Modelling of inverted U beams bolted through legs

[kww2008]

Hi,

I am modelling an existing bridge as a grillage, and welcome advice on how I can model the structure more accurately to improve load sharing between the beams.

The RC bridge has several inverted precast U beams with their side faces shaped to form a longitudinal shear key between beams with no overlay/topping. Legs of beams are connected together on site by a single M20 bolt placed at 600mm spacings.

My present model consists of:
(1) longitudinal members, each representing a beam with the inverted U beam properties,
(2) transverse members at regular interval along the length of the bridge, each with properties based on the contributing "top slab" of the inverted U beams,
(3) a hinge (with zero moment) is inserted at mid length of each transverse grillage members to represent the shear key By doing this, I am assuming that the shear key can transmit shear but not moment, and that the bolts hold the units together so that the longitudinal shear keys between the beams are effective.

The above model is probably conservative as the bolts are likely to contribute, given their 600mm spacing, to improving moment transfer laterally across the width of the bridge. I am not sure how to model the effect of these bolts, or whether it is worthwhile to do so.

Please advise.

 

[ishvaaag]

Most surely they introduced this practice because they had seen it effective somewhere else; so one is inclined to think the standing bolts were enough for the expected point loads at the bridge when designed.

You may produce a maybe more correct model (or at least an alternative one) by using small plates to build the inverted U beams, then where a bolt is present use a (for a start) rigid link. Most likely, giving the torsional distortions at the Us, that it is not as rigid a to be a notional rigid link won't be much a problem. Then you have forces at the bolts (reactions at rigid links) and general behaviour. Maybe a minor problem can be to represent variable thickness in the legs but you can deal with it by subdivision or using a program directly able to take the tapered plates.