The URL can be used to link to this page

Home My WebLink AboutAttach C.3 - 10 Pct Structural Basis of Design MEMO Boeing South Bridge Seismic Retrofit – Design Criteria Date: May 29, 2025 To: Jared McDonald, P.E. (City of Renton) From: Aaron Olson, P.E.; Jacob Chekal, P.E. Subject: Panther Creek 10% Bridge Design – Basis of Design Structural Summary The proposed bridge shown in the 10% design drawings replaces the existing culvert for the Panther Creek crossing of Talbot Road South. The bridge superstructure is a concrete deck supported by Wide Flange (WF) Precast Prestressed Concrete girders. The superstructure supports vehicle and pedestrian traffic as well as multiple bridge utilities. The nominal span length is 100 feet. The bridge span length provides a minimum 3-foot inspection clearance below the girder soffits in accordance with the WSDOT Bridge Design Manual (BDM) recommendations with a maximum finished grade slope of 2H:1V from the edge of the typical stream section. The span length was selected as a balance between maximizing wall heights (within typical values) to minimize bridge length. In general, minimizing the bridge length results in lower construction and long-term maintenance costs as walls are more cost effective to build and maintain. Additionally, minimizing the span length results in lighter girders which facilitate easier crane picks during setting of the girders. The span length can be adjusted in future design phases as needed to accommodate changes in hydraulic and geotechnical information. The bridge foundations use drilled shaft deep foundations. Walls are provided behind the abutments parallel to the roadway for grading and scour protection requirements. The following sections of this Basis of Design Memo discuss Stream, Geotechnical, Roadway, Utility, and Right of Way considerations that affect the bridge layout. Stream Considerations The proposed stream alignment for Panther Creek runs perpendicular to the roadway alignment. The stream section is 20 feet wide with an 18-foot-wide minimum hydraulic opening (MHO). The structure clearance at the stream section is based on the following Structure Free Zone (SFZ) requirements. The structure free zone requirements are provided by GeoEngineers. - SFZ Width: 18 feet, matches MHO. - Controlling Bottom Elevation: 3 feet below thalweg, based on scour ATTACHMENT C.3 MEMO - Controlling Top Elevation: 8 feet above the thalweg, based on a 6ft maintenance clearance above the maximum finished grade stream elevation within the MHO. The structure span length and vertical clearance are not governed by SFZ requirements due to the height of the roadway above the stream. The design flood and check flood scours are assumed to be 3 feet for 10% design. The scour angle of repose is 26 degrees per Geotechnical recommendations. The scour catch point begins at the edge of the stream section. Lateral migration is a potential concern for this stream crossing and will need to be assessed in future design advancement. The top of shaft caps are placed below the scour line in accordance with WSDOT BDM 7.1.7 and the bottom of wing walls are placed 2 feet below the scour line or finished grade in accordance with WSDOT BDM 8.1.10. Geotechnical Considerations The geotechnical recommendations for 10% design are based on three borings taken at the site location. Spread footings are not a consideration for this bridge due to the slopes in front of the abutments and the depth of competent bearing soils. Driven piles are also not considered considering the presence of the bedrock layer. Typically, it is not feasible to anchor, or “socket”, driven piles into bedrock. Without the restraint provided by socketing, piles are not expected to provide enough lateral stability for the bridge. Additionally, driven piles induce significant amounts of vibration which may be undesirable for the adjacent utilities and/or the community. Drilled shafts are used to get into the bedrock layer for both axial and lateral capacity. It is assumed for 10% design that rock sockets will likely be required to drill into the rock layers per Geotechnical recommendations. Drilled shafts will use the oscillator method of drilling due to site specific soil conditions. This will result in shafts being specified and built based on metric shaft sizes. Shaft sizes shown for 10% design are called out as nominal diameter for simplicity. The axial capacity of the drilled shafts is based on shaft capacity plots provided by GeoEngineers. These plots show a minimum factored strength capacity of 600 kips of end bearing resistance for a 3-foot diameter shaft. This results in an 85 ksf factored bearing capacity in the bedrock layer at the strength level. Shaft lateral behavior is based on the following soil parameters in Table 1. Table 1. Shaft Lateral Analysis Parameters MEMO Soil Unit Approx Layer El (top to bottom) Soil Model Effective Unit Weight (pcf) Friction Angle (degree) Stiffness Parameter, k (pci) Fill 86 to 69.5 Sand (Reese) 115 30 90 Weathered bedrock 69.5 to 62 Sand (Reese) 67.6 35 125 Intact Bedrock 62 to 50 Sand (Reese) 77.6 42 125 The length of shaft is based on the depth required to stabilize deflections due to lateral loading. The shaft length is extended an additional two shaft diameters beyond the minimum depth for stability to account for potential variability in soil layer elevations. The ground slope at the top of the shaft is used to model the shaft behavior regardless of the thalweg depth per Geotechnical recommendations at 10% design. The site acceleration coefficient (As) used is 0.426g. Earth pressures for design are based on the following factors. - Soil Unit Weight: 125 pcf - Active Earth Pressure Coefficient Ka = 0.32 - At-Rest Earth Pressure Coefficient Ko = 0.47 - Passive Earth Pressure Coefficient Kp = 3.25 - Seismic Earth Pressure Coefficient Kae = 0.38 Roadway Considerations The bridge width is based on the following roadway section at the bridge and approach slabs as shown in Figure 1. - (2) 5’-6” Sidewalks - (2) 5’-0” Bike Lanes - (2) 11’-0” Traffic Lanes - (1) 12’-0” Turn Lane MEMO Figure 1. Typical Roadway Section at Bridge Pedestrian barriers with BP rail provide a 54” fall protection height for the structure. The use of the standard pedestrian barrier is acceptable at this bridge location due to having a minimum of 5ft of raised sidewalk and roadway speed that does not exceed 35 mph. The roadway grade will be changed to provide a constant grade with a minimum slope of 0.5% in future design phases to facilitate drainage. This change is assumed to result in negligible change to the structure layout. Utility Considerations There are multiple existing utilities at the proposed bridge location including overhead power, telecom, gas lines, wastewater line, water lines, and underground power. The following utilities are proposed to be supported by the bridge. - Telecom Line - 2” Gas Line - 16” Gas Line - 21” Wastewater Line - 12” Water Line It is assumed that all utilities will need to be temporarily relocated to allow for bridge construction. Right of Way Considerations There is limited space in the City’s existing right of way lines adjacent to the proposed structure. Temporary access beyond the City’s Right of Way limits will be needed for bridge construction. Codes/References The structural design criteria for this project are based upon the following design standards: MEMO - AASHTO Guide Specifications for LRFD Seismic Bridge Design, 3rd Edition (2023) - AASHTO LRFD Bridge Design Specifications, 9th Edition (2020) - WSDOT Bridge Design Manual, M 23-50.23 (July 2024) - WSDOT Standard Specifications for Road, Bridge, and Municipal Construction, M 41-10 (2025)