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Highway Question
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ghporter
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Join Date: Apr 2001
Location: San Antonio TX USA
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Mar 4, 2018, 04:35 PM
 
So they're building a "flyover" connection between two local, controlled access highways (I-410 and SH 151 on San Antonio's Northwest side for anyone curious). It only connects between 151 southeast bound to 410 northbound, and between 410 southbound and 151 northwest bound, making it only a quarter of a "real" highway interchange, but hey, at least it's a start. 25 years late, but a start...

They've finally gotten to the point where they've started on the curved connections for both directions. The connection between southbound 410 and 151 has concrete beams supporting the roadway, but the (newly installed) support for the 151 to 410 connection is steel beams.

Here's the question: Why the heck do they use two different types of materials to support what should be (from my perspective anyway) identical pieces of elevated roadway?

Inquiring - and taxpaying - minds want to know... Anyone have a clue?

Glenn -----OTR/L, MOT, Tx
     
Thorzdad
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Mar 4, 2018, 05:01 PM
 
When did they build the sb 410-to-151 section? My guess is it’s just a difference between what one engineering contractor specced and another contractor specced.
     
Waragainstsleep
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Mar 4, 2018, 05:12 PM
 
Could the ground be different? At a guess concrete would spread the weight more than steel.
I have plenty of more important things to do, if only I could bring myself to do them....
     
subego
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Mar 4, 2018, 05:53 PM
 
My guess is two different sets of regulations. One interchange is federal, one is state.
     
ghporter  (op)
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Mar 5, 2018, 04:14 PM
 
The ground is limestone for the whole thing. (Basically that’s true for most of San Antonio, and why we don’t have basements.). The connection is from a state highway to a state-maintained stretch of an Interstate loop, in both directions. That’s the curious part; it’s essentially the same road on both sides, so why steel for one and concrete for the other.

Glenn -----OTR/L, MOT, Tx
     
andi*pandi
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Mar 5, 2018, 04:23 PM
 
the left hand doesn't know what the right hand is doing?
     
subego
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Mar 5, 2018, 04:44 PM
 
What I’m saying (guessing) is with the 410 to 151 interchange, travel on the interstate doesn’t officially end until you’ve left the interchange.

In the other direction, it’s reversed. Travel on the interstate doesn’t officially begin until you’ve left the interchange.

In other words, the 410 side needs to follow all federal interstate regulations, but the 151 side doesn’t.

Presumably the federal regulations are tougher, because they need to work just as well in Alaska or Florida, while the state regulations need only apply to Texas.

Likewise, no state highway is going to see action like an interstate will, so specs are less beefy.
     
ghporter  (op)
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Mar 5, 2018, 08:45 PM
 
Well it looks like my hope that someone had a background in this kind of civil engineering has been dashed.

I've contacted the state highway department with this very question, and (if they get back to me), I'll post what they have to say.

Glenn -----OTR/L, MOT, Tx
     
   
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