civl 331 - introduction to lsd

8/10/2019 CIVL 331 - Introduction to LSD

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AN INTRODUCTION TO

LIMIT STATES DESIGN

CIVL 331Steel and Timber Design

By:

Ferya Moayedi,

Salman Soleimani

January 16, 2013


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Analysis Vs. Design

Analysis is understandingthe structure anddetermining how theeffects of globally applied

forces are resisted byindividual structuralmembers

Design is an iterativeprocess to create a safeand reliable structurewhich is aesthetically

pleasing in an economicalmanner

2CIVL 331 - Intro to LSD by F. Moayedi, S.Soleimani


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Putting Thoughts Into aDesign Problem

Design is an iterative process

Structural layoutAnalyze the structure Find member forces

Assume structural membersAnalyze the structure again OK ??!

Modify member sizes

Analyze again 3CIVL 331 - Intro to LSD by F. Moayedi, S.Soleimani


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Structural Design

Tools available to engineers:

Codes

NBCC, IBC, BCBC

CSA-A23.3, CSA-S16-01

Software SAP2000, ETABS, Perform 3D

S-Frame, RISA 3D, Opensees

ANSYS, ABAQUS, CANNY

Specifications & Standards

ASTM, ACI, AASHTO, ANSI, CGSB Guidelines

Seismic Retrofit Guideline (SRG)

CIVL 331 - Intro to LSD by F. Moayedi, S.Soleimani 4


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Structural Systems

Moment resisting frame

(Beam-column system)

Braced frame

Un-braced frame

Other available

Structural Systems

Bearing-wall

Rigid frame

Long-span

High-rise

CIVL 331 - Intro to LSD by F. Moayedi, S.Soleimani 5


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Structural Design Approaches

Different design approaches

ASD

Allowable Stress Design

Also called Permissible Stress Design

WSD

Working Stress Design

LRFD

Load and Resistance Factor Design

LSD

Limit States Design



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Limit States Design (LSD)

LSD is a design method in which the performanceof a structure is checked against various limitingconditions at appropriate load levels

In a structural steel design, the limiting conditions tobe checked are:

Ultimate Limit States (ULS)

These states concern safety

e.g. load carrying capacity, overturning, sliding, fatigue

Serviceability Limit States (SLS) These states concern performance under normal operations

e.g. excessive deflection, vibration, or permanent deformation

Note: LSD is referred to as Load & Resistance Factor Design (LRFD) in US



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General Design Equation

Factored ResistanceFactored Load

R S

Where,

= resistance factor

R = nominal resistance of structural element

= load factor

S = specified load



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Design which is based on the probability and mode of

failure, or limit of usefulness, and the probability of the

occurrence and variation of the load

Characteristic resistance:

645.1

0.2

0.1 68%

95%

98%



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Probability of Failure

S

R2

R1

Failure occurs when R S

The hatched area = probability of failure

The global safety factor:

s

R

o

m

m



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Probability of Failure

Although the distance between the mean values mR and mSis thesame, i.e., the same o, the probability of failure greatly depends on

the scatter of both S and R values, i.e., the magnitude of their

respective standard deviations

In order to achieve a consistent probability of failure, the global

safety factor will have to be adjusted depending on the scatter (i.e.,

reliability) of load and resistant values

Exact knowledge of the distribution function in the asymptoticregions of the curves is essential, but unfortunately data for those

regions is usually not available or is too sparsely spaced



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Safety Index

Safety Index

Limit StatesDegree of Damage

small

medium

severe

Serviceability 2.0 2.5 3.0

Ultimate 4.2 4.7 5.2

22

sR

sR

z

z mmm



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Reliability and Safety

The purpose of the design process is to maintain anacceptable difference between the load S and the resistance R

Relationship between Safety Index and Safety Factor

95thPercentile of

load ( )

5thPercentile of

resistance ( )

645.1

645.1



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Advantages of Limit States Design

Utilizing Load Factors

Using distinct load factors for different loads, makes

the probability of failure more consistent in different

loading conditions

However, the Working Stress Design (WSD) approach uses asingle factor of safety

Utilizing Resistance Factors

Applying different resistance factors in order todetermine resistances of different members with

different materials, would result in a more uniform

reliability in general



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Advantages of Limit States Design



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Design Equations

Ultimate Limit States

Where,

= performance factor

R = nominal member strength, or resistance

i= load factor for specified loads (Table 13)

Si= specified loads (Clause 6.2: D, L, S, W, E)

Serviceability Limit States

Where,

= performance factor

R = nominal member strength, or resistance

i= 1

Si= specified loads (Clause 6.2: D, L, S, W, E)

R iSi R iSi

Note:D = dead loadL = specifiedlive loadS = snow load

W = specified wind loadE= specified earthquake load


*Ref. Clause 7.2.1


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Globally Applied Loads

Gravity Loads (vertical) Dead Load (DL)

Permanent structure loads

e.g. building weight, partition

walls, Live Load (LL)

Temporary loads

Varying loads

Use and occupancy loads Snow and rain loads

e.g. weight of the people

occupying the building,



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Lateral Loads (horizontally applied)

Earthquake Load

Wind Load

18

May cause uplift

CIVL 331 - Intro to LSD by F. Moayedi, S.Soleimani


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Load Combinations Structures, components, and foundations shall be

designed so that their design strength equals or exceedsthe effects of the factored load combinations

Analyze the structure considering all of the different

possible load combinations. Use the worst case scenarioas the governing load case

This will be the topic of next lecture



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Useful Tips

&

Housekeeping Notes



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Try to become a master in using your HSC !!!

Mark up your handbook very well, as you go

Refer to your handbook as much & frequent as you can

Read the book called Limit States Design in Structural

Steel, by Kulak Grondin

Try to utilize different software for differentapplications as much as possible!!! (Take it as a hint)

Familiarize yourself with many software and make sure you

know how to use them efficiently and effectively

e.g. Cheops, S-Frame, RISA 2D, Excel Spreadsheets,

Useful Tips for CIVL 331



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Some Housekeeping Notes:

Educational licences availablefor the S-Frame software S-Frame request for licence

Deadline:Jan 31st2013

Licence Agreement Form needs tobe signed (Obtain from Salman)

Other available software

CHEOPS RISA 2D (Educational Version)

http://www.risa.com/forms/2deducational.html

Dr. Frame, Dr. Beam, ?!

http://www.risa.com/forms/2deducational.htmlhttp://www.risa.com/forms/2deducational.htmlhttp://www.risa.com/forms/2deducational.html


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Any Questions?

Salman Soleimani:

[email protected]

Ferya Moayedi:

[email protected]

23CIVL 331 Intro to LSD by F Moayedi S Soleimani
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civl 331 - introduction to lsd

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