Phased Bridge Construction 1

[71corvette]

I'm currently in the process of designing a single span, simply supported highway bridge to be constructed in two phases (one phase construction is not possible). The proposed superstructure will be a concrete deck supported by steel girders.

My concern has to do with the concrete deck placement for the second phase. Because one half of the new bridge will be open to traffic during the deck placement, I have concerns about the differential deflection between the two sections of deck slab during concrete placement & curing. To put it another way, the newly placed phase II section of deck will see no live load, and therefore no deflections, while the phase I section of deck will likely see thousands of live load deflection cycles during the cure period. Typically, we have tied the two secitons of deck together using mechanical rebar splicers.

I'm interested to find out how others have approached this problem in the past.

Thanks,
Tim

 

[SlideRuleEra]

Existing bridges that are being widened have the same conditions. The deflection differential between the two sections during concrete placement is a theoretical concern, but I am not aware of it causing any problems in practice. The wheel loads cannot be too close to the construction joint for safety reasons anyway.

Traffic cones and human flaggers are sometimes used, to "squeeze" traffic away from the temporary barrier at the longitudinal construction joint. An added benefit is to slow down vehicles during concrete placement. After the concrete has taken it's intitial set, even these measures can be discontinued.

 

[TTK]

It is also common practice to specify loose connections of the cross frames/diaphragms in the bay immediately next to the construction joint (on the Phase II side) until the new deck has reached a specified strength level.

Phased decks likley loose some of their service life due to live load during construction, but this has to be weighed against the disruption of traffic and public safety if the bridge were closed completly.

 

[crossframe]

Are you using a closure pour? I think most states do. This would take care of differential deflections due to DL, and (hopefully) isolates Phase II to some degree to allow for better curing. This way, the crossframes and diaphragms can be installed with (limited) access from the top and enables the two phases to "share" the LL deflections during the closure pour.

 

[71corvette]

Crossframe - I assume by closure pour you mean placing the deck in 3 sections with the third and last section being cast in a single bay between the first two pours. This would likely not be an option as it would increase the concrete pay item $ significantly and puch back the project completion date. Expediting construction is important as the project will likely result in traffic jams on the highway.

TTK - wouldn't you want your cross frames/diaphragms located at the construction joint to be fully torqued? I would think that this would help transfer LL deflections to the phase II superstructure, thereby reducing differential deflections.

 

[fuchsjr]

Crossframe has it correct. Most states use closure pours to isolate the stages. The diaphragms are then bolted solid in the closure pour bay to try to limit the differential vibrations/deflection due to one stage having live load.

Another option is to close the bridge over a weekend using the Friday before and the Monday after. Using accelerators.

If you do not do either option, that longitudinal "joint" is already going thru failure. You might as well schedule a deck rehab at the joint now. In which case, you will be cutting out that strip and puitting in a delayed closure pour.

 

[71corvette]

I spoke with a more senior engineer in the office who has dealt with these types of structures before. The local DOT does suggest the use of what they call "zipper strips", and they have been used in the past. I was hoping there was an alternate way to mitigate the differential deflections. Contractors have been frequently using precast panels during construction and specifying a zipper strip or closure pour may preclude or complicate their use.

I think the bottom line is this, good sound engineering decisions should come first with economy and schedule being a secondary consideration. Fuchsjr, I agree. Its no sense saving 10% if the deck only lasts 10 years.

 

[bridgebuster]

tinytim22,

I'm curious, what state are you working in? In New York, the DOT allows precast panels but I've never seen them used; same goes for the other states I've worked in.

Also, in NY closure pours are normally used with staged [phased] construction.

 

[71corvette]

Bridgebuster,

The bridge is being constructed in Maine. The precast panels work well from a construction standpoint, they install quickly and eliminate the need to shield the interior bays of the bridge during construction. The use of the panels has increased a fair deal over the past 5 years or so. On projects I've seen I would guess they've been used on about 40-50% of the bridges. However, it should be noted that the use of panels is not required by the contract documents, it is purely a contractors option. The bridge deck is designed as CIP, but a note is added stating that panels may be used at the contractors option.

Its interesting that they seem to be less common in your area. Are you aware of specific reasons? Perhaps contractors have yet to develop a confort level with them? Most panels used in Maine are shipped from New Brunswick Canada, but I'm sure there are suppliers in your area. Generally speaking, are precast / prestressed concrete structures uncommon in your area?

 

[crossframe]

tinytim22,

The "more senior engineer" in your office is correct. "Zipper strip" is another name for the closure pour to which I was referring. The sizes vary, but typically they're as small as practical, allowing for transverse rebar splices and so on. I've seen them as small as 2 feet. Another criteria I've used is to try to "balance" the DL deflection in the girders adjacent to the phase line so the cross frames and diaphragms will fit easier.

For further information, see the Maine DOT Bridge Design Guide, Section 6.2.2 (p 6-10) It's available for free download at

http://www.maine.gov/mdot/technical-publications/technical-publications.php

 

[crossframe]

I don't know why the link above was cut off. The missing part is "echnical-publications.php"

 

[HgTX]

bridgebuster: Texas uses precast panels all the time.

crossframe: TGML only displays the first so many characters in the link, but the link itself should work anyway.

Hg

 

[bridgebuster]

tinytim22, HgTX,

Precast concrete superstructures are not as prevalent in New York City and the surrounding area as steel bridges.

NYSDOT specs allow precast panels but here metal pans are the most common type; in some instances DOT requires removable forms. My guess is that we don't see panels because of the erection costs. It's not easy to build here because of the traffic restrictions.

 

[HgTX]

TX uses precast concrete panels on straight steel bridges too. My understanding is they minimize road closure time. Where they can't use the panels they use permanent metal deck forms.

Hg

 

[DRC1]

I have never seen a closure pour. Have replaced several decks. The actual deflection of the deck under actual traffic conditions is minimal. Biggest problem is vibration due to trucks hitting joints at high speeds. Decks have generally been poured at night with reduced lane widths and police for for traffic control. This reduces LL vibration. Dowel bar splicres are often used to continue reinforcement through the joint.

 

[fuchsjr]

DRC1,

You have been fortunate in your past experiences. We have tried the low speed routine; didn't work, still ended up with a weaker deck at those locations.

As for the dowels, you must have a real stiff superstructure, otherwise, the dowels in the already cast deck will "move" upwards in the new deck as it is being placed; or the bars in the new deck being placed, will "move" downwards under the increasing deck dead load as it is being placed.

You must have used some accelerators as well, otherwise, you could not even come close to any kind of significant strength by the morning.

 

[DRC1]

Most bridge decks are stiff. Deck pours are divided into 5 sections with the outside and center portion poured first and the intermediate sections poured about a week later.This is done to limit differentail deflection between the slabs. Most of the deflection occurs before the concrete has an inital set. Usually a bridge replacement is done in stages with each stage taking 2 months or more, therefore accelerators are not needed. We use a 4,0000 psi concrete w/ 3/4 in. stone. If you do not have speed control the area near the joints may be weaker than the rest of the bridge from the vibration. I think this would be an issue even with the zipper strips