In the early days of the Interstate system, it was fairly common for longer bridges to have narrow 4-foot shoulders instead of the full-width (4' left and 10' right) shoulders matching the cross-section of the adjacent roadway. Some agencies still build narrower bridges on off-Interstate highways. WVDOH has used 4' shoulders on many of the newer Corridor H spans and on WV 43 built within the last 15 years.
How much money is saved by having the narrower shoulders? I would have to think the cost savings from building a 32' deck instead of a 38' wide one is a lot less than the 19% difference in deck width.
Quote from: Bitmapped on January 16, 2024, 10:35:11 PMHow much money is saved by having the narrower shoulders? I would have to think the cost savings from building a 32' deck instead of a 38' wide one is a lot less than the 19% difference in deck width.
Bridges are priced (for estimating purposes) by cost per square foot of deck. FHWA has a table giving average costs per square foot in each state. (https://www.fhwa.dot.gov/bridge/nbi/sd2020.cfm)
Assuming the national average cost of $228 per square foot (can be half in some states, double in others), the amount saved by omitting the extra six feet of right shoulder is $1,368 times the length of the bridge deck in feet. That can be easily $600,000 for two bridges to form an overpass over a surface road with four through lanes and left and right turn lanes, or $1.2 million for two bridges to cross a river roughly the size of the Arkansas at Wichita.
Personally, I would spend the money. But there are definitely tradeoffs.
Quote from: J N Winkler on January 16, 2024, 11:01:37 PM
Quote from: Bitmapped on January 16, 2024, 10:35:11 PMHow much money is saved by having the narrower shoulders? I would have to think the cost savings from building a 32' deck instead of a 38' wide one is a lot less than the 19% difference in deck width.
Bridges are priced (for estimating purposes) by cost per square foot of deck. FHWA has a table giving average costs per square foot in each state. (https://www.fhwa.dot.gov/bridge/nbi/sd2020.cfm)
Assuming the national average cost of $228 per square foot (can be half in some states, double in others), the amount saved by omitting the extra six feet of right shoulder is $1,368 times the length of the bridge deck in feet. That can be easily $600,000 for two bridges to form an overpass over a surface road with four through lanes and left and right turn lanes, or $1.2 million for two bridges to cross a river roughly the size of the Arkansas at Wichita.
Personally, I would spend the money. But there are definitely tradeoffs.
Those charts appear to be for rehab/replacement work, rather than new work.
And looking at NJ, they appear to be low. Very low. But maybe those figures are looking solely at the concrete replacement, and not all the other associated work often involved in bridge repair.
In IL, on non-interstate highways, there is an ADT breakdown for allowances of 32', 36' and 40' bridge deck widths. This was based on an FHWA study many years ago that examined the tradeoffs between crashes and shoulder widths and the cost effectiveness of risk-based crash cost vs. extra bridge width cost. For reconstructed bridges in the "3R" policy, with ADT's less than 1000/day, sometimes a 28' width can be justified, but 32' is usually the default minimum. 3000-4999 ADT is 36', and 5000 and above ADT calls for 40'. I remember the local criticism from a few truckers and farmers we got at IDOT when we replaced a bridge on a low volume section of IL 17 with the 32' width while many adjacent bridges that were constructed before the "cost effective" policy had a 40' width, and why we didn't just match them.
Quote from: J N Winkler on January 16, 2024, 11:01:37 PM
Quote from: Bitmapped on January 16, 2024, 10:35:11 PMHow much money is saved by having the narrower shoulders? I would have to think the cost savings from building a 32' deck instead of a 38' wide one is a lot less than the 19% difference in deck width.
Bridges are priced (for estimating purposes) by cost per square foot of deck. FHWA has a table giving average costs per square foot in each state. (https://www.fhwa.dot.gov/bridge/nbi/sd2020.cfm)
Assuming the national average cost of $228 per square foot (can be half in some states, double in others), the amount saved by omitting the extra six feet of right shoulder is $1,368 times the length of the bridge deck in feet. That can be easily $600,000 for two bridges to form an overpass over a surface road with four through lanes and left and right turn lanes, or $1.2 million for two bridges to cross a river roughly the size of the Arkansas at Wichita.
Personally, I would spend the money. But there are definitely tradeoffs.
How accurate that "per sqft" approximation? I would expect a lot of costs to not scale that simply with dimensions.
Quote from: kalvado on February 11, 2024, 02:12:19 PM
Quote from: J N Winkler on January 16, 2024, 11:01:37 PM
Quote from: Bitmapped on January 16, 2024, 10:35:11 PMHow much money is saved by having the narrower shoulders? I would have to think the cost savings from building a 32' deck instead of a 38' wide one is a lot less than the 19% difference in deck width.
Bridges are priced (for estimating purposes) by cost per square foot of deck. FHWA has a table giving average costs per square foot in each state. (https://www.fhwa.dot.gov/bridge/nbi/sd2020.cfm)
Assuming the national average cost of $228 per square foot (can be half in some states, double in others), the amount saved by omitting the extra six feet of right shoulder is $1,368 times the length of the bridge deck in feet. That can be easily $600,000 for two bridges to form an overpass over a surface road with four through lanes and left and right turn lanes, or $1.2 million for two bridges to cross a river roughly the size of the Arkansas at Wichita.
Personally, I would spend the money. But there are definitely tradeoffs.
How accurate that "per sqft" approximation? I would expect a lot of costs to not scale that simply with dimensions.
Well, he did say "for estimation purposes", which implies it's not going to be accurate, but rather a start.
And there's a lot to go with that. If the right of way is there and the structure is relatively simple, it may be fairly accurate. If they have to buy land, move utilities, change how the structure will be built, etc, that estimate will be mere dollars of what the actual cost will be.
State DOTs need to employ Occam as a consultant. He just shaved himself and decided that if it were cheaper in the long run to build lanes now, they would.
Quote from: Rick Powell on February 11, 2024, 01:16:17 PM
In IL, on non-interstate highways, there is an ADT breakdown for allowances of 32', 36' and 40' bridge deck widths. This was based on an FHWA study many years ago that examined the tradeoffs between crashes and shoulder widths and the cost effectiveness of risk-based crash cost vs. extra bridge width cost. For reconstructed bridges in the "3R" policy, with ADT's less than 1000/day, sometimes a 28' width can be justified, but 32' is usually the default minimum. 3000-4999 ADT is 36', and 5000 and above ADT calls for 40'. I remember the local criticism from a few truckers and farmers we got at IDOT when we replaced a bridge on a low volume section of IL 17 with the 32' width while many adjacent bridges that were constructed before the "cost effective" policy had a 40' width, and why we didn't just match them.
Interesting. Does IDOT take into account the width of the shoulder on the approach roadways or is the deck width strictly based off the traffic counts?
Quote from: Bitmapped on February 14, 2024, 09:31:41 AM
Quote from: Rick Powell on February 11, 2024, 01:16:17 PM
In IL, on non-interstate highways, there is an ADT breakdown for allowances of 32', 36' and 40' bridge deck widths. This was based on an FHWA study many years ago that examined the tradeoffs between crashes and shoulder widths and the cost effectiveness of risk-based crash cost vs. extra bridge width cost. For reconstructed bridges in the "3R" policy, with ADT's less than 1000/day, sometimes a 28' width can be justified, but 32' is usually the default minimum. 3000-4999 ADT is 36', and 5000 and above ADT calls for 40'. I remember the local criticism from a few truckers and farmers we got at IDOT when we replaced a bridge on a low volume section of IL 17 with the 32' width while many adjacent bridges that were constructed before the "cost effective" policy had a 40' width, and why we didn't just match them.
Interesting. Does IDOT take into account the width of the shoulder on the approach roadways or is the deck width strictly based off the traffic counts?
There are situations in the policy where, if the existing approach roadway shoulder is less than the bridge shoulder, the bridge width can go to the next lower width. Sometimes a bridge can go to a wider width if using modular components that are wider than the standard, or for construction staging purposes. There is no blanket "if the roadway shoulder is wide, the bridge can match it" policy. Design exceptions are considered, but must be justified.