Looking at concrete road bridges, I notice that although the vertical supports are pretty thick, most of that width isn’t actually directly supporting anything: the actual connection to the bridge they are supporting appears to be only a small fraction of the cross sectional area.
The size of the bearing has only to be sufficient to not crush the concrete underneath it. The size of the pier column has to be such that it can transmit the full load from the superstructure down to the footing without buckling or exceeding its moment capacity due to lateral wind loads on the bridge deck.
There’s no particular advantage to oversized bearings. You size them as big as you need to and no more.
Those two small points are bearings of some sort. Either a pin to hold the beams, or a slider to let it move. Obviously, this is implemented on pairs of piers, and allows for thermal expansion. The pier is wider at the top to allow for stability of the deck, but soon narrows as you get closer to the ground because the narrower part is sufficient to hold the load.
I have always wondered about thiskind of system. I couldn’t find a better pic in a hurry. But the column seems rather narrow for the with of the box section it is holding and the actual point of contact is even smaller.
I have seen bridges where I am almost certain that it is not a sliding bearing of some sort the connecting metal is attached to both members.
Huh, I have always assumed they over engineered it in anticipation of future road widening! So they don’t have to rebuild the bridge from the bottom up if they need to widen the road.
I think it’s because it creates multiple arches (turn your head sideways to look at your linked pic and you’ll see what I mean), and arches bear weight much better than the more obvious solution of having stuff either side. This is particularly true of concrete since it contracts and expands more than brick, so needs more space to move into. Brick bridges have a similar structure with less overhang.
According to Saint-Venant’s Principle, a load is only evenly distributed at a significant distance from the application of that load. So the full cross section would not be utilized at the point of application (the top of the column) anyway. It would be fully utilized somewhat further down the column, which is where the cross section becomes uniform. This principle drives column design in many situations.
Take what this guy said, and consider, the bearing has to be a special part. It needs to be steel to be resilient enough to withstand the seismic* and road vibration/bouncy forces (*“seismic” includes typical things like wind, it doesn’t strictly refer to earthquakes), plus it needs to be some kind of tie-down for the roadway. So it needs to be a special manufactured part.
So that’s why a bigger size is a disadvantage: bigger would be more expensive
And like he said, bigger also isn’t any mechanical advantage. As long as you never exceed the forces on the concrete that would cause it to split or crack, you’re fine, and the whole cross-sectional area of the column will work to support the weight of the bridge
