STRUCTURAL ENGINEERING

What Does a Structural Engineer Do?

Roles of a Structural Engineer Lead engineer/Project engineer Consultant for an architect Consultant for another engineer, insurance companies, lawyers, etc. As well as: –Aerospace design. –Product design, etc. for industries. –Facilities engineer.

Lead or Project Engineer Defines project goals –Costs –Performance requirements Supervises design based on these requirements. Outlines tasks –What needs to be done & who will do it Organizes Project –Calendar –Sequence

Palm Valley Interchange

Lower Granite Dam Lock Repair

Consulting for an Architect or Engineer The architect works with the client to establish project requirements: –space requirements and relationships –siting –aesthetics –lighting –finishes –budget

Consulting for an Architect or Engineer The engineer’s job is to make the architect look good. –Ensure integrity of structure –Provide economical solutions. –Develop innovative ways to solve new problems and use new materials.

Boise Air Terminal

Design Process Conceptual design –Layout (location, size, shape, spans) –Materials (steel, concrete, masonry, timber) –Performance requirements –Cost estimates

Design Process (cont.) Preliminary Design –Layout framing –Rough sizing of members & foundation –Interaction with mechanical, electrical, etc.

Design Process (cont.) Final Design –Detailed analysis –Final member sizes –Preparation of bid documents Inspect and review construction process.

Day-to-Day Tasks “Beginning” –Quantity take -offs –Checking shop drawings –Inspecting construction –Repetitive simple designs

Day-to-Day Tasks “Intermediate” –Developing complex computer models Analyzing wind or earthquake loads –Creating complex designs

Day-to-Day Tasks “Advanced” - Working directly with client –Establish project performance criteria –Select structural framing system and layout –Estimating costs

Design Loads Design loads include: –Dead loads Self-weight, “Permanent” contents. –Live loads Occupants, Transient contents –Environmental loads Wind, snow, earthquake, etc.

Uncertainty Dead loads can be predicted with some confidence. Live load and environmental load predictions are much more uncertain. –E.g., it is nearly impossible to say what will be the exact maximum occupancy live load in a classroom. –It is also difficult to say how that load will be distributed in the room.

Uncertainty (cont.) Structural codes account for this uncertainty two ways: –We chose a conservative estimate (high-side estimate) for the load: E.g., a “50-year” snow load, which is a snow load that occurs, on average, only once in 50 years. –We factor that estimate upwards just to be sure.

Load Factors Newer codes have separate load and resistance factors: –Load factors “overestimate” the load. –Resistance factors “underestimate” the strength of the structure. Dead load factors range from 1.1 to 1.4 –Smaller uncertainty. Environmental and live load factors range from 1.7 to 2.0 and higher. –Higher uncertainty

Design Loads Since we can’t predict exactly the maximum load a given structure will experience, the design codes provide: –Rational procedures for estimating a reasonable maximum value –Procedures for arranging the loads on the structure. Experience has shown that if the engineer follows these procedures he/she can expect the structure to perform properly (I.e., not collapse, etc.)

Wind Loads What factors should the wind design loads consider?

Wind Loads Current codes consider: –Maximum wind speed expected at the location in question Maximum speed in a 3-second gust with a 50-year return period. –This is based on historical data Coastal regions, such as Florida, have higher design wind speeds than most inland areas.

Topography Codes consider the effects of general topography. –E.g, inland vs. exposed coast. They also consider local terrain –Exposure factors account for shelter provided by surrounding buildings and trees. Standard wind speed measurements are based on “moderate” topography and terrain.

Building Height Codes also account for the fact that wind speeds tend to increase with height. –Standard measurements are made 33 ft (30 m) above the ground.

Aerodynamics Codes must also consider the drag forces generated by the wind. The drag coefficient is based on –The shape of the building Rectangular vs. rounded, etc. –Whether building is “open” or “closed” Based on the number of windows and doors –Windward vs. leeward forces.

Importance Factor Codes also consider how important the building is. –If the building is a hospital that must remain functional during a hurricane, the design wind loads must be increased. –If the building is an agricultural storage building that doesn’t endanger anything nearby, the wind loads can be decreased.

Summary Design loads used by engineers represent rational estimates of loads that we should consider in our design. –Experience has shown if we design for these loads, the building should survive for a reasonable amount of time (50 years or more).

Summary (cont.) The models try to consider situations that will have a significant effect on the design load. –Max wind speed, topography and terrain, building height and shape, etc. The maximum loads estimated by the design codes are then factored to add a safety margin to our calculations.