Min free distance between girder and wall

[M-Z]

Hi
Is there any required minimum free length between edge of girders and wall of abutment, when using sliding elastomeric support.(This free length seems to should be larger than displacement due to lateral loads.)

 

[bridgebuster]

If you're designing for particular DOT check their standards. Yes, the distance from the end of the girder should be larger than the displacement plus some fat.

 

[M-Z]

Thanks.
But would you please refer me to a specific article of AASHTO or any other formula so that I can precisely calculate the required distance between EDGE OF GIRDERS and BACKWALL OF ABUTMENTS.

 

[bridgebuster]

I'm not aware of any AASHTO provisions. On new NYSDOT bridges, centerline of bearing to face of backwall is usually 1-3" or 1'-6". The girder standards call for the centerline of bearing to be 8" from the end of the girder and the deck standards call for the deck to overhang end of girder by 6", add in the movement, you're somewhere between 1'3" to 1'-6". There's a little more too it: Minimum girder seat to meet AASHTO seismic requirement and keeping the formwork in 3" multiples.

 

[BridgeSmith]

I am also not aware of any specific provisions in the AASHTO spec regarding distances. It specifies the temperature range for calculating the movement to be accommodated by the bearings. Implicit in those provisions is that there should be enough space for the girders to expand at least that far without hitting the abutment. Any potential movement of the abutment should be accounted for, also.

 

[OSUCivlEng]

The main DOT I do work for requires at least 3" plus the calculated change in length for thermal expansion between the end of the beam and the abutment backwall.

If you have a skewed bridge you have to account for this as well as the corner of the beam/girder and the edge of the bearing.

 

[M-Z]

Thanks for your great understanding of my question.

So, you have detected some sort of implicity in AASHTO.
Accordingly,article (4.7.4.4) in AASHTO LRFD BRIDGE DESIGN SPECIFICATION 2012 , proposes a support length for girders(N).But, regarding the article's figure, this length just accomodates the outward movement of abutment (relative to girder) ,and in the case when the abutment is moving towards the gider, there would be a collision between backwall and girder.
So, could one infere that there should be a distance as much as N between backwall and edge of girder.

 

[BridgeSmith]

AASHTO allows impact and damage to components during a seismic event. Life-safety is the design criteria for bridges (except for those deemed to be essential for access by emergency vehicles after an earthquake). So, as long as the girders remain seated on the abutment through the design-level event, even if they damage the backwall, the AASHTO requirements are met. The owner may impose more restrictive criteria.