Bracing of Welded I-Girder 1

[hdn32]

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#top

Several threads in

http://www.eng-tips.com

on Lateral Bracing of I-girder/Beam

Thank you in advance,

hdn32

 

[gwynn]

Late to the party, but provided you can get adequate capacity from your Hilti anchors I would do things a little differently than any of the options on your attached pdf.

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.

 

[Agent666]

You don't have to let the fact that you aren't allowed to drill holes in the existing girder limit your options with respect to connecting to the steelwork.

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!

 

[Agent666]

Another different approach would be to lift first and second girder up together complete with diaphragm bracing, and splice girders. This actually seems like a nicer approach erecting two girders simultaneously, but of course we don't know the complete configuration or site limitations that might prevent this approach.

 

[gwynn]

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!

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.

 

[SlideRuleEra]

It is time to review the facts:

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[gwynn]

Thank you for the summary, and my apologies, I must have missed a couple of posts when first reading the thread.

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.

 

[hdn32]

Thanks Sliderulera for the summary and everyone for joining the discussion.

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

 

[SlideRuleEra]

hdn32 - You have done a good job of furnishing information about this project, and have received a wide range of thoughts from ET members. From what you have told us, this is a major project. IMHO, it is time to confer, again, with colleagues in your firm to reach a solution. Members of any online website can contribute only up to a certain level - beyond that, the numerous site specific details become critically important. Explaining those details with sufficient clarity over the internet is unlikely.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[hdn32]

Sliderudeera: Yes, thank you for your advice