Bridge Design Challenge

[2bighorn]

How did you come up with that so fast?  Do you have a spread sheet handy or something?

Your whole chart is 23 ft in height, you have to over-bridge about 400 ft only, and cost is concern. Not many options left, are they?

[grizz441]

How do you know a steel truss is cheaper than prestressed girders?

[2bighorn]

How do you know a steel truss is cheaper than prestressed girders?

It's structurally more efficient and design is cheaper at these lengths. As for the economical efficiency for the lifespan of the bridge, I don't know. That depends on many factors.

[AAJagerX]

I know it was supposed to be at 187m, but the giant cat keeps drinking all the water.

[grizz441]

It's structurally more efficient and design is cheaper at these lengths. As for the economical efficiency for the lifespan of the bridge, I don't know. That depends on many factors.

That's contrary to all I've read and been told.  Anything from 100-150ft spans I've heard that prestressed dominates.

[2bighorn]

That's contrary to all I've read and been told.  Anything from 100-150ft spans I've heard that prestressed dominates.

Steel bridges were always and still are, structurally more efficient at all but shortest spans. That's why they build so many. Of course, they used to have many disadvantages and limitations as well (fabrication, lifespan, maintenance, environmental impact, aesthetics, etc), although with technology advances that is changing too.

Remember I'm not talking about life-cycle economics here, which (today) isn't so clear cut as it has been in the past.

[VonMessa]

Wish I had gotten here sooner.  Most of the good ideas have already been pitched, including the giant cat idea.

I dated a girl who went to VA Tech and was part of the steel bridge team.  There were some very interesting ideas that I got to see.

Here is a beautiful one near me.

2300ft span 

[Killer91]

(Image removed from quote.)

rpm and killer, I lol'd 

kilo I'm definitely going to look into that sloped crossing.  It looks steeper than it is based on the scale differences.

Grizz is correct. The slope would not be steep in the slightest.

If i did my math correctly then the road would only drop 0.1368 inches per foot or keeping it in metric units 0.01125 meters per meter traveled.

 

[Dichotomy]

Side note:  I was talking to one of the PE's I work with and he said that 90% of the short bridges in TX that he's dealt with are fabbed from precast concrete whereas the larger spans tend to be fabricated more of steel beam and girders.  Didn't have time to get into a long winded discussion with him for a lot of detail. 

[Babalonian]

If you adjust for scale (height/length) you can quickly figure out that any type of arched design is probably not suitable.

So, if you go by the cost, the cheapest would be three section pratt truss (or variation of thereof) steel bridge.

If you account for the maintenance, than two section (one center pylon) pre-stressed concrete balanced cantilever design would suit you perfectly (aesthetics).

You're right about maintenance costs, but have you looked up the prices of structural grade steel in the last decade?  After shipping, permits, securing enough qualified union steelworkers, etc. etc. it's easily over 3-times the cost of on-site or off-site prestressed concrete fabrication.  Prestressed concrete is the popular standard now in everything from bridges to hospitals.

For a while the price was rising so steeply that contractors would buy all the steel they would need years in advance (as soon as they could get their first bid documents) and let it sit.  This caused problems in itself because beams ordered from the foundry 2-3 years ago may not be needed or have been modified during the permiting or VE process.  Even on conrete jobs they can't leave the exposed or stock rebar unsecure these days else scrappers will come by and pinch it all in a second.

How do you know a steel truss is cheaper than prestressed girders?

He doesn't because I'll tell you right now and up-front he's so wrong in today's market it's not funny.

It's structurally more efficient and design is cheaper at these lengths. As for the economical efficiency for the lifespan of the bridge, I don't know. That depends on many factors.

Please, just stop or present to us your sources.  Yes, it's apears to be structuraly less mass (a few steel beams instead of slabs, beams and blocks of concrete) , but far from effecient or economic.

[Dichotomy]

steel prices are currently trending up again. 

[2bighorn]

You're right about maintenance costs, but have you looked up the prices of structural grade steel in the last decade?  After shipping, permits, securing enough qualified union steelworkers, etc. etc. it's easily over 3-times the cost of on-site or off-site prestressed concrete fabrication.  Prestressed concrete is the popular standard now in everything from bridges to hospitals.

For a while the price was rising so steeply that contractors would buy all the steel they would need years in advance (as soon as they could get their first bid documents) and let it sit.  This caused problems in itself because beams ordered from the foundry 2-3 years ago may not be needed or have been modified during the permiting or VE process.  Even on conrete jobs they can't leave the exposed or stock rebar unsecure these days else scrappers will come by and pinch it all in a second.


He doesn't because I'll tell you right now and up-front he's so wrong in today's market it's not funny.

Please, just stop or present to us your sources.  Yes, it's apears to be structuraly less mass (a few steel beams instead of slabs, beams and blocks of concrete) , but far from effecient or economic.

Again, I said steel bridges trump concrete bridges when it comes to structural efficiency. If you'd bother to read, you could see that I said I don't know which would be ahead for the life cycle economics.

But again for you:

1. There is no such thing as one type fits all, especially when it comes to bridges
2. Every type of bridge design has its optimal span length which vary by the type. Out of that optimum it will most likely be less efficient then type within the range.
3. The above will influence type selection
4. Price of the material and fabrication is only one aspect of life-cycle economics of a bridge
5. At given span length of 400-500 ft, and other data given by Grizz, best choices are a)over-deck steel truss b) segmental pre-stressed concrete (mentioned in one of my previous posts)
6. No engineer worth his salt will tell you which is more economical without 'case by case' life-cycle cost analysis (everything else is just a guess). 

PS
my sources:
Publications of American Society of Civil Engineers (ASCE)
Issues of The Journal of Structural Engineering by ASCE
Issues of The Practice Periodical on Structural Design and Construction by ASCE
Issues of The Journal of Management in Engineering by ASCE
Publications of American Iron and Steel Institute (AISI)
Publications of American Concrete Institute (ACI)
Publications of American Institute of Steel Construction (AISC)

[Babalonian]

Again, I said steel bridges trump concrete bridges when it comes to structural efficiency. If you'd bother to read, you could see that I said I don't know which would be ahead for the life cycle economics.

But again for you:

1. There is no such thing as one type fits all, especially when it comes to bridges
2. Every type of bridge design has its optimal span length which vary by the type. Out of that optimum it will most likely be less efficient then type within the range.
3. The above will influence type selection
4. Price of the material and fabrication is only one aspect of life-cycle economics of a bridge
5. At given span length of 400-500 ft, and other data given by Grizz, best choices are a)over-deck steel truss b) segmental pre-stressed concrete (mentioned in one of my previous posts)
6. No engineer worth his salt will tell you which is more economical without 'case by case' life-cycle cost analysis (everything else is just a guess). 

PS
my sources:
Publications of American Society of Civil Engineers (ASCE)
Issues of The Journal of Structural Engineering by ASCE
Issues of The Practice Periodical on Structural Design and Construction by ASCE
Issues of The Journal of Management in Engineering by ASCE
Publications of American Iron and Steel Institute (AISI)
Publications of American Concrete Institute (ACI)
Publications of American Institute of Steel Construction (AISC)

If you adjust for scale (height/length) you can quickly figure out that any type of arched design is probably not suitable.

So, if you go by the cost, the cheapest would be three section pratt truss (or variation of thereof) steel bridge.

If you account for the maintenance, than two section (one center pylon) pre-stressed concrete balanced cantilever design would suit you perfectly (aesthetics).

Since we're now talking about structural effeciencies and not costs of design, fabrication or construction, then yes, I do agree that in general steel is a superior material.  Then again, so would titanium be to steel (if it were plentiful enough and thus, cheap).

[Dichotomy]

Here's one for you guys.

Two years ago I designed an 80 ft pole with 18 fixtures for 72 EPA in a 90 MPH zone under AASHTO 1994 as defined by the customer.  The pole calculates out to be at 96 percent capacity under full load.  A few months ago I was asked if they could add another 18 fixtures.  Knowing the answer was no but also knowing the customer I ran the calcs and it came out 30% overstressed and adding wall thickness (new pole) didn't help because the bolt circle was required to be increased by 4 inches and the foundation was beyond max capacity.

Sorry Mr customer it wont work you'll have to do a new install if you want that much EPA.

This morning I received a request to see if they could install a 100 ft pole on the existing foundation if I designed the new pole to fit over the existing anchor bolts and they installed chem bolts to fit the new bolt circle and used commercial code...   

[NormH3]

I know the answer to this..  B