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Copyright 1996 by Leonard G. Barton

680/24 Construction Details

Pylons supporing the structure are built in several stages. At this location the soil is compact silt and sand. [Wall_ditch]

First, an excavation is made for the walls of an in-ground box structure, by digging four deep connected trenches. [Pylon_Form]

Reenforcing bar and forms are placed in the ditches and then the concrete walls are poured. Later, the earth in the center of the box is excavated. Precast concrete pilings are then driven into the soil in a precise pattern. [Pylon_Form]

Reenforcing bars connect each pile with a reinforcing mat that distributes the pylon load and ties the pilings together. The mat also has projecting bars that will join to the pylon. The mat is then poured. Atop the mat, a prefabricated tube of rebar is erected and held in place with a crane. Then the reusable form for the pylon is erected. [Pylons]

The form will now suport the pylon reenforcing and the crane is removed. The walkway seen at the top of the form is for the worker who will guide the hose from a concrete pumper. Great care and considerable skill is required to ensure that there are no trapped air pockets in the finished pour. The design is fundamentaly the same as that used in earlier designs. The significant difference is that the horizontal bands of rebar are now much stronger and closer together than in older freeway construction. This extra strength is intended to prevent bursting and failure of the pylon in the most severe earthquake expected. This particular pylon consists of two cylindrical structures that interlock to form an oval cross section.


Finished pylons are ready for construction of the falsework that will support the forms for the deck structure.


The white line seen below the bottom of the deck is a flexible plastic caulk sealing a gap between the deck and the foundation. Flexiblity is required since the length of the deck changes with temperature. The complex keying design prevents the end of the deck from slipping off of the foundation during earthquakes. Unlike some earlier designs that consisted of hinged segments, the entire crossing is a single monolithic piece from end to end. This ensures the integrity of the structure and reduces bending loads on the central pylons when seismic waves pass by.


This is the location of the recent falsework collapse. The collapsed section extended further to the right of the picture. The finished portion constructed here will take traffic from 24 east to 680 north. The view is from the corner of Mt. Diablo Blvd. and Boulevard Way, looking west. Further work involving steel erection will now take place at night and the streets and ramps under the construction will be closed during that time.


Right: View looking north (January). This temporary two lane flyover carries 680 south over the construction area. Some of the materials from this will be reused in the Olympic Boulevard exchange. It is built in three reenforced concrete sections connected with steel spans. THese spans can be removed quickly when the permanent structures are opened. This "decoupled" structure is also less prone to respond unfavorably to earthquake waves than a single long continuous structure. During the Loma Prieta earthquake the Cypress Viaduct structure in Oakland behaved in unexpected ways due to its continuous structure which could not be analysed as a whole at the time of its design, as well as a deffecient attention to certain design details.


Left: Another view of the flyover form a location to the right of and toward the previous view (April). The connector falsework supports will soon be joined.


Right: Straw bales are used to catch silt washing off of the earthworks and so prevent polution of our creeks. Our unusually warm winter and recent rains makes it look like spring this January day. Permanent landscaping will be installed as a finishing touch.


On an April-shower day, we see the view from the top of a dirt pile. View is southward, with the 680 South flyover above the horizon. Just below is the 24 East to 680 North connector, coming from the right and toward us. This connector splits on each side of one of the temporary flyover pylons, with the driver's right side (view left) forming the Ygnatio Valley Road off ramp, terminating at the grade level wedge seen at the left. This will continue descending into an underpass seen in the next view.


The trace of the Ygnatio Valley off ramp leads to this underpass, parallel to the Bay Area Rapid Transit (BART) tracks, seen at the left of this view.


To the right is a view of the Ygnacio Valley Road off ramp falsework. This is constructed atop adjustable posts.


At the left, a close view reveals the telescoping end pieces, topped with a screw jack.


Right, we have a view of the top of a pylon (in this case a very short one). At each pylon a transverse beam will be constructed to carry the load from the longitudinal beams.


View from the falsework deck of an overcrossing. The wall seen at in the middle distance is a transverse beam, connected to a pylon top similar to that seen in the preceding picture.

The structure of a deck consists of an upper and lower plate connected by a number of continuous beam webs. A short portion of these beams have been constructed here.

These longitudinal beams contain tubes for cables, tensioned after the concrete cures and before the falsework is removed.

This complex structure reduces weight and cost. Light weight is also important to reduce inertial effects during an earthquake. These reenforced concrete structures are actually quite elastic under moderate shaking. This observer was walking under the previous structure at this location during the 1989 Loma Prieta earthquake. Tall rectangular columns were seen to deflect almost a foot (30cm) by bending as the structure swayed, without damage, but with an alarming "whump" sound as the jointed deck sections banged together. (I did pick up the pace a bit just in case a vehicle came over the side.)

One of the techniques of seismic retrofiting in older structures is to lace the deck sections together with steel cables in order to prevent sliding to the point where the deck would fall, as happened during the Silomar and Northridge California earthquakes. Another retrofit is to place a continuous steel jacket around each critical column and fill the interval with concrete grout. This is being done at the Highway 24 Rockridge Station overpass in Oakland and is intended to prevent the kind of catastrophic pylon failure seen in the viaduct collapse in Kobe, Japan.

Earthquake effects on large structures

January 25th,1995 Kobe (Hyogoken-Nanbu), Japan earthquake.

October 17, 1989 Loma Prieta, California earthquake. (A promotion and sampler for a photo-CD.)

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