Bridge Problem

[kieran1]

I'm working on remedial works to an existing bridge an Africa. It looks like it was constructed in the 1960's. Anyway I don't have any details on the existing design. I tried to analyse the bridge for the existing spans with current truck loading etc and it comes no where near to working although large trucks use it every day.

I have to provide two new carriageways underneath the bridge each at 5.0m wide. See attached existing and proposed.

I measured the beam on site and worked out a rough load based upon a moment capacity for the existing span. My new support arrangement will not result in any larger moments. Based upon this load. My brief is to ensure we don't weaken the existing structure.

I have analysed the new structure as two cantilevers at the ends with the existing splice connections taking no moment. This is under uniform loading. I know non uniform loading will occur. I'm concerned about load transfer across the splice. Anyone got any simple ideas to resolve this. Materails and skilled labour are very rare. I do have some old beams 500mm deep but only 2.0m long. I'm thinking of bolted these to the underside of the existing beams across the splice.

Any recommendations?

Kieran

 

[kieran1]

Proposed details

Kieran

 

[kieran1]

Existing splice detail

Kieran

 

[BAretired]

Bolting 2m long beams to the underside of existing is going to look like an afterthought. It would not be easy to achieve a good connection that way either and would require considerable skilled labor on site.

Are the twelve existing bolts adequate to carry shear? Can the outer ends of the 3410 span be anchored down to resist uplift when a heavy concentration is placed on the splice point?

Could you replace the existing bolts with larger, stronger bolts, i.e. 3/4" A325 bolts to 1" A490 bolts?

BA

 

[kieran1]

thanks for your response BA. I will need to investigate a little more as to the condition of the existing bolts. I think the splice will carry the shear load that the beams are capabale of supporting. The beam under is really a "belt & braces" solution to carry the shear from the traffic loading specified in the code. Even though the existing beams would not be adequate to resist the bending moment from these loads.

The outer ends of the 3410 span will be anchored down to resist uplift. I also need to do a little more investigation into the size of the end abutments and will apply a ballast if necessary.

With regards to the beams underneath, the only traffic using this will be haulage trucks from a mine. I guess they don't care what it looks like. I also realised the 500mm beams will restrict headroom so i may have to cut them in half and weld a plate to the cut end.

Kieran

 

[bridgebuster]

If I understand things correctly, your initial model assumes the splices to be pins. If you tie the flanges together you're introducing some degree of continuity.

Take a look at page 22 of the attachment. Perhaps this is what your thinking? If your strucutre is modelled with pins, the pin & hanger retrofit shown maintains that condition. If you can make things work with the materials you have the capacity of the splice becomes a non-issue.

 

[kieran1]

Bridgebuster for some reason i was not able to open your attachment. My current thinking is that no matter about my assumed structural model, this connection will try to develop moment. My current proposal is to replace the existing splice connection with M24 g8,8 bolts and 15mm plates both sides of the web. I also intend to fix a 20mm bottom flange and some 15mm plates welded to the underside of the top flange. I will also weld the ends of the existing beams together to transfer the shear load. This will enable the connection to develop some moment

Kieran

 

[bridgebuster]

Kieran - for what it's worth,I uploaded the file again. If not the link below gives an illustration:

http://www.pbworld.com/news_events/publications/network/issue_28/28_18_chartersd_pinshangers.asp

I think a moment connection is a better way to go. Hopefully, you can find a qualified welder. Doing a full pen groove weld in the field will be difficult.

 

[kieran1]

Thanks again Bridgebuster good article. Im currently trying to bring in experts from outside the country. I think this is too important an issue to put faith in unqualifed workers.

Kieran

 

[bridgebuster]

No problem Kieran. Good luck.