Why don't highway overpasses have columns in the center ?

Question

So often, bridges spanning a divided highway (say, 4 lanes total with a grass strip in the middle) span the whole thing. There are no columns in the middle, which would make the I beams much lighter. From what I recall in mechanical engineering, the stress on the I beams goes up dramatically (more than linearly) with the span. And deflection goes up as a fairly high power (3rd, 4th, 5th, more?) of the span. Also, the supports typically are slanted away from the shoulders, requiring even more span. It looks like these bridges are crying out for a set of poles in the middle.

Answer 1

The bending moment is proportional to the square of the length, however the bending strength is proportional to the square of the depth for a rectangular section. So if you double the span you need double the depth. The deflection is proprtional to the 4th power of the length assuming a constant load along the beam or the 3rd power for a point load in the middle, e.g. a heavy truck. The stiffness, resistance to defelction is proportional to the depth cubed. A continuous two span beam also has a negative bending moment at the middle further reducing the maximum bending moment and deflection. So you are correct, doubling the width means at least doubling the depth. However if you save costs by having the span, then you have additional costs in building central columns. So there is balancing act between more columns and foundations and bigger beams.

Differential settlement can be limited and coped with by using additional steel in the bridge, or by using two simply supported spans.

The cost of the central columns will be much higher if the bridge is being built over an existing road and there is a narrow median strip. The foundations may not fit within the strip and part of the road may need to be closed for a long time to build the columns. Traffic management is not cheap either. In that case a single span can be lifted into place quickly, perhaps even in one night, or the bridge "launched" from one side with traffic still flowing underneath. This is often done with precast concrete bridges with a casting dcok on one side, the bridge is then incrementally pushed out after each segment is cast. Separate pre-stressing cables have to be installed to cope with the launching loads and the final loading.

The design is further complicated in areas prone to earthquakes where the abutments will respond differently to earthquake vibration to a central column.

So at the end of the day its really a compromise between the costs of the beams vs. columns and/or the practicality of having central columns.

Answer 2

Well, if three is better than two, why not go to four? Adding additional supports is an engineering decision. At some point, as the width to be spanned increases, it becomes more economical to make a multispan overpass. But this adds complexity to both the design and the construction, so the engineer will use a single span whenever possible. If the spans you are seeing look too long to you, either your local highway engineers don't know what they're doing, or your intuition of either the support system or the strength and rigidity of the beams is faulty.

Answer 3

I believe the main reason that there are no center supports is because a three point support system would make it impossible to determine for sure what the reactions at each support would be.

There is no way to determine how much differential settling at the base of each support there would be, and if one of the end supports footing, or the middle support footing settled just a litttle too much it would change the load distribution and the structure could fail.
We used to design many large cylindrical storage tanks (i.e. propane tanks, etc.) and they are always designed with only two supports, without regard to their length, for the very reason that with three supports there is no way for sure to determine that the support loads will remain constant.

With only two supports there is no doubt what the support and foundation loads will be.

If you make a sketch of a three point beam support system, including the footings, and assume a distributed load on the beam, and get into designing the footings and supports you will see what the problem is. There is no such thing as a totally fixed footing.

Answer 4

Like a lot of designs in engineering, the decision is one of economics. Some of the other respondents brought out some valid points, but the main reason I believe is that someone determined the design would be cheaper to do with a heavy beam as opposed to one with a center column.

Answer 5

A few things to consider:
They design bridges to be flexible. They must move with heat/cold dimensional changes, they must move under heavy and changing loads.
Another thing to consider is safety. More columns = more chances of a car hitting it head-on.

In general I agree with you. As a tax-payer, I want reduced cost on the bridges and I would assume it's cheaper to put a few more posts in versus having beams that are twice as thick.