Bracing of Welded I-Girder
Bracing of Welded I-Girder
(OP)
Dear forum member,
Currently I am putting together details of temporary bracing that will be used during the erection of the first new I-girder of a new bridge.
We have quite a few debates/discussions within our office about lateral torsional buckling mechanism, bracing types/configurations, and practical design & detail for temporary/permanent bracing systems. But we did not come to an agreement on this topic.
Therefore, I would like to put sketches of bracing schemes on this forum with hope to get additional opinions.
Please take a look at the attached PDF files and let me know your comments about whether or not the details make sense to you.
Following are my references:
“Is Your Structures Suitably Braced?” by professor Yura
“Guide to Stability Design Criteria for Metal Structures” by Ronald D. Ziemian
“Bracing System Design - Volume 13” by FHWA
http://www.steelconstruction.info/Bracing_systems#...
Several threads in www.eng-tips.com on Lateral Bracing of I-girder/Beam
Thank you in advance,
hdn32
Currently I am putting together details of temporary bracing that will be used during the erection of the first new I-girder of a new bridge.
We have quite a few debates/discussions within our office about lateral torsional buckling mechanism, bracing types/configurations, and practical design & detail for temporary/permanent bracing systems. But we did not come to an agreement on this topic.
Therefore, I would like to put sketches of bracing schemes on this forum with hope to get additional opinions.
Please take a look at the attached PDF files and let me know your comments about whether or not the details make sense to you.
Following are my references:
“Is Your Structures Suitably Braced?” by professor Yura
“Guide to Stability Design Criteria for Metal Structures” by Ronald D. Ziemian
“Bracing System Design - Volume 13” by FHWA
http://www.steelconstruction.info/Bracing_systems#...
Several threads in www.eng-tips.com on Lateral Bracing of I-girder/Beam
Thank you in advance,
hdn32






RE: Bracing of Welded I-Girder
If the beam's compression flange is laterally restrained, then it is braced.
RE: Bracing of Welded I-Girder
RE: Bracing of Welded I-Girder
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RE: Bracing of Welded I-Girder
I propose the torsional bracing detail below. I believe that it would have the following features:
1) Horizontal braces again.
2) Doesn't damage the concrete.
3) Convincing LTB restraint for both positive and negative bending regions (not sure if this is a simple span or continuous girder.
4) Ought to be about as economical as your details.
I'm assuming that the bottom flange of the existing girder either is, or could be, braced laterally.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Bracing of Welded I-Girder
Please find the attached PDF file of the plan showing the additional info.
Yes, the the existing bridge is in good conditions and can be of our advantage.
Thank you for your comments and please let me know if you have additional question/request.
hdn32
RE: Bracing of Welded I-Girder
Thank you for your comments and preference.
However, we are not allowed to drill and/or weld into the new I-girder.
For my curiosity, could you please let me know why you want to add the stub (wide flange) and have the new strut go perpendicular to the existing girder instead of connecting new strut to the lowest bolt hole on the gusset plate of new girder?
Regards,
hdn32
RE: Bracing of Welded I-Girder
RE: Bracing of Welded I-Girder
I used horizontal bracing between girders, arranged as shown, for three reasons:
1) limited transmission of vertical displacement between girders
2) healthy torsion lever arm on existing side.
3) healthy torsion lever arm on new side.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Bracing of Welded I-Girder
RE: Bracing of Welded I-Girder
Details 2, 3 and 4 rely on concrete anchors. The safe load rating of any reasonable size concrete anchor is going to be a (very) weak link in the temporary bracing. Also, if possible, temporary connections to the underside of bridge deck should be avoided - constructability issues. IMHO, all three of those details should be rejected.
For Detail 1, the comments above apply for use of a concrete anchor. Use of a thru bolt has possibilities. Drilling through an existing bridge deck is a challenge - a lot of heavy longitudinal rebar to miss - but it can be done. I'm not wild about having to work with the bolt under the overhang, but believe it is the best way. In Detail 1, suggest omitting the lower pipe brace assembly, just use one - the upper one. If you would be more comfortable using both pipe braces, it won't hurt - but I don't consider it necessary. Make use of the detail at each and every available gusset plate.
When the second girder line is set, the permanent diaphragms can be installed between girder line 1 and girder line 2. This should take care of LTB issues when the new deck is placed.
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RE: Bracing of Welded I-Girder
Again, thank you for getting back to me with your thought/comments.
Yes, we are leaning toward using detail 1 as you suggested.
As far as frequency of bracing, the I-Girder is okay with maximum 50ft & 30ft unbraced length for straight girder and curved pieces respectively. However, I will calculate the required strength & stiffness of the temporary braces, then I'd look at the capacity of the anchor/thru bolt connections and availability of gusset plate to make my decision. I will definitely be on the conservative side.
Regards,
hdn32
RE: Bracing of Welded I-Girder
Read about this one, with three fatalities, very similar to your project. It was national news 12 years ago: Link
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RE: Bracing of Welded I-Girder
There have been some good and important points made in the posts above. Are you going to erect the two lines of girders kind of in unison (side by side) from pier to pier or are you going to erect girder line #1 full length, then girder line #2 next? Tagging on to SRE’s last post, I like your scheme #1 best also. I would offer the following suggestion. Given the existing bridge curb detail, you could fabricate steel shoes which would fit over the top of the curb and down its inside slope and then have a second part bolted to the top curb shoe. This outer (2nd) part would have a horiz. leg that fit up under the curb, locking the shoe to the curb, maybe some shimming needed, and it could have a vert. limb which allowed bolting (pinning) of your adjustable length braces, at a more favorable distance apart, but still pretty much in a horiz. orientation. Then a few concrete anchors would be used just to fix the shoes to the curb. The anchors would not be taking all of the primary bracing loads. If the side by side erection scheme is used, you could just jump 6 or 8 of these curb shoes on down the line as permanent bracing frames are installed btwn. the girders and erection progressed.
RE: Bracing of Welded I-Girder
RE: Bracing of Welded I-Girder
@ Dhengr: There are actually 5 girder lines and we are doing "side by side" erection. The 2nd line will be picked with permanent X-bracing/Diaphragms installed on both sides. Thank you for your opinion and suggestion of the bracket/shoe detail.
@ BridgeEI: There may be a couple locations along the entire girder line only. Thank you for your question. Should you have additional comments and/or questions, please let me know.
RE: Bracing of Welded I-Girder
RE: Bracing of Welded I-Girder
Please be cautious with any lateral bracing arrangement - you have to be sure that the steel structure can deflect as anticipated when the deck is placed. KootK had suggested a moment connection between the stub and the (existing?) girder stiffeners. That may not be wise unless I'm coming into this thread a bit late and am missing something. Perhaps all of the temporary connections to the existing bridge are to be removed before the deck is placed, making my comment moot.
Ralph
Structures Consulting
Northeast USA
RE: Bracing of Welded I-Girder
You'll have to be more specific RHTPE. I recommended a detail specifically designed to allow differential girder deflection. It involved a stub wide flange moment connected to the new girder as shown in my sketch above.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Bracing of Welded I-Girder
@ Ralph (RHTPE): We are concerning about the stability of the 1st piece after crane releases. Yes, all temporary braces (to existing bridge) will be removed prior to the pouring of deck. Thank you.
RE: Bracing of Welded I-Girder
If the edge of deck is thick enough, attach a single vertical anchor plate to the outside edge of the deck. Run pipe braces from the top and bottom holes in the stiffener to the same bolt hole on the anchor plate - landing the foot of each brace on opposite sides of the brace and stiffener. Deflection in the girder will also cause a slight rotation of the girder, so you will need to check the extent of this to make sure you can install the permanent diaphragms once the next girder is in place. If your sketches are to scale you may need to use angle iron in place of pipe braces, as the braces look shorter than what is commonly available.
This avoids the underside anchor (which is a bit of a pain to install), reduces the number of anchor plates required and eliminates the extra steel plate bolted to the existing stiffener shown in option 1. If you go with one of your attached options, go with number 1.
RE: Bracing of Welded I-Girder
For example you could use lindapter clamps and just remove them afterwards to facilitate a connection.
Link
I would be worried in your option 1 as sketched that the two horizontal struts are too close, Any slackness in the system from bolt holes etc will mean you would still likely get some rotation until the bolts are engaged in bearing. Better to space them out as far as possible vertically, similar to KootK's detail.
Agree with all the other sentiments to brace all points irrespective of what the calcs tell you. No point saving a few, dollars on braces, contractors have a way of building things in a way you didn't think of and which might not be in line with your design intent!
RE: Bracing of Welded I-Girder
RE: Bracing of Welded I-Girder
I strongly disagree with this sentiment. As the construction engineer you should be providing the contractor with most economical design that meets the design requirements. I routinely see bridge girders up to 50m in length that require one brace point or two (or none). Going from one brace point to bracing at every diaphragm (which may be 8 or 9 brace points) is not insignificant and overly conservative. All of the braces need to be installed prior to releasing the crane. Best case is that the bracing work is being done from a zoom boom, and they have two on site. Worst case it is being done in a manbasket and the bracing is heavy enough to require a stinger or crane to install.
If it was a case where I knew the contractor tended to be a bit of a cowboy, then I might apply some extra conservatism based on my engineering judgement, but not to that extent.
The OP has already stated that they cannot lift a girder pair. Maybe they could launch a girder pair, but that usually isn't very cost effective. With an existing structure to brace to, crane erection is probably the better option.
RE: Bracing of Welded I-Girder
1. I requested, and the OP provided a good partial plan.
2. Girder Line 1 (which requires the initial temporary bracing) is approximately 1045 feet (318 meters) long. It is six-span continuous and is composed of eleven (11) separate girder segments that are spliced longitudinally. None of the individual eleven girder segments are simply supported.
3. Two or three of the eleven Girder Line 1 segments appear to be curved. The OP stated that the acceptable unbraced length for the curved segments is 30 feet (during structural steel erection).
4. The remaining eight or nine of the eleven Girder Line 1 segments appear to be straight. The OP stated that the acceptable unbraced length for the straight segments is 50 feet (during structural steel erection).
5. There are approximately 50 each, potentially suitable diaphragm (brace point) locations distributed along Girder Line 1. Using the OP's numbers, approximately 30 of the 50 brace points are needed to meet the minimum acceptable unbraced length criteria. This minimum criteria is for best case conditions - the 11 segments of Girder Line 1 are fully assembled and there are no external loads (e.g. wind).
6. No erection of the similar Girder Line 2 will occur until Girder Line 1 is fully assembled and (temporarily) braced to the existing bridge.
7. I provided a link to the National Transportation Safety Board (NTSB) "Highway Accident Brief" concerning the collapse of a smaller scale, but almost identical steel girder in 2004. The accident occurred because of lack of planning and poor execution by the Contractor. Three members of the general public were killed. The findings and recommendations from this report are possibly the underlying reason for the OP's firm is performing this temporary bracing design.
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RE: Bracing of Welded I-Girder
If ~30 of 50 brace points are required, prior to including wind loads, then the question of whether or not to add redundant bracing may be moot. Once the girders are checked with wind loads, it may be that all available brace points are required anyway.
RE: Bracing of Welded I-Girder
I just take a closer look at the posts & details and realized that our preferred detail will not work on horizontally curved I-girders because of of the tendency to rotate about the line/axis (connecting the two ends). Normally, to stabilize single curved I-girder, we would use a holding crane or shoring tower (placed as close as the girder's gravity center as possible - when pair lifting is not allowed/possible).
Let's take a fresh look at the details and see what can we use them for:
- For LTB: Kootk's detail, Detail 1, 4 would be okay.
- For weak axis bending (from wind loads, in case of disruption in erection and no possible tie-down): Kootk's detail, Detail 1, 3 & 4
- For rotation due to horizontally curved profile: Detail 4
Please comment,
hdn32
RE: Bracing of Welded I-Girder
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RE: Bracing of Welded I-Girder