Demolition of Paseo Bridge

• Demolition analysis of a self-anchored suspension bridge

• Staged construction modelling of existing structure including all renovations made

• Staged demolition analysis using global and local models to ensure safe dismantling

Genesis Structures was appointed by the Paseo Corridor Constructors joint venture to carry out a detailed demolition analysis of the Paseo Bridge in Kansas City, Missouri. Using LUSAS Bridge analysis software, Genesis Structures built a model of the existing structure incorporating all major renovations made to it during its lifetime. This was further developed to include all proposed demolition/removal steps. An additional model was developed to assess the lowering of the main suspension cables and another model investigated detailed stresses and effects upon the pylon base and anchor bolt system. All analyses proved that the intended demolition sequence could be undertaken safely.

Overview

Paseo Bridge was designed by Howard, Needles, Tammen and Bergendoff in the early 1950's, and built by American Bridge Co. in 1954. It was a four-lane, self-anchored suspension bridge that carried I-29/35 and State Route 71 over the Missouri River in Kansas City, Missouri. Suspended spans of 308, 616 and 308 feet, south approach plate girder spans of 90 and 174 feet and north approach plate girder spans of 219 and 110 feet carried two 13-ft traffic lanes in both directions separated by a 4-ft median. When first opened to traffic it was the longest self-anchored suspension bridge constructed in the world.

As part of the kcICON project, funded by the Missouri Department of Transportation (MoDOT), a new iconic river bridge - the Christopher S. Bond Bridge - was constructed adjacent to, and to replace, the existing Paseo Bridge. Design and construction for the new bridge was carried out by Paseo Corridor Constructors (PCC), a joint venture partnership of Clarkson Construction Company, Massman Construction Co. and Kiewit Construction. PCC retained Genesis Structures to perform a demolition analysis of the existing bridge.

Bridge Construction

The 136-ft tall towers were of plated steel construction and were braced with upper and lower struts. Suspension cables passed through holes in the upper struts and sat on cable saddles that were supported on a set of 12, 6" diameter by 30" long rollers that rested on a substantial steel plated support. Pairs of hangers suspended along each cable supported the deck superstructure. For the deck itself, a 7" non-composite concrete roadway slab sat on 21" deep stringer beams that spanned between 6-ft deep transverse floor beams which, in turn, spanned between the massive 10-ft deep longitudinal plated stiffening girders. Hinged connections in these girders at mid-span and at each tower position used 13" diameter steel pins to accommodate rotational movement and thrust during construction. Following the placement of the concrete deck, temporary bolted connections previously made at these hinge positions were removed and cover plates were riveted to complete the connection and make the stiffening girder continuous. During construction the movement of the saddles was restrained by the use of locking angles which were removed at particular stages to allow the saddles to move longitudinally and allow re-positioning of the top of the towers. After the construction of the concrete deck and on completion of construction, the tops of the towers were adjusted for the final time and permanent locking angles were fitted to restrict any further movement.

Prior to carrying out a demolition analysis a detailed staged construction model of the existing bridge needed to be created. Original construction documents provided a suggested erection sequence and based upon this a preliminary evaluation of the required demolition was performed. During this evaluation, PCC was able to obtain the original erection plans that were developed by American Bridge Co. This provided a great deal of additional information to assist with the modelling process and included the actual constructed geometry of the stiffening girders and the main suspension cables as well as the exact fabrication lengths used for the suspension hangers. 

From all the construction information available, Genesis Structures was able to develop a very detailed and accurate model which not only represented the actual construction sequence but also incorporated all renovations made to the structure during its lifetime. This included an additional wearing surface and the replacement of the main stiffening girder bearings which resulted in an approximately 3" lowering of the deck system. As a result of this modelling process the displacements and stresses seen during the bridge's construction could be appreciated and the forces in the main cables and hangers could be obtained for its final in-service condition. A survey of the existing structure confirmed the accuracy of the LUSAS modelling process. Major milestones in the as-built analysis model included:

• Lifting and lowering the primary stiffening girder during construction
• Connection of the suspension hangers at the appropriate time step
• Modelling and locking/unlocking the main cable roller saddle
• Lowering of the superstructure to accurately account for a bearing replacement
• Modelling the addition of a wearing surface overlay.

Demolition modelling

After developing a comprehensive demolition sequence with PCC, Genesis Structures then appended all of the proposed demolition/removal steps to the erection model to assess all the effects on the structure. The wearing surface, slabs, stringers and selected floor beams were removed from the superstructure to leave a minimum structural configuration. To assist with the demolition four falsework towers, each comprising 36" diameter pipes braced in a six-pile configuration, were installed under the centre span. Jacks on these towers and a strand jack system supported on the existing bridge piers raised the longitudinal girders and removed the remaining superstructure load from the main suspension cables. Major milestones in the demolition analysis model included:

• Evaluating the effects of unlocking the main cable roller saddle
• Monitoring of the locked-in forces in the main towers
• Sequential removal of the existing superstructure
• Lifting analysis of the superstructure to remove the load from the main suspension cable
• Detailed analysis of both the lowering of the main suspension cables and the existing bridge pier anchor bolt system

Cable lowering

Following the removal of the superstructure load from the cables, a separate LUSAS model was developed to evaluate the lowering of the main cable to the deck level for removal. This was done by using strand jacks fixed to the upper struts of the towers to lower the upper strut and saddle in stages by amounts specified by Genesis Structures as a result of the LUSAS analysis. Additional loadcases were defined for the cable lowering model with pin or roller supports being inserted as necessary at the positions where the cable touched down on the frame, falsework, girder or floor beam supports. By this process the lowering of the cables was modelled and the displacements in the main cables and reactions at each stage could be seen.

Cable lowering model for Paseo Bridge

Animation of cable lowering for Paseo Bridge

Tower base modelling

To investigate the effects on the tower base system from dead and wind loading effects, superstructure lifting, cable lowering operations and for situations when the saddles were either released or locked during particular demolition stages yet another model was used. For this, the varying geometric properties of a pylon were modelled with shell elements for the lowest 24-ft with the remainder of its height being represented by thick nonlinear beams. Solid elements modelled the 5" thick steel pylon base plate and friction slidelines were used on their contacting surfaces to represent the interaction of the base plate with the concrete pier. Beam elements modelled the 10-ft long, 2.5" diameter anchor bolts cast into the concrete piers. Concrete bearing stress, base plate bending stress and maximum tensile forces in the anchor bolts were obtained for a set of considered loadcases.

In summary

Using LUSAS, Genesis Structures was able to accurately model construction techniques used in 1954 to determine the state of stress and geometry of this major self-anchored suspension bridge as the contractor prepared for the demolition. Knowing the exact state of stress in the structure allowed for a complete understanding of the demolition process during all stages including critical unbalanced lifting and lowering operations. David Byers, President of Genesis Structures said: “Being able to construct and deconstruct a major nonlinear structure like the Paseo Bridge in a single analysis model was critical in the success of the demolition.”

"Being able to construct and deconstruct a major nonlinear structure like the Paseo Bridge in a single analysis model was critical in the success of the demolition."

Dr David Byers, President, Genesis Structures

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