico connectors work well. Even Erico will say that NMBs develop a larger overcapacity (150% instead of 125%) of the strength of the bar, which may be an issue in a high-seismic area.
As for your comment that NMB Splice Sleeves "Sure makes connections come together easily," our erection crews could not disagree more. They put up with them where they have to (e.g., shear walls), but they sure prefer anchor rods. And they prefer enough anchor rods to hold the column in place so the hook can move on to the next piece while others grout the Splice Sleeves.
Most EORs with whom we work leave the design details to a designated (specialty structural, etc.) engineer. Every precaster has run across the same problems and devised different ways to solve it and cannot bear to think of the way another company would do it. You may want to allow that to happen here. You'll get a better executed detail because the low bidder will do it the way he likes, and your owner will get a lower price because the low bidder will do it the way he likes.
James L. Getaz III, P.E.
Precast Concrete Engineer
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