fabricating and welding up a 1250mm x 600mm internally stiffenened rectangular box girder

[jonathanpye]

I have a 1250mm x 600mm box girder to be made out of 25mm thk plate, for a bridge structure.

It requires 15mm thick intermediate internal stiffeners.

At this size, it's not going be easy to weld in the internal stiffeners... but I'm sure there must be an accepted way of fabricating boxes of this size, which is not too uneconomic.

Can anyone enlighten me ?

Many thanks in advance.

UK based Chartered Civil and Structural Engineer

 

[racookpe1978]

Small welders.
many access ports.
Much time and money - particularly if preheat is needed.
Many safety and hazards inside the small box (fumes generated, welding gas spill-off and shield gas bleed-out, cooling air supply and return, haz grit and debris across and in the welder's clothes.

How long a bridge girder?

 

[jonathanpye]

thanks for the comments... there are two spans of 30m each

UK based Chartered Civil and Structural Engineer

 

[racookpe1978]

Automated (linear-track) welders using wire-feed and remote TV controllers could help - if in a pre-fab type shop situation.

 

[drawoh]

jonathanpye,

Can the 25mm plates be fabricated in sections corresponding to the 15mm stiffeners?

Do you absolutely have to weld the stiffeners on both sides?

--
JHG

 

[jonathanpye]

yes, I would expect the plates to be welded in sections between stiffeners.

Stiffeners can be elded from one side only I believe

UK based Chartered Civil and Structural Engineer

 

[Tmoose]

Do the folks who designed the girders have anything to say?

===================

THE DESIGNER

The designer bent across his board,
Wonderful things in his head were stored,
And he said as he rubbed his throbbing bean,
"How can I make this thing hard to machine?
If this part here were only straight,
I'm sure the thing would work first rate.
But would be so easy to turn and bore,
It never would make the machinist sore.
I better put in a right angle there,
Then watch them babies tear their hair.
And I'll put the holes that hold the cap,
Way down here where they're hard to tap.
Now this piece won't work, I'll bet a buck,
For it can't be held in a shoe or chuck,
It can't be drilled or it can't be ground,
In fact, the design is exceedingly sound.
He look again and cried, "At last!
Success is mine, it can't even be cast."

 

[jonathanpye]

I'm the designer... that's why I'm asking the questions... I don't want to design something now that creates problems for the bridge fabricators !

I've found something similar done on another project, and I think they must have welded the cheek plates on in 2 sections (each cheek section 612.5mm high - overall height 1250mm)... this would mean that the welder only has to reach 600mm inside the box at a time, and with a width of 600mm to reach in through, this should be acceptable I think ??

There would be a longitudival weld running halfway down the side of each side of the box girder, bt this could be ground flush I guess.

Any other comments appreciated.

UK based Chartered Civil and Structural Engineer

 

[dhengr]

Jonathanpye:
I’d need to see a plan and elevation view of that box girder, and a few cross sections; along with some explanation of how it is used and loaded, any special spec. items; with dimensions, sizes, stiffener locations, etc. You have to be able to see this and understand its end use to know how to design and fab the girder so that stiffeners are where they need to be, and properly attached, and that you must do little or no internal welding. Generally, you try to design and fab something like this so you can work your way out of the box in all directions. That is to say... you usually build the girder upside down. You can do the weld prep. on all four outside pieces; lay the top flg. down (with its top face, down), on a stiff fab table (a strong back assembly fixture). You then stand the two web pieces up on the top flg., weld them longitudinally to the flg. and apply (fit and weld) the stiffeners to these three plate pieces. You want to be able to rotate the fab table for ease of welding, all down hand welding. You have to pay attention to camber and movement caused by welding when fab’ing. something like this. Then you stand this three sided assembly up on the bottom flg., which is now laying with its bottom face down, and make the final longitudinal welds from the outside only. A 100' (90m) girder will have to have several field splices to be transportable. There are some tricks to doing these splices too, so they fit together when you get them out in the field.

 

[paddingtongreen]

You might try welding the internal stiffeners to the full length vertical plates, I assume these are the 1250mm plates, punch holes in the top and bottom plates, at the stiffener locations and use plug welds to make the attachment, then weld the plates along the edges.

The precautions suggested by dhenger should be followed to end up with a straight box.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[Duwe6]

Final note: the As-Fitup camber of this girder is the most it will EVER have. Every operation, to include the cross-country transportation of this "tub girder" will cause it to loose camber. The good bridge engineers will specify a generous maximum As-Fit camber, and a minimum Installed camber that will work for the next 40-60 years.