Mat-Ch36

1Portal Frame Teaching Resource in Design of Steel Structures IIT Madras, SERC Madras, Anna Univ., INSDAG

PORTAL FRAMES

PROF. V. KALYANARAMANDepartment of Civil Engineering

Indian Institute of Technology MadrasChennai 600036

email: [email protected]


PORTAL FRAMES

OUTLINEINTRODUCTIONGABLE PORTAL FRAMESGENERAL DESIGN PROCEDURESECONDARY DESIGN CONSIDERATIONSSUMMARYDESIGN EXAMPLE


INTRODUCTIONApplications

Single storey Industrial BuildingsMulti-storey Building Gantry Crane StructuresSuspension Bridge Towers

Types of FramesRectangular Portal FramesGable Portal FramesHaunched Portal Frames


INTRODUCTIONGable Portal Frames (Normal ranges)

Span 9 m to 25 mRoof slope 1 in 3 to 1 in 10Spacing 4 m to 7.5 mStanchion height 3m to 15 mBase fixed or hinged

SpacingStanchion heightSpan

Slope


INTRODUCTIONMethods of Analysis & Design

Elastic AnalysisPlastic AnalysisNon-Linear Limit Analysis

Allowable Stress DesignPlastic DesignLimit States DesignOptimum Design (Plastic or Limit States)

Advanced analysis & Design


GABLE PORTAL FRAMESTypes of Gable Portal Frames

Prismatic Haunched

Tapered Latticed


RELATIVE MERITSPrismatic

Simple to fabricateInefficient use of materialAppropriate for short spans

TaperedEfficient use of materialFabrication cost is highAppropriate for large spans

HaunchedRelatively Simple to fabricateEfficient use of materialAppropriate for all spans

LatticedVery efficient use of materialFabrication cost is relatively

highAppropriate for large spans


GENERAL DESIGN PROCEDURE

Steps in Limit design Using Plastic AnalysisDetermine possible loading conditionsCalculate factored load combinations Estimate plastic moment capacity ratiosAnalyse for required plastic moment capacity Select the appropriate sectionCheck the section for secondary failure modes


ESTIMATION OF PLATIC MP RATIOS

Optimum Plastic Design To Obtain Mp

Simple Approximate MethodDetermine the absolute plastic moment

Do this for all the load combinationsArrest all joint rotations but not the swayThe actual plastic capacity required would be greater

Select appropriate plastic moment ratioAt joints establish equilibriumFor columns use above corner connection momentsFor beams use the MP from absolute plastic moment


SECONDARY DESIGN CONSIDERATIONS

•Influence of Axial Force on MP

•Influence of Shear Force on MP

•Local Buckling of Flanges and Web•Lateral Buckling of Flexural Members•Column Buckling


INFLUENCE OF AXIAL FORCE ON MP

18.1yP

P18.1

pMpcM

yP/P1Z85.0

pcMpM

ZreqZ

Fcl /Pcs

O

Long columnsloading curve

AB

Failure envelope

Stub columnloading curve

1.0

1.0

P0/Pcs

Mo/Mp

Fc/Pcs

M/MpM/Mp


INFLUENCE OF SHEAR FORCE ON MP

Premature Failure Due toGeneral Shear Yielding due to high V / M Partial Plastification Under M & Shear yielding at N. A.

Vym = 0.55 Aw.fy.

Increase in MP due to strain hardening compensates for V


LOCAL BUCKLING

• Flanges• Web

Section Classification–Plastic Sections–Compact Sections–Non-Compact Sections–Slenderness Sections


LATERAL BUCKLINGThe Compression Flanges Should Be Adequately Laterally Supported – At plastic hinge locations– Within 640 / fy from plastic hinges– Within 960/ fy from the above bracing– Balance as per elastic design.

f y/yrυ960 yf/yr..640 f y

/yrυ960yf/yr..640

Lateral Supports to compression flange

Mp

Mp


COLUMN BUCKLING

In long columns consider the failure of stanchions as beam-columns


CONNECTIONSConnections between members should meet

Strength RequirementsStiffness RequirementsDuctility RequirementsEconomical

R1

R2

HAUNCH POINT, H

r1

r2R1

R2

H


TYPES OF CONNECTIONS Corner Connections

(a) (b)

(c) (d)

t

tf

d'

d

HAt

Bt

C

(e)


TYPES OF CONNECTIONSInterior Connections

(a) Top (b) Side (c) Interior


SUMMARY•Portal frames can take different shapes •Ductility of connections is important in

plastic designs•Effect of axial compression, shear, local buckling and lateral buckling should be considered on plastic strength

Documents

Mat-Ch36