Catalog Description Elementary Structures II Text Gradingcourses.washington.edu/cee380/intro.pdf ·...
Transcript of Catalog Description Elementary Structures II Text Gradingcourses.washington.edu/cee380/intro.pdf ·...
CEE380
Elementary Structures II
Introduction
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Catalog Description
• Structural design concepts, approaches,
procedures, and codes. Characterization and
determination of loads (dead, live, seismic, wind,
etc.) Structural systems and system behavior (load
paths, lateral and vertical response, failure modes
and limit states). Structural component behavior
and design (composite action, inelastic bending,
column stability, member capacities). Prerequisite:
CEE 379.
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Text
• Minimum Design Loads for Buildings andOther Structures, ASCE7-05.
• CEE380 design aids.
• Powerpoints
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Grading
• Homework Assignments 25%
• Quizzes (3, includes final) 75%
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ASCE7-05
• Become familiar with the layout and
mark sections
• Read the commentary as well as the
chapters
• Be prepared to use on the quizzes
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Factors influencing structural
design
• Primary function of the structure
• Inflexible requirements
– Code
– Topography
– Foundation
• Secondary requirements
• Materials available
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Structural Design Procedure
• No unique solution = art + science
• General Steps:
1. The function of the structure is clearly defined
2. The general layout is established to fulfill 1.
3. Several solutions are proposed. For example,
steel vs. concrete vs. combination
4. Preliminary structural designs of the solutions
proposed are undertaken.
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Procedure continued
5. The solution which appears to be the most
satisfactory is selected.
6. A detailed structural design of the solution
from 5. is undertaken.
– Frequently, steps 2, 3, and 4 merge.
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Stages in Preliminary and
Detailed Design1. Approximate structural system
2. Loads determined
• Design codes and specs
• Assessment
• Weight estimation
3. Stiffness evaluation
4. Structural analysis
5. Members
(Check 2 and 3.)12/29/09 10
Design Codes and
Specifications
• Provide minimum standards for design
and construction
• Health and safety
• Lower standards not permitted
• Higher quality may be provided
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IMPORTANT
STANDARDS DO NOT
REPLACE ENGINEERING
KNOWLEDGE,
EXPERIENCE OR
JUDGMENT.
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Types of Codes
1. Minimum requirements for structural detailsand methods of analysis and design
2. Specify performance -- materials oriented
3. Comprehensive• Particular type of structure
• Different materials
• Specifies design loads
• Fire protection requirements
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Examples
• ASCE7 Standard
• International Building
Code (IBC) [2000];
prior to that Uniform
Building Code [UBC],
BOCA Building Code
and Standard Building
Code
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Materials Oriented
•Specification for the Design, Fabrication and
Erection of
Structural Steel for Buildings:
American Institute of Steel Construction
[AISC]
•Manual for Engineered Wood Construction;
American Forest and Paper Association,
American Wood Council
•Building Code Requirements for Concrete:
American Concrete Institute [ACI]
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Legal Status of Codes
• Determined by governmental bodies
• Consensus codes such as ACI document
• Membership balance1. Designers
2. Material suppliers
3. Government officials
4. University researchers
• US distribution
• Public meetings
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Structural Design
Philosophies
• Basic objective: To providesafety and serviceability witheconomy.
• In order to achieve this goal,strength AND stiffness must beadequate.
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Three loading categories
typically considered
1. Daily functions without excessivedeflection, cracking or vibration
2. Moderately high loads such as minorearthquakes without permanentdamage
3. Major overloads such as a severeearthquake sustained without collapsebut some damage acceptable
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Two major viewpoints
historically
1. Allowable stress design [ASD]
uses the concept of allowable
stress.
2. Load resistance factor design
[LRFD] uses load and capacity
reduction factors.
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Allowable Stress Design
[ASD]
• Linear elastic behavior
! Hooke’s law
! Stress for service loads
• Members proportioned so that
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ASD continued
�
! actual "! allowable
! allowable = 0.60! failure
• The failure stress may be the yield
stress, the fatigue limit stress stress or
the buckling stress.
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LRFD Method
�
!Q " #R
Q $ loads; R $ resistance
! is a load factor; # is a capacity factor
Typically ! %1; # "1.
• Moments and forces determined using plastic design
of steel and strength design of concrete
• Timber recent convert to LRFD
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Limit States Design:
ASCE7
• Definition [p. 1]: A condition beyond
which a structure or member becomes
unfit for service or is judged either to be
no longer useful for its intended function
.. Or to be unsafe…
• In design, the behavior of the structure
is checked at “limit states.”
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Examples
• Serviceability limit states: deflection
and crack width limits specified for
short and long term loads
• Collapse limit state -- a margin of
safety above service load is
required
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Example of deflection limits
• Vertical: Span Length/ 360
• Horizontal: 0.003 times
building height
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Material Behavior: Steel
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Material Behavior : R/C
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Structural Safety
Considerations
• Primary purpose: protection of life and
property
• Reasons for a margin of safety
1. Uncertainty as to the actual loads that will act on
the structure
o Actual loading is randomly distributed
2. Uncertainty as to the actual effects produced in
the structure by the applied loads
o Structural behavior is not fully linearly elastic; supports
are not truly pinned
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Structural safety continued
3. Uncertainty as to the actual strength of the
materials
o Variability in actual material strength; quality
control issues.
4. Uncertainties as to the cumulative effect of
small adverse variations in workmanship
o Individual variations are considered
acceptable.
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Example of
building frame
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ASD: Required margin of
safety
! allowable (at service load) = a
b
"#$
%&'! failure
where a
b ( margin of safety
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LRFD
• Load factors
o Design load = load factor [for DL] times dead load
+ load factor [for LL] times live load, etc.
o Critical combinations are considered as dictated
by ASCE7.
• Capacity reduction factors
– Material strength reduction factors
– Example from ACI
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Typical ! values from ACI
Flexure 0.90
Shear and Torsion 0.85
Spirally reinforced compression
members
0.75
Tied compression members 0.70
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Ties and Spirals…
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Failure occurs from
1. Overstressing (Insufficient strength)o The material is of insufficient strength and it
actually crushes, tears, rips or breaks.
2. Excessive deformation (inadequatestiffness)
o The structure deflects too much, has excessivecurvature, vibrates extensively or buckles.
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Demand ! Capacity
Demand " loading - induced stresses or displacements
Capacity " material strength
Demand side of equation has load factors;
Capacity side of equation has capacity reduction factors.
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References
• ASCE7-05
• Design Aids
• Structural Systems Design, Coleman.
• Fundamentals of Structural Design, L.A.
Hill, Jr.
• Army Manual TM 5-809-10