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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:
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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
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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”
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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.