Bridges close to each other

Factual Questions
1

On I road I drive on there are two bridges within a mile. One has a paved roadway and the other is an open grate type.
They are both about the same length.
Can anybody tell my who decides which type and why?

2

I work in a road agency on, among other things, historic bridges. Hard to say without a better idea of the bridges involved. Some of the reasons why two bridges might not be the same or similar where their setting is identical:

  • cheapness - I always think US bridges are under-engineered and look like they are saving money (based on a rigorous sample of abandoned, backwoods and picturesque bridges that appear in movies and TV). The decision to not seal a road surface might be entirely cost-based, either for construction or perceived future maintenance spend and serviceability [if you build it you have to fix it and that means closing the bridge for a period]

  • part of a trade-off to accommodate another engineering issue. Once you begin to skimp on any engineered structure, the risk of different failings increase, and you have to modify your standard design to reduce the likelihood of bad things. Deciding not to seal the road deck may be part of that trade-off of weight / budget / construction cost / maintenance liability / performance / etc. across the entire design

  • built at different times - either originally or one is a replacement - and they reflect the preferred and often most economical material and form of their time

  • live loads [vehicle weight and mass] loads they were intend to carry are different - one may be designed for higher mass vehicles

  • different structural design engineers with different experience and different budgets and road network needs

  • a paved road (do you mean an asphalt black top?) can add considerable weight to the bridge and there may be reasons why a lighter bridge is required (if the construction authority had finite budgets they may have chosen a less massive bridge to begin with, with its own inherent design constraints).

  • a paved roadway means that you can control run-off (although usually it pours down scuppers to drainage next to the bridge anyway). Don’t think that’s a big deal for non-major bridges, but is a normal design requirement for bridges over waterways that supply water for human consumption.

3

I know in the Cleveland area, there’s a lift bridge over the Cuyahoga that, to save weight (important for a lift bridge!) has a metal grate roadway… but it’s right next to the bike co-op, and bikes have a very difficult time with grate roadways, and so the co-op managed to convince the right people to put in a solid-surface bike lane each way. Which was probably still a lot cheaper than a solid-surface car lane, both because it’s narrower, and because it didn’t need to support as much weight.

4

The Mackinac Bridge has four lanes; the inner two are metal grids so that air can blow through instead of lifting the bridge deck.

5

Could the open grate type could also be used to keep (some) animals out ?

6

Supposedly one of the downtown bridges was open grate due to mayflies (a/k/a dayflies). When they hatch there can be enough that the road surface is quite slick.
The nearby hotel complained about the noise of the open grate bridge. (even though the bride was there decades before the hotel).
I’m not sure if it was due to that, but the bridges is now paved. I think they plow it when the mayflies get thick.

Brian

7

I assume too there’s less of a snow buildup or frozen puddle issue on a grate deck. (And frozen puddles lead to the ice-thaw cycle degredation of asphalt or concrete.)

8

These bridges were not in a city and were on the same road, They seemed to be fairly new.

9

I can tell you that when I was on the bank taking photos, directly under this open grate bridge, I practically jumped out of my skin the first time I heard a car driving over. Yet, I had just driven over the same bridge in my own car, but of course hadn’t noticed any great amount of noise. I think it’s just another example showing how we’re in a totally different world when we’re driving.

Imgur

10

Early morning cheerfulness is extremely obnoxious

11

I’ve spent the last two weeks trying to find a definitive answer for the OP, because I lived in Paducah, KY, home of the infamous U.S. 45 bridge across the Ohio River. Finally I found the Steel Bridge Design Handbook, which says this about metal grate decks.

Metal grid decks, both open and filled with reinforced concrete, have been used in bridge
construction since the 1930’s. These deck systems can be considered for both new construction
and bridge rehabilitation projects, where weight reduction and/or speed of construction are
important considerations. Metal grid decks, filled or unfilled, are typically lighter in weight than
a conventional reinforced concrete deck slab with similar flexural capacity. Weight savings can
be especially important in movable bridges as well has rehabilitated structures in which a lighter
weight deck may reduce the need to strengthen a structure for increased live load capacity. In
many cases, filled grid decks have proven to be quite durable. Filled grid decks on bridges such
as the South 10th Street Bridge in Pittsburgh, the Walt Whitman Bridge in Philadelphia, and the
Mackinac Bridge in Michigan have all provided a service life of 50 years or more.

TL;DR

  1. The steel deck weighs less, which can increase the weight capacity of the bridge
  2. It’s faster to install a steel grate deck

Interestingly, the bridge at Paducah originally had a conventional concrete deck, but when it was rebuilt in the 1970s the concrete deck was replaced with a steel grate.