Transcript Structures

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
Tie Zong
©2006 WashDOT
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:
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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
Mark Hedge,
Jess Haldeman,
Riley Mahaffey
Lochsa
Engineering
Forensic Engineering
• Finding out what went wrong.
– Insurance companies
– Lawyers
Construction Management
• Etc.
Major Graves
Major Graves
Research
Nick McDowell
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” 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
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:
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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
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Agriculture Buildings
“Typical” Buildings
Hazardous Buildings
Essential Facilities
Category I
Category II
Category III
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 (gable buildings).
– 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
terrain
Kzt = 1
Wind Loads
• Calculate Wind Loads
– Wind From East
– Wind From West
9.80
5.65
5.20
9.05
8.06
6.89
6.89
5.20
15.48
14.25
13.26
12.09
12.09
15.48
14.25
13.26
12.09
12.09