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| March 2011 (-43.540 Degrees, 172.637 Degrees) Moorhouse Overbridge |
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March 2011 (-43.541 Degrees, 172.637 Degrees) Durham RR Overhead |
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| March 2011 (-43.573 Degrees, 172.695 Degrees) Horotane Overbridges |
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March 2011 (-43.573 Degrees, 172.695 Degrees) Horotane Overbridges |
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March 2011 (-43.573 Degrees, 172.695 Degrees) Port Hills Overbridges I took this photo of the Port Hills Overbridges while standing between the bridge decks. At-grade road crossings require a stop sign, a traffic light, or a traffic circle. However, if one of the roads is an expressway you need to separate them with a bridge (or maybe a tunnel). If they are both expressways, you usually need an interchange. Typically, to carry vehicles onto a bridge you can use mounds of earth (embankments), retaining walls (like we saw on the Colombo Street Overbridge), or the bridge can start at grade and climb steeply with short and tall piers. However, Port Hills Road is descending from hills, so instead of an embankment, Route 73 is in a cut section below the bridge. Note the crib wall holding back the toe of the slope. As I've said before, I've seldom seem earthquake damage to this type of earth retaining structure. Also note the steel keys keeping the girders from moving transversely. At Caltrans, our philosophy is to let the bridge move, and design for the displacement. |
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| March 2011 (-43.573 Degrees, 172.695 Degrees) Port Hills Overbridges Driving south from Christhchurch to Lyttleton are these six span precast, voided slab bridges on hammerhead bents. At least one of these bridges was built in 1970 and the other may have been built later. If you look closely you can see the bridges were retrofitted with braces and brackets to hold it together during an earthquake. Another nice detail is the column by the far abutment, which is the same length as the others but isolated from the surrounding embankment with a hollow casing. |
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| March (-43.605 Degrees, 172.622 Degrees) Oxford Street Bridge Looking toward Lyttleton Harbor and the Oxford Street Bridge. |
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| March (-43.605 Degrees, 172.622 Degrees) Oxford Street Bridge We are in the port town of Lyttleton where 61% of imports and 34% of the exports to and from the South Island are handled. It is also the closest location to the February earthquake, although we could find little damage to this bridge. That unfortunately was not the case for the town of Lyttleton, which looked battered by the earthquake. The Oxford Street Bridge is a long, winding, steel plate girder bridge on pier walls across roads and rails that goes between the hills and the Port at Lyttleton. |
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| March 2011 (-43.541 Degrees, 172.637 Degrees) Colombo Street Overbridge We'll finish our tour of Christchurch with some railway and roadway overcrossings. The Colombo Street Overbridge crosses over the railway in the middle of town. It is a long, continuous T-girder bridge on pier walls that was built in 1940. These types of bridges go from street level to 25 feet above the ground in as short a distance as possible and have piers of varying height. That makes the short columns susceptible to earthquake damage, although there was none during the February earthquake. Note the retaining walls at the two ends to bring the bridge down to grade. |
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| March 2011 (-43.430 Degrees, 172.646 Degrees) Chaney's Overpass One last photo of Chaney's Overpass. Its odd that the precast girders extend beyond the sides of the pier wall. However, the bridge deck is haunched downward to sit on the pier wall, so its actually a solid cap sitting on the wall. It seems like it would have been easier to have set the ends of the girder segments on the wall, but maybe it's easier to design it with the ends near the inflection points of the spans. The wall looks kind of thin to be supporting this big superstructure. Even though it is well-seated at the abutments we could see cracks in the soil where the pier foundation had moved back and forth during the earthquake. |
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March 2011 (-43.430 Degrees, 172.646 Degrees) Chaney's Overpass Another look at Chaney's Overpass. There are cracks, holes, and ejected material on the ground from February's earthquake. My friend Kawashima-san (in the yellow vest) is so well-known in Japan that he brings his own film crew on earthquake investigations to record his thoughts. The bridge barrier rail is bolted to the underside of the deck overhang. I wonder how effective it is at deflecting vehicles? They painted the bottom half of the pier walls for some reason. Maybe it makes them less reflective to oncoming traffic at night? The barrier rails are pretty close to the pier walls. I wonder if they have rules on how close the piers can be to traffic? It looks like the slope paving on the far embankment has slid downward. Also, it appears that the wingwall goes straight out instead of running parallel to the roadway. |
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| March 2011 (-43.430 Degrees, 172.646 Degrees) Chaney's Overpass There are just a few highway bridges around Christchurch. Chaney's Overpass is a three span precast box girder bridge that carries southbound Route 1 over the new road that connects northbound Route 1 to the Main North Road. This bridge site has some nice sand boils and other signs of liquefaction from the series of earthquakes that started in September of 2010 and has continued sporadically ever since. New Zealand has made an art and science of building precast girder bridges that are continuous for several spans. This bridge is supported on big pier walls. If you look closely, you can see a construction joint at the top of the walls, but with the wall reinforcement continuing up into the superstructure.The girders are cantilevered over the pier walls and made continuous with drop-in spans over the road and at the abutments. We'll take another look at this interesting bridge tomorrow. |
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| March 2011 (-43.451 Degrees, 172.606 Degrees) A photo of Groynes Footbridge showing the cable anchors buried in the ground. |
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| March 2011 (-43.451 Degrees, 172.606 Degrees) Groynes Bridge A groyne, according to a book on 'Bridge Scour' by Melville and Coleman, is a projection into a channel to reduce flow velocities through sedimentation. There is a rocky projection on the south bank of Otukaikino Stream just to the west of this bridge; perhaps to prevent scouring around the tower legs. Note the rectangular steel frame a little past the tower to keeps the cables in position. The ends of the cables are swaged into eye bars that are attached to fancy turnbuckles and connected to steel rings that are cast into the anchors. This is quite different than the cables on the suspension footbridges in Ecuador that were attached to the anchors by unravelling the ends of the cables and weaving them back further up the cable to form a loop. |
March 2011 (-43.451 Degrees, 172.606 Degrees) A steel suspension footbridge with concrete cruciform towers in 'The Groynes,' a meandering wetlands a little south of the Waimakariri River. As can be seen in the photo, the deck twisted due to the shaking during the February Earthquake. Every bridge has its characteristic way of responding to earthquakes. Fortunately, the shaking during this earthquake was small enough so we could see the initiation of damage at each bridge. This is better (in many ways) than when there is too much damage, which hides the evidence of how it occurred. We'll take another look at this bridge tomorrow. |
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| March 2011 (-43.381 Degrees, 172.656 Degrees) Mandeville Footbridge Due to frequent earthquakes during the last year this old, formerly elegant suspension bridge has become sadly disheveled. Fulton Hogan, an expert in cable bridge repairs, has begun re-tensioning the cables. The Waimakariri District Council has evaluated several approaches for the repair and hopeful they can return this symbol of Christchurch and Kaiapoi to its former attractiveness and functionality. Five spans with short, wooden towers and only two anchorages means these cables have to resist a lot of tension to hold up the deck without overloading the towers. Suspension footbridges are interesting structures, so we'll take a look at another one tomorrow. |
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| March 2011 (-43.381 Degrees, 172.656 Degrees) Mandeville Footbridge The Kaiapoi River joins the Waimakariri just before it empties into the Pacific. A mile upstream from the river's mouth is the town of Kaiapoi and a five span timber suspension footbridge, During the September 2010 and the February 2011 earthquakes the banks moved towards the river carrying the bridge anchorages with them. Consequently the deck sags and the bridge was closed when I visited it. Anchorages need to be buried deeply so they aren't effected by lateral spreading during earthquakes. This bridge was built in 1874. |
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| March 2011 (-43.412 Degrees, 172.651 Degrees) Old Waimakariri River Bridges |
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| March 2011 (-43.414 Degrees, 172.646 Degrees) New Waimakariri Bridge The Waimakariri River is quite a way south of this photo. Perhaps the owners of the Christchurch Northern Motorway wanted to provide the smallest possible grade for their bridges. Or maybe it was essential to provide a couple of roads north of the river. A braided river constantly regrades its path to the sea (as it drops sediment along its path). Perhaps long bridges were necessary over a river that periodically changes its channel. The new Waimakariri River Bridges are precast I girder bridges on pier walls with very long bent caps. Perhaps the extra large caps are to accommodate a bridge widening at some future date? |
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| March 2011 (-43.558 Degrees, 172.606 Degrees) Smartlea Street Bridge This is my last photo of a bridge across the diminutive Heathcote River. The river's source (an underground spring) is about a mile west of this location. After spending two months looking at two small meandering rivers, readers might think that New Zealand's South Island is a flat meadow. Actually, there are large braided rivers just north (the Waimakarira) and south (the Rakaia) of Christchurch. Also an enormous mountain range to the northwest and an extinct? volcano to the southeast. Very few highways and very few highway bridges though. We'll take a look at a bridge across the Waimakarira River tomorrow.  |
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| March 2011 (-43.565 Degrees, 172.639 Degrees) Malcolm Avenue Bridge A steel girder bridge that more closely resembles a Vierendeel deck truss. It was built in 1960 and suffered only minor damage during the February earthquake. |
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| March 2011 (-43.569 Degrees, 172.639 Degrees) Sloan Avenue Bridge Another damaged footbridge across the Heathcote River. My hosts from the University of Canterbury, Simone and Alessandro are standing on the bridge. I like the big round piers on each side of the river. They support a prestressed concrete T girder main span and side spans that descend to the ground. During the earthquake, the two piers moved closer together which cracked open the top of the main span at the ends (see figure below).  They put up tape to stop people from using the bridge, which apparently wasn't a very effective disincentive for people needing to cross the river. |
