shear studs - negative moment

[skjoex]

Hi,

...just simple question. Let us consider composite (steel and concrete) one span beam.

In case of simply supported beam I design number of shear studs from min(Ac*0.85*fc, Aa*fy). Then divided by length of the beam and by number of studs in one transversal row. Then I can this longitudinal distance lower in region of supports and higher in rest region.

In case of beam with both its ends fixed, in supports there is maximal shear force, but negative moment so concrete is in tension and does not contribute to resistance of the beam.

So how to generaly design number of studs and spacing between them in this case ? Specially in regions where concrete is in tension ?

Thanx in advance

 

[kelowna]

"so concrete is in tension and does not contribute to resistance of the beam"

Concrete maybe not, but reinforcement in concrete does, does it not? You still have bo be able to transfer that contribution through shear.

 

[Qshake]

For a single span beam, I find it difficult to believe you'd have anything more than residual negative moment at the ends.

If the bridge is integral and supported on a pile cap with a single row of pile, the location of the stiffness change will provide the necessary rotation for a "pin" support.

Also for single span bridges, we usually detail the shear studs from centerline bearing to centerline bearing. Max spacing at the ends and smaller spacing at the mid span.

Regards,
Qshake

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

 

[skjoex]

...ok, I am quite new one in field of composite structures, so just some additional questions please.

Actually it is a bridge with lower deck, so I have longitudinal steel girders only and composite transversals (12m long, spacing 3m) going into them. And for some reason we do not plan to tied reinforcement with webs of main girders via let us say perfored plate welded to webs of main girders.

So I am not quite sure if I can count with reinforcement in this case....Can I ?

And anyway I design number of studs from min(Ac*0.85*fc, Aa*fy) and not from forces in reinforcement - so this is what is not clear to me.

For simply supported beam on support regions there we put max spacing and in the mid span smaller. It is effective to do the same in beam with both its ends fixed ?

Thanx in advance

 

[Qshake]

As I see it you're dealing with a 'through girder bridge' where two main girders (12 m apart) support the transverse beams, 3m apart spaced longitudinally.

The deck in this case need not be made composite with the main girders nor should it because if the main girders were reliant on the deck for structural capacity and the composite connection compromised this system is not redundant like a multi-girder system, with al girders spanning in the longitudinal direction.

Still the objective is the same for composite action of the transverse members, to reduce the required beam size and weight.

The formula you have is based on the ultimate condition at failure of either the concrete or the steel. It is not based on the actual 'forces' generated by the loading. So you need not determine any horizontal shear between the beam and concrete.

Also, there is a reason for not making the deck composite with the main girders. If the main girders were reliant on the deck for structural capacity and the connection were compromised this system of two girders is not redundant like a multi-girder system and could lead to collapse. Same applies if there were ever work required that would remove that connection with the deck and leave the main girder without a key resisting element.

I hope this helps. Good luck

Regards,
Qshake

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

 

[bridgeart]

There is a shear studs design spreadsheet based on AASHTO LRDF 3rd Edition (2004), Art. 6.10.10 that can be downloaded from

http://www.designspreadsheets.com/shear-studs

The picture on rows 95 to 102 and the strength check on rows 110 and 111 of the spreadsheet explain how the negative moment region affects the required number of shear studs for the strength check.

-bridgeart

 

[Qshake]

Back to the matter of fixed end moments. Typically the connection of the transverse beams to main girder is a shear connection only. Even if it has full web and angles on top and bottom flange (if it does) the rotation is provided by the out of plane movement of the main girder web.


Regards,
Qshake

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

 

[skjoex]

Thank to all of you for comments of yours. I appreciated it.