Evaluate economic factors
How do we evaluate our preliminary design?
Preliminary design is the first indicator of the project cost. The client will evaluate the preliminary design based on economic factors.
Steel is priced by weight. Naturally, clients evaluate the project cost using the weight of the main structural members selected during preliminary design. Structural engineers face pressure to choose the lightest members in design — the low-weight solution. But the low-weight solution is not the most cost effective solution. Carol Drucker’s presentation “Do’s and Don’ts of Steel Design (A Construction Friendly Perspective)” outlines evidence that supports this conclusion. One of the main concerns is the narrow flanges and thin webs common on low-weight members. These properties make connections difficult. Every length of weld, every bolt, and each extra connecting plate has an associated cost. And each new connection detail or construction sequence adds labour cost and complexity. Refer to the References section for more resources that discuss how to make cost effective design decisions.
When we evaluate our preliminary design, we identify actions that help manage project costs.
Minimizing fabrication labour cost
Cutting, welding, and fitting each piece of steel are three parts of the fabrication process. The fabricator sequences the process so that the amount of work is organized and completed error-free. Although parts of the fabrication process are automated, human labour still needs to weld, fit up, and inspect the material before it is shipped to the project site.
Consider how each member gets prepared in the shop. Every reinforcement plate needs to be cut, marked, and sorted. Then each plate — a flange stiffener, shear tab, or end plate — is fitted to the member. Welds are placed in multiple passes. Inspectors need to check each weld before fabrication is complete. Understanding how steel members and connections are fabricated makes us aware of the cost of fabrication.
The low-weight beam has low material cost. But, the additional labour will add substantial cost to the project.
Actions to help minimize fabrication costs:
- Using the minimum required weld
- Choosing stockier members
- Limiting coped beams
- Reducing column reinforcement
- Designing connections that are accessible in the field
- Making smarter framing arrangements
- Simplifying construction methods
Simplifying construction methods
Construction costs are related to the simplicity of the construction methods. Simplifying can make construction faster and safer.
- Actions to help simplify construction methods:
- Avoiding field welded connections
- Using field bolted connections
- Choosing deeper beams and columns, providing more access to insert and tighten bolts
- Allowing one-sided connections so the bolt can be inserted and tightened from a single position
- Planning an efficient construction sequence
Communicating accurate beam end forces
Beam end forces affect the connection design. We can control the accuracy of the force by using an organized method to our design. Clearly communicate the direction, magnitude, and load combination of each beam end force.
Structural engineers use convenient strategies to report the beam end forces, such as:
- Specifying a shear force based on the beam’s maximum moment resistance
- Specifying a percentage of the beam’s shear resistance
- Specifying end forces based on grouped beam sizes
These strategies make ultra-conservative connection designs. Although they are convenient for the structural engineer, the fabricator’s workload and costs increase.
Communicating accurate beam end forces simplifies the connection designer’s workflow. The task is simple with integrated analysis and drafting software. Indicate the beam end forces clearly on the drawing or in a table accompanying the drawings.
Actions to help communicate accurate beam end forces:
- Understand equilibrium in the beam and structure
- Trace the loads path through the structures
- Calculate pass through forces at joints
- Review force envelopes from all load combinations
- Indicate load combinations used
- Organize the governing loads
- Work with the drafting team to communicate the loads on the drawings
Summarizing cost estimation
Economic factors such as fabrication cost, construction methods, and design communication will help evaluate our preliminary design. Each factor exposes methods to manage costs and make smarter design decisions.