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STRUCTURAL ENGINEERING What Does a Structural Engineer Do? 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.