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

– Jarrod Milligan

Consulting for an Architect or Engineer

• The architect works with the client to establish project requirements:– space requirements and relationships – siting– aesthetics– lighting– 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

Forensic Engineering

• Finding out what went wrong.– Insurance companies– Lawyers

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, say, 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 (LARGE

estimate) for the load:• E.g., a “50-year” wind load, which is a wind 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

Simplified Wind Loads• Since we can’t predict exactly the

maximum load a given structure will experience, the code provides:– 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?

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, building height and shape, etc.

• The maximum loads estimated by the design codes are then factored to add a safety margin to our calculations.

Example Building

Design Methods

• Method 1 – Simplified Procedure:– Simple diaphragm building,– Low-rise,– Enclosed,– Regular geometry, symmetric,– Not flexible, prone to flutter/vortex shedding,

torsion etc.

• Method 2 – Analytic Procedure.

• Method 3 – Wind Tunnel Procedure.

Wind Loads on Structures

Gust Factor, G

• G = 0.85 for rigid, low rise buildings

Wall Pressure Coefficients, Cp

Wind Velocity Pressure

Importance Factor

• Agriculture Buildings Category I• “Typical” Buildings Category II• Hazardous Buildings Category III• Essential Facilities Category IV

Wind Load Map: Western US

Wind speeds in MPH (kph)

Velocity Pressure Exposure Coefficients

Exposure B, Case 2

Velocity Pressure Exposure Coefficients Notes

• Case 1– a. All components and cladding.– b. Main wind force resisting system in low-rise

structure designed using Figure 6-10 [Method 2].

• Case 2– a. All main force wind resisting systems in

buildings except those in low-rise buildings designed using Figure 6-10.

– b. All main wind force resisting systems in other structures.

• We will use Case 2.

Velocity Pressure Exposure Coefficients (cont.)

Exposure Categories• Exposure B:

– Urban and suburban areas, wooded areas… Exposure B shall be assumed unless the site meets the definition of another type of exposure.

• Exposure C– Open terrain with scattered obstructions…

• Exposure D– Flat unobstructed areas exposed to wind

flowing over open water for a distance of at least one mile…

Directionality Factor Kd

Topographic Factor Kzt

If flat terrainKzt = 1

Wind Loads

• Calculate Wind Loads– Wind From East– Wind From West

5.66

5.66

6.89

6.898.06

9.05

9.80 6.14

5.66

12.54

12.54

13.7214.71

15.97

12.54

12.54

13.7214.71

15.97