Connection to plate girder web

[GerhardSA]

Hi everyone,

Doing some third party connection checking I came across a potential problem, would like to confirm my suspicion.

I have a bracing connecting to a Plate girder at 45 degrees 1400x500x50x20, the bracing has a tensile force of 190KN and it connects to the girder's web 390mm from the top flange. (see attachment)

Currently there is no web stiffners.

I am concerned that the web will deflect eccesively, like bending deflection, under the force perpendicular to the web. Not really finding anything in the code that pertains directly to this situation, but this seems pretty obvious to me unless I am missing something simple.

Any guidance or literature appreciated.

 

[kelowna]

I understand what you mean. The detail is not good, that is sure. I would not know where to start to see if it works or not. I would just not accept it even if somebody could prove it does.

Maybe you could make a comment to the Designer and ask how he designed it (a polite way to tell him that he screwed up)

 

[bridgebuster]

A brace with tensile force - sounds to me like a cyclic load. If so, it's potential fatigue problem: prying action that could result in cracking of the connection angle or web, and/or tensile failure of the bolts.

I've come across a similar situations on bridge inspections:

Stringers riveted to a steel pier cap that is supported on two steel columns. Various connections failed but the web of the pier cap was intact - but it had stiffeners.

Flexible cross-frame connections that resulted in cracks in the girder webs.

AISC/ National Steel Bridge Alliance publishes a book on fatigue design; it could be worthwhile to obtain a copy.

 

[Qshake]

Agree with Brigebuster, assuming, of course, this is for a bridge girder. If not, cyclic is not a problem.

A better detail would be to place a full depth stiffener on both sides of the plate girder's web and then using a horizontal plate, with a slot to straddel the new full depth stiffener, weld the horizontal plate to the siffener. Seat the bracing member atop the horizontal plate.

the above is done quite a bit for lateral bracing with longer plate girders.

Good luck.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[apsix]

Just ask the designer for confirmation, by calculation, that the web is adequate.
Like you, I also have my reservations.

 

[rowingengineer]

Agreed, it dosn't look good on it's own.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[paddingtongreen]

I just got hung up on why the plates have four dimensions, I was trying to figure out the dimensions in ft and ins, units I can "feel".

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

GerhardSA,
I think you are absolutely correct. The web cannot carry the perpendicular component without excessive bending.

BA

 

[GerhardSA]

Thanks all,

This is not very severe cyclic loading but some can be expected, it is for a big power plant that is being built.

Paddington, the dims are 55x20x2x0,8 in inches.

Most of these connections are already manufactured, so the changes has to be simple, what I will suggest is that 2 web stiffners should be welded to one side of the web.

I did model the web plate as a seperate unit in staad for what is was worth, with the sides pinned the deflection is only 5mm, but of course this will greatly increase if the flanges move closer together due to the force.

Is it better in a situation like this to model the connection in solidworks or ANSYS?

I did a bit of searching and I dont find a lot of literature on plate bendings in girders.

 

[ToadJones]

It just seems like a poor structural layout.

We have done a lot of work on power plants and normally you would try to "back-up" this type of brace, with another brace or beam....essentially a horizontal truss that would in-turn take the loads to the lateral force system.....in the case of a power plant, veritcal braces.

The forces in that brace not only have to be withstood by the beam, but must be carried by the beam to another location.

 

[ToadJones]

I guess what I am suggesting is that although the connection detail may not be desirable, the member layout is even less desirable.

 

[paddingtongreen]

In a power plant, this is probably in the TG building, if so it is rarely used at full capacity and then, slowly and gingerly.

I agree it doesn't look like good member placement, but I would think that a stiffener on the back of the web would handle it, I think 190KN translates to 43kips, about 30kips parallel and 30 perpendicular.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[ToadJones]

Pad-
Out of curiousity, what is the TG building?
A lot of the work we did was on the main boiler house and "back end" retrofits....SCR's, Baghouses, Lime solos...