ce 382 l1 structural analysis introduction

7/28/2019 CE 382 L1 Structural Analysis Introduction

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CE 382

Structural Mechanics Blue text is important course

material

Green text is importantdefinition material for thecourse

Red text is important conclu-sion or caution material for thecourse

Text in bold is more signi-

ficant than non-bolded text


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STRUCTURAL

ANALYSIS

As a structural engineer, you willbe required to make manytechnical decisions about

structural systems. Thesedecisions include:

(1) selecting an efficient, econo-mical and attractive structuralform;

(2) evaluating structural safety,that is, its strength and stiffness;

and(3) planning the erection of thestructure under temporaryconstruction loads.


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In order to perform structuraldesign, you need to learn to carryout structural analyses.

Structural analysis involves the

prediction of the performance

of a given structure underprescribed loads and/or other

external effects, such as

support movements and

temperature changes.

The fundamental purpose of a

structural analysis is to deter-

mine the magnitudes of force

and displacement for each

element of a design system for

a given set of design loads.


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Force is used to represent

stresses or stress resultants,e.g., axial forces, shear forces,bending moments, and torsionalmoments along with theirassociated stresses.

Designers determine the internalforces in key members in order tosize both members and the

connection between members.

Deflection calculations are carriedout to ensure a serviceablestructure one that does notdeflect or vibrate excessivelyunder load such that structuralfunction is impaired.


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Stated differently, structuralengineering is the science and

art of planning, designing, and

constructing economical

structures that can safely

resist the forces to which they

are subjected to with

elegance.


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Structural

nalysis/Design

Flowchart


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DESIGN OBJECTIVES

Factors which influence design

include: safety, serviceability,

feasibility, and esthetics.

Safety Normally, structuralengineers design safely in casesomething unforeseen orunknown happens. To properlyassess safety, the structuralengineer must:


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A. Understand the environment,

use of the structure, behavior ofthe construction materials,structural behavior, appropriateallowances for the uncertainty

involved in the prediction ofload effects and structuralresponse, and any otherinfluences that may influence

the design or designperformance.

B. Determine the internal forces

and failure load.


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A number of approaches tostructural safety are possible. Onemethod is referred to asAllowableStress Design (ASD).

ASD is based on the assumption

that if the stresses under workingloads are limited to valuessubstantially smaller than stressescorresponding to failure, then

safety is assured.

Other approaches include:Ultimate Strength Design,

Plastic Design, Load and

Resistance Factor Design andothers.


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These limit state proceduresincorporate safety by multiplyingdead loads and live loads by loadfactors greater than one but lessthan two. The live load factor istypically larger than the dead loadfactor since dead loads are moreaccurately assessed.

Furthermore, all design procedures

typically reduce the load carryingcapacity or strength parametersassociated with the structure.Typically, the structural strength

parameters are reduced by 0.60 to0.90.


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Serviceability All aspects of

performance must be acceptablefor the intended use. Somespecific performance consider-ations that the structural engineer

must consider include: deflectionand cracking must be limited, i.e.,not visible; vibration and noiseshould be controlled; liquid and

gas containers should not leak;foundations must not settleimproperly; etc.


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Feasibility Construction of aproposed structure must be

economical as well as plausible(feasible). A basic rule in designis that the designer shouldcontemplate at least one method

of construction. The structuremay not be built in exactly themanner anticipated, but still thefeasibility of the structure is

assured.

Esthetics (or Aesthetics) Thelooks of the structure are of great

concern to the community and thedesigner.


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STRUCTURESTruss: A truss is a geometrically

stable arrangement of slendermembers which primarily supportaxial load.

Frame: A frame is a stablestructural form consisting of two ormore flexural members that canresist bending moment, shear,

and axial forces. A frame isclassified as a rigid frame whenits members are joined togetherby moment resisting connections;

i.e., where joint translations androtations occur without relativerotations between the connectedmembers.


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Arch:The arch can be definedas a curved structural shape that

is usually configured to supportgravity loads in a manner thatresults in uniform compressiveresistance. Gravity loading tends

to flatten the arch and push itssupports outward.

Cable: Cable supported struc-

tures are very common and areoften expressed in the form ofsuspension and cable-stayedbridge construction.


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Membranes are thin-walledstructures such as air supported

stadium roofs and weatherballoons which provide tensileresistance in two directions.

Flat plate structures canprovide bending, tensile, andcompressive force resistance,e.g., floor slabs.

Shells are often defined ascurved plates. Roof domes,water and fuel storage tanks, and

grain silos typify some forms ofshell structures.


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Computer Analysis

Most structural computerprograms perform a first-orderanalysis: (1) linear elasticbehavior, (2) member forces are

unaffected by deformations(change of geometry) of thestructure, and (3) no reduction inflexural stiffness.

Computer programs have tremen-dously reduced the labor of per-forming structural analysis and

design.


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The designer must still possessbasic insight into all potential

failure modes in order to assess

computer solution reliability.

Furthermore, the engineer mustprepare a mathematical model

that adequately represents the

structure and the engineer mustinterpret the results correctly.


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CE 382 StructuralCalculations

Prepare a clear sketch of thestructure, showing all loads anddimensions.

Include all steps of yourcomputations.

Check your results .

For computer generated results:

Verify that the deflection direc-tions are consistent with theapplied forces computer

programs typically include post-processing features that includescaled displacement plots of thestructure.

ce 382 l1 structural analysis introduction

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