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  • Writer's pictureSaqib Khan

Jacking a Century-Old Truss Bridge: Post #3 Detailed Design

For a background on this series review Post #1 here.


The deck rehabilitation process for the truss spans involved a full replacement of the concrete slab and all central truss span longitudinal stringers. Additionally, the deck was widened to accommodate a new pedestrian sidewalk, as depicted in the bridge section of Figure 1's final construction stage. At this stage, the roller mechanism of the bearings at Pier 1 was replaced. There are three truss planes, labeled as A, B, and C.

Figure 1. Sketch depicting Final stage of construction (Source: St. Andrews Lock & Dam Bridge Deck Replacement Drawings)
Figure 1. Section at Final Rehabilitation Stage

Part of the larger scope for Spannovation was to increase the number of work fronts for replacing permanent bracing members in the truss spans, without necessitating substantial temporary works. This required our team to create a 3D model of a typical truss span. We used this model to conduct an in-depth analysis during the replacement of permanent bracing members. We updated this model for the final construction stage and analyzed it to calculate the jacking and blocking loads during the expected construction stage at the time of Pier 1 jacking, which included lateral loads from the wind.


The replacement process involved lifting the Pier 1 bearing onto temporary blocking and then lowering it onto refurbished bearings. Throughout this process, the bridge was supported on blocking, and both traffic lanes remained operational. However, usage was restricted to trucks weighing no more than 36 tons. Figures 2 and 3 below illustrate the controlling cases for the maximum vertical reaction at service and ultimate limit states. These occur at Truss Plane B, which receives tributary contributions from both the roadway and the sidewalk decks.

Figure 2. A screenshot of the FEM model displaying the vertical Permanent Bearing Load (Service)
Figure 2. A screenshot of the FEM model displaying the vertical Permanent Bearing Load (Service)
Figure 3. A screenshot of the FEM model displaying the vertical Permanent + Transitory Bearing Load (Ultimate)
Figure 3. A screenshot of the FEM model displaying the vertical Permanent + Transitory Bearing Load (Ultimate)

Jacking Load (Per Jack)

The Jacking load is calculated under the assumption that traffic is closed during jacking operations. The reactions for minimum jacking loads are as follows:

  • Reaction Load per jack = 1638 kN/ 2 jacks = 819 kN (Assuming jacks are positioned equidistant from CL post/bearing)

  • Hence, the Minimum Jacking Load = 1.5 x Reaction Load = 1.5 x 819 kN = 1229 kN


Load Adjustment from Movements & Tolerances

The adjusted jacking load is determined based on the following assumptions:

  • Allowance for site placement tolerance in the positioning of the jacks = 50 mm

  • Longitudinal movement of CL Post/Bearing relative to jacks = ~50 mm

  • Total offset of CL Jack from CL Post/Bearing = e = 100 mm

  • The Adjusted (Increased) Jacking Load is calculated as: (L/2 + e)/L x P = (600mm + 100mm)/1200mm x (2 x 1229 kN) = 1434 kN per jack


Figure 4. Sketch depicting loading and reaction locations on Jacking Beam arrangement
Figure 4. Sketch depicting loading and reaction locations on jacking beam / blocking assembly

Blocking Load (Per Bearing)

The blocking load acting on each bearing for the entire bridge load is calculated based on the following assumptions:

  • Traffic is open while supported on blocking, with North and South lanes restricted to a 36 Ton truck each.

  • Both the blocking and the entire jacking frame are designed to withstand the greater of 1.5 times the Service and Ultimate Bearing Loads.

  • The blocking load is calculated as follows: Max (1.5 x Service, Ultimate) = Max (1.5 x 2118, 2795) = 3177 kN


Jacking Beam

The jacking beam was conservatively treated as a simply supported beam during blocking, even though it was rigidly connected to the frames at both ends. The figure below depicts this static system. The jacking beam was configured with five 25mm thick plates placed side by side, composed of 490MPa steel.

Figure 5. Statical Diagram for Design of Jacking Beam
Figure 5. Statical Diagram for Design of Jacking Beam

Longitudinal Loads

The maximum longitudinal load restrained by the stools anchored to the pier cap during blocking is limited by the Teflon on steel coefficient of friction, which is 4% of the vertical load. Four Hilti Kwik anchor bolts, each 3/4" in size with 4" embedment, were required per stool to transfer the longitudinal forces.


Transverse Loads (Wind) The maximum transverse load during blocking is calculated based on the demand from the maximum of a 10-year wind event (with traffic) and a 50-year wind event (without traffic). To counteract transverse loads from wind events, an external strut, connected to the pier wall, was installed to stabilize the truss in both directions.

Figure 6. Sketch depicting Transverse Restraint to Transfer Transverse Wind Forces
Figure 6. Sketch depicting Transverse Restraint to Transfer Transverse Wind Forces


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