Eng-Tips is the largest engineering community on the Internet
Intelligent Work Forums for Engineering Professionals
-
Congratulations waross on being selected by the Tek-Tips community for having the most helpful posts in the forums last week. Way to Go!
Miami Pedestrian Bridge, Part II 55
- Thread starter hokie66
- Start date
- Status
-
1
- #42
-
1
- #45
GregLocock
Automotive
Lnewqban - you raise two points I was interested in. It is essentially an I beam with holes poked in the web, which is not a great recipe for torsional stiffness, and the loads while installing or constructing a bridge have always fascinated me. In this case trundling down the highway with the thing on a couple of platforms seems like a design case nobody could really pin down without measuring the road profile etc.
As an expansion, if the I beam was twisted at some point, would that stress the tendon anchorages in ways that would not normally be considered in the design process?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
As an expansion, if the I beam was twisted at some point, would that stress the tendon anchorages in ways that would not normally be considered in the design process?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
-
1
- #47
I think #11 failed because one of the tensioning rods ruptured, leaving an asymmetric preload. It may not have helped that the bottom end of the rod blew out some concrete. One thing that isn't clear to me is the way loads are transferred from the tendons in the deck to members #2 and #11.
-
2
- #49
EPCI-Steel
Structural
@ Lnewqban
I have a bit of experience with SPMTs. They are really a wonderfully designed piece of equipment, there are several manufacturers now but they all operate the same way. All of the axle lines during a normal transport are on the same hydraulic circuit which allows each axle to stroke up and down independent of adjacent axle lines while maintaining a constant bearing, so crossing normal bumps and slight elevation changes are no problem. But the axles only have 1ft of up/down stroke so there are limits. Also they talk about them in terms of the number of axles but there really are no axles, they have independent, two tire hubs on either side of an axle line.
And just because I think they are so cool, here's some more. Any hub can be isolated and raised, for instance if a tire blows out. They can turn the hubs on either side of the axle line independent from each other so you can walk the trailer nearly 90 deg from its long axis, or almost pivot in place, wonderful things.
I have a bit of experience with SPMTs. They are really a wonderfully designed piece of equipment, there are several manufacturers now but they all operate the same way. All of the axle lines during a normal transport are on the same hydraulic circuit which allows each axle to stroke up and down independent of adjacent axle lines while maintaining a constant bearing, so crossing normal bumps and slight elevation changes are no problem. But the axles only have 1ft of up/down stroke so there are limits. Also they talk about them in terms of the number of axles but there really are no axles, they have independent, two tire hubs on either side of an axle line.
And just because I think they are so cool, here's some more. Any hub can be isolated and raised, for instance if a tire blows out. They can turn the hubs on either side of the axle line independent from each other so you can walk the trailer nearly 90 deg from its long axis, or almost pivot in place, wonderful things.
-
1
- #50
OSUCivlEng
Civil/Environmental
It's pretty clear from the pictures they had a hydraulic jack attached to the PT rod of a compression member. I understand why it was needed for transport, but once it was placed on the permanent supports it went from a tension member to a compression member.
They were jacking a PT rod in a compression member because in order to de-tension it you have to actually increase the tension first? Isn't that just putting more compression force into a compression member? If,during post tensioning, you damage the member or the member's connection to the rest of the truss you're screwed because there isn't any redundancy. How many times have you heard of something getting "blown out" during a post tensioning job?
It seems to me it would take a pretty darn sophisticated structural model to account for the affect of all these post tension forces in addition to the forces in the truss to due self weight.
Build a steel truss, if you need extra weight use an oversized concrete deck. Sometimes stuff seems really cool on paper, but it sucks in real life.
They were jacking a PT rod in a compression member because in order to de-tension it you have to actually increase the tension first? Isn't that just putting more compression force into a compression member? If,during post tensioning, you damage the member or the member's connection to the rest of the truss you're screwed because there isn't any redundancy. How many times have you heard of something getting "blown out" during a post tensioning job?
It seems to me it would take a pretty darn sophisticated structural model to account for the affect of all these post tension forces in addition to the forces in the truss to due self weight.
Build a steel truss, if you need extra weight use an oversized concrete deck. Sometimes stuff seems really cool on paper, but it sucks in real life.
OSUCivlEng
Civil/Environmental
First lawsuit to be filed Monday.
-
1
- #54
OSUCivlEng, the point you make about the PT in #11 probably being intended for the transport phase and then detensioning for the permanent phase makes sense and has been assumed by several of us here. The timing seems odd to me, if that was the intent of the design and the construction sequencing, wouldn't they have detensioned #11 before Thursday if the bridge was set on Saturday. There is a video of someone from the construction company mentioning detensioning 2 cables on top of the bridge after the bridge was set. I think the video is from the day the bridge was set (Saturday). It is not clear whether he was referring to the 2 PT rods in #11 or 2 of the longitudinal tendons in the canopy/top chord, but either way it seems like it would have been done before Thursday. Does that mean that the work being done Thursday was not the normal design/construction sequence, but was some sort of remediation for another problem, maybe the cracking that has been reported? Any thoughts?
Following on from my last post... if the intent was to detension #11 then wouldn't #2 probably get the same treatment? The construction company video mentioned detensioning 2 cables, but we think that #11 and #2 had 2 rods each, 4 total, so did he mean 2 members, or was he referring to 2 longitudinal tendons in the canopy/top chord and not the rods in the diagonals?
A lot of unknowns. It will be interesting to learn the details when they come out.
A lot of unknowns. It will be interesting to learn the details when they come out.
-
1
- #56
Correct me if I'm wrong, but, with a prestressed compression member, the prestressing does not increase the compression loading... The member is loaded in compression until the prestressed value is reached before the stress increases in the section. Is that not correct?
Dik
Dik
racookpe1978
Nuclear
Nobody important (Computer)
Nobody Important said:
That (rough) compression-tension FEA sketch is logical, and explains how the top members are in great compression across greater length/limited thickness elements = classic buckling failure at the odd-angled connections of each member that is in compression. But - Why would the "missing" center vertical temporary vertical pier NOT be installed?
Does the analysis/design team truly think that a cable-stayed bridge only needs the cable stays and central pier for "looks"?
- Status
Similar threads
- Locked
- News