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