Understanding the process for the Willamette River bridge has been difficult.
As we did in the first phase when we were evaluating various bridge types, TriMet utilized management best practices and relied upon the National Constructors Group (NCG) for an independent cost estimate to build the wave frame and cable-stayed bridge type. NCG is an industry leader and a firm of retired bridge engineers and contractors who have been hired by government agencies throughout the U.S. to do independent analysis on the cost to build major infrastructure, including bridges.
When we began detailed estimates back in December on the wave frame and the cable-stayed, they provided the cost estimate for how much it would cost from a contractor's perspective. Based on NCG's estimates, WRBAC selected the cable-stayed design type because the wave frame was more expensive and with a lot of unknowns and risks associated with it. NCG is an objective third party that has no investment in the design.
The process starts with the engineering team developing a computer model of the bridge and then applying load scenarios (how it will be used) to the model to determine how big structural members need to be. After the structural member sizes are known, the engineering team develops a list of quantities for each material such as concrete, cables, etc. This information is then given to the NCG who uses it to develop a full workup of labor, equipment, material costs and production rates to generate an independent cost estimate.
NCG did not ask the architect for cost estimates as they have experience providing full cost estimates from the ground up for both cable stayed and suspension bridges since they have built these bridge types around the US. This methodology provides a more accurate estimate compared to engineers and architects who typically develop estimates by square footage.
Admittedly, I was wrong about some of this. There's a learning curve when it comes to understanding bridge selection process versus buildings.