B#10 OLD & NEW

[BCota]

above is the cover sheet for the present B.10 bridge.  It was cast in place sometime shortly after these drawings were approved by chief engineer H.S. Sargent on May 31st, 1932.  The plan set for present B.10 consists of 10 hand drafted sheets (the new bridge plan set is 100+).

link to plan set from 1932:

http://filebeam.com/5198d0fee0cb54fe72f55d62070d6304

a .pdf viewer is necessary to open the file.  click the download button in the center of the screen after the new window opens; the file is titled: "Fairfax Bridge, 1932".

[BCota]

Notice the slope from left to right; that is a prefabricated crown for the roadway so that the bridge will shed water to the outsides.  Typically this is accomplished by one of two methods: either sloping of the bearings at each abutment or a variable depth HMA (hot-mix asphalt) wearing surface.  In the interest  of reduced on site construction time, neither of these methods were selected and instead the beams were cast with a slope along the width -- looked significantly easier on paper than it was to actually cast (especially for the 2nd beam from the edge on each side where rebar is projecting out of the surface to connect the sidewalk & curbing).







The white pvc pipe creates a duct that will allow for high tensile, woven steel wire called strands to be fed through each beam across the entire deck once it is in place.  The 1/2" diameter steel strands (one at each end and one in the middle) will then be tightened via hydraulic jack to impart 30,000# of force at each of the 3 locations.  This will make the 9 separate beams work together even if a load is applied to the outermost edge of the deck -- and no, this does not mean that you should drive on the sidewalks...



The first photo of this post are units 3 & 4 of the above drawing.  The circles that you see in the beams are actually hollow tubes that run nearly the entire length of the beams.  This reduces the weight of the beams without sacrificing much strength.  This type of structural element is considered a "precast, prestressed, full depth voided beam".

[BCota]

If any viewers have been around long enough (or know someone else who has) to remember construction of the present B.10 I'm interested in a few facts: how long it took to build (Estimates are about 10 months) and who the contractor might have been at the time?  If the contractor is still around or the documents of the business still exist they may contain the bid price of the current bridge as well as who paid for it.  The construction took place nearly 78 years ago so this theoretical witness would have to be nearly 100yrs young and have been a resident of the area for an awfully long time -- if you're out there, you're rare .  I imagine the construction back then would've been a significantly more memorable event than this time around will be so maybe more people will remember it than I think -- hope so!

[Mike Raburn]

Look at that EPOXY coated rebar!!
Waste waste waste......


Nice looking precast!!!
Excellent job!

[BCota]

preaching to the choir with me on that one, Mike.  The only reason we use it is because it is required for all state jobs (any state, not just VT).  By the sarcasm I'd bet you know this too, but if not kept completely free of flaws, epoxy rebar will get localized rust concentrations (provided that moisture can infiltrate through the concrete) which is actually worse than balanced rust formation like what has happened to the steel bridge on McNall Rd.  The reason Mike considers it wasteful is because once the steel is encased in 'crete it wont matter whether it is epoxy or not because the beams will never crack and the concrete is extremely dense which will prevent moisture from penetrating anywhere close to any of the mild steel reinforcement or prestressing strands.  Epoxy coated rebar is almost always required when bars must project our of the beam (in this case to attach the sidewalks and curbs) because they are not protected by the concrete until the sidewalk is cast.  It is expected that this bridge will be around for quite some time so the increased upfront expense of the coated bars will be offset by increased longevity of the structure.