Our tilt-up contractor has requested to eliminate panel to panel connections. Our connections are lap plates that connect to each panel separately so there is no differential out-of-plane movement, but the panels can contract and expand. It does seem that the panels will move together with wind and seismic forces so there is no load transfer across the joints.
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Tilt-up panel to panel connections 1
- Thread starter rlmpellc
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Seismic maybe - if the participating mass of each panel is exactly the same (that's the panel and everything connected to it, of course)
Wind - not a chance. Wind isn't a uniform force. We analyze it that way out of convenience, but the actual loading is random, chaotic, and uneven.
What forces did you design those connections for? Where did that come from?
Wind - not a chance. Wind isn't a uniform force. We analyze it that way out of convenience, but the actual loading is random, chaotic, and uneven.
What forces did you design those connections for? Where did that come from?
I'd only be okay with this if:
a) You don't need in plane shear transfer capacity between panels and;
b) The owner has agreed, in writing, that they don't care about the architectural and building envelope consequences of adjacent wall panels flexing differentially out of plane. Like phamENG said, the assumption of uniform load is a myth in many cases.
a) You don't need in plane shear transfer capacity between panels and;
b) The owner has agreed, in writing, that they don't care about the architectural and building envelope consequences of adjacent wall panels flexing differentially out of plane. Like phamENG said, the assumption of uniform load is a myth in many cases.
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- #4
1. Are these single wythe panels or sandwich panels?
2. What is the overall height?
3. Many times when have been involved in these discussions there are multiple floors and the EOR has rigidly connected their diaphragm assemblies to the wall panels.
4. Is there a interior wall with finishes or is the concrete exposed on the inside?
5. I would expect more surface imperfections with tilt v. precast. I am not certain how much difference the connections will make aesthetically on the exterior face. In our region we do not usually see the wind load deflections we predict in our analysis, but in some cases we see deformations due to temperature differences. Does the owner have an opinion or is there a meaningful savings.
I prefer connections, but our precast panels are never as wide, so that is usually driven by seismic req'ts.
2. What is the overall height?
3. Many times when have been involved in these discussions there are multiple floors and the EOR has rigidly connected their diaphragm assemblies to the wall panels.
4. Is there a interior wall with finishes or is the concrete exposed on the inside?
5. I would expect more surface imperfections with tilt v. precast. I am not certain how much difference the connections will make aesthetically on the exterior face. In our region we do not usually see the wind load deflections we predict in our analysis, but in some cases we see deformations due to temperature differences. Does the owner have an opinion or is there a meaningful savings.
I prefer connections, but our precast panels are never as wide, so that is usually driven by seismic req'ts.
We used to spec the lap plates as OP describes. Many many dozens of big box stores and warehouses. Client requested we remove them and they agreed to essentially Koot's point b). I was skeptical at first, mainly because I was miffed about the penny pinching. Then I came around to realize that there really isn't that much benefit to them unless you have some sort of brittle finish crossing panel joints. Otherwise a plain panel joint with backer rod and sealant can tolerate whatever the minimal differential movement is under service wind....probably. Hell, I wouldn't be surprised to see more movement due to thermal effects than you'd see from service wind, on the off chance you have a panel painted black next to a panel painted white, for an extreme example. Just my 0.02.
Not OP's question, but in-plane panel-to-panel stitch plates (ostensibly to increase overturning resistance by tying panels together) are something we never budged on. Contractors realllly hate these types of fully welded "rigid" connections because they have to wait weeks/a month (?) before the concrete shrinks until they can re-mobilize their welders to finish up. Otherwise, they'd have already installed insulation and started interiors. Sometimes we'd give them the option to replace those with hefty uplift connections to the foundations, which typically involved an obscene amount of steel. But that's a big can of worms. I've argued many times in the office(s) that the forces on these connections are faaaaaarrrr higher than one might calculate with a quick FBD or something. R3D models have substantiated my heretical claims. It's kindof a brain-teaser when you start looking at the assumptions you'd make to set up the calculation. I've read a couple of articles and discussions by TCA folks discussing this. They lay out various methods of calculating OTM and RM, but I've never found a definitive 'answer' by the experts. This opens up an even deeper rabbit hole of the true seismic response of tilt-up construction which could be an entire thread on it's own...
To illustrate OP's situation:
Not OP's question, but in-plane panel-to-panel stitch plates (ostensibly to increase overturning resistance by tying panels together) are something we never budged on. Contractors realllly hate these types of fully welded "rigid" connections because they have to wait weeks/a month (?) before the concrete shrinks until they can re-mobilize their welders to finish up. Otherwise, they'd have already installed insulation and started interiors. Sometimes we'd give them the option to replace those with hefty uplift connections to the foundations, which typically involved an obscene amount of steel. But that's a big can of worms. I've argued many times in the office(s) that the forces on these connections are faaaaaarrrr higher than one might calculate with a quick FBD or something. R3D models have substantiated my heretical claims. It's kindof a brain-teaser when you start looking at the assumptions you'd make to set up the calculation. I've read a couple of articles and discussions by TCA folks discussing this. They lay out various methods of calculating OTM and RM, but I've never found a definitive 'answer' by the experts. This opens up an even deeper rabbit hole of the true seismic response of tilt-up construction which could be an entire thread on it's own...
To illustrate OP's situation:
- Thread starter
- #10
The detail posted by dold is the connection we have drawn for all of our tilt-up projects. There are no objections to the detail other than the contractor does not want to have any connections, loose or fixed. He stated "most engineers" say that no connection is necessary. I am looking to see what "most engineers" think about not having any connections at the panel edges, other than corners.
Thanks for the replies.
Thanks for the replies.
I don't find the PCI example overly helpful except for the very rare case where the wall has not been butchered by archs. The wall lines we see usually look like the image below. The topic of determining connection forces is very poorly covered in my view.
Brad: Agreed. PCI does not cover connection very well considering the importance they play in precast. However I assume you use the second method in PCI for analysis since you appear to have the panels connected and the openings centered in the panels.
If these are shear walls, I would want a mechanical mechanism to transfer the in plane shear load from one panel to another. Otherwise, "in theory", one panel would have to tilt until it came in contact with the adjacent panel setting off a chain reaction to transfer shear. I've never seen tilt up panels without these plates. They also do a pretty good job keeping the panels aligned out of plane.
rowingengineer
Structural
Typically we don't have these plates on our panels, we on have a bracing angle is similarat the top of the panels to ensure the panels work together in shear.
Does thermal expansion/contraction (horizontal direction) really happen to 8~15ft wide walls?
I was surprised to hear that welding should be done weeks later after erection.
I wonder temperature & shrinkage should be considered for wall-wall connection design.
Generally, this is not considered for precast walls.
This topic could be related with expansion joint.
When these exterior walls are 500ft long, do we need to consider expansion joints?
As this connection is relatively flexible (for example, JVI Mini V connectors at 10ft spacing),
I think expansion joint is not required.
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- #19
I rarely connect panels together. I learned my lesson the hard way when I first started designing with tilt-up. I connected a series of panels together, thinking it would be a benefit (though I did not have anything specific to back up that thought). I ended up having the connections fail due to shrinkage. If I need a longer section of panel due to shear demand, I will tie a couple together, other than that, I do not connect at all (except in the corners of the building). I have never seen an issue of panels deflecting out of plane enough to cause a problem. We do specify that joints in finishes align with the panel joints, but that is more for shrinkage concerns than out of plane.
How much out of plane deflection do you think you will have? Will your detail resist the load required to create such deflection? You say yourself that you have not designed for any specific load.
How much out of plane deflection do you think you will have? Will your detail resist the load required to create such deflection? You say yourself that you have not designed for any specific load.
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