S13 Steel Structures 1

URP S-09 TrainingModule S13

Steel Structures 1Date: 22 June 2021

Raquib Ahsan, PhDProfessor, Department of Civil Engineering, BUET

Relevant Chapters

Part 6: Chapter 2

Part 6: Chapter 10

S. K. Ghosh Associates LLC International Code Council (ICC)

www.skghoshassociates.com 1

Bases of BNBC 2006 and BNBC 2020

BNBC 2006 was based on Specification for Structural Steel Buildings-- Allowable Stress Design and Plastic Design – 1989 and Supplement No. 1 to the Specification (LRFD) adopted September 1, 1986 –1989.

BNBC 2020 is based on Specification for Structural Steel Buildings (ANSI/AISC 360-05) – 2005.

Comparison of TOCs of BNBC 2006 & 2020BNBC 2006 BNBC 2020

10.1 Scope10.2 Definitions and Notation10.3 Material10.4 Types of Construction10.5 Frames and Other Structures10.6 Design Requirements10.7 Working Stress Design Method10.8 Load Factor Design Method10.9 Connections, Joints and Fasteners10.10 Serviceability Requirements10.11 Fabrication, Erection and Quality Control10.12 Surface Treatment10.13 Design Documents

10.1 General Provisions for Structural Steel Buildings and Structures10.2 General Design Requirements10.3 Stability Analysis and Design10.4 Design of Members for Tension10.5 Design of Members for Compression10.6 Design of Members for Flexure10.7 Design of Members for Shear10.8 Design of Members for Combined Forces and Torsion10.9 Evaluation of Existing Structures10.10 Connections10.11 Design of HSS and Box Member Connections10.12 Design for Serviceability10.13 Fabrication, Erection and Quality Control10.14 Direct Analysis Method10.15 Inelastic Analysis and Design10.16 Design for Ponding10.17 Design for Fatigue10.18 Structural Design for Fire Conditions10.19 Stability Bracing for Columns and Beams10.20 Seismic Provisions for Structural Steel Buildings



Low Seismic and High Seismic Applications

Low Seismic Applications: R <= 3; Compliance to seismic provisions (Sec. 10.20) is not necessary

High Seismic Applications: R > 3; Seismic provisions (Sec. 10.20) must be complied.

System R SDC B SDC C SDC DHeight Limit (m)

B. BUILDING FRAME SYSTEMS1. Steel eccentrically braced frames, moment

resisting connections2. Steel eccentrically braced frames, nonmoment-

resisting connections3. Special steel concentrically braced frames4. Ordinary steel concentrically braced frames

8

7

63.25

NL

NL

NLNL

NL

NL

NLNL

50

50

5011

C. MOMENT RESISTING FRAME SYSTEMS1. Special steel moment frames2. Intermediate steel moment frames3. Ordinary steel moment frames

84.53.5

NLNLNL

NLNLNL

NL35NP

D. DUAL SYSTEMS: SPECIAL MOMENT FRAMES1. Steel eccentrically braced frames2. Special steel concentrically braced frames

87

NLNL

NLNL

NLNL

E. DUAL SYSTEMS: INTERMEDIATE MOMENTFRAMES1. Special steel concentrically braced frames

(Ordinary and Intermediate frames are allowed for SDC B and C)

6 NL NL 11

G. STEEL SYSTEMS NOT SPECIFICALLY DETAILED FOR SEISMIC RESISTANCE

3 NL NL NP



Material

Regular Structural Steel (Hot-rolled shapes, tubing, pipes, plates, bars and sheets): Only ASTM and BDS standards are allowed.

Undefined Steel: Only for unimportant members and details.

Rolled or Built-Up Heavy Shapes: Charpy V-Notch (CVN) Impact Test is required.

Cold Form Sections: Refers to AISI standard.

Steel Castings and Forgings; Bolts, Washers and Nuts; Anchor Rods and Threaded Rods: Only ASTM standard is allowed.

Design Basis

Load and Resistance Factor Design (LRFD)

Allowable Strength Design (ASD)

Required strength (LRFD)

Resistance factorDesign strength

Nominal strength

Required strength (ASD)

Nominal strength

Safety factor

Allowable strength



Load Combinations

ASD Method LRFD Method

1. D + F

2. D + H + F + L + T

3. D + H + F + (Lr or R)

4. D + H + F + 0.75(L + T ) + 0.75(Lr or R)

5. D + H + F + (W or 0.7E)

6. D + H + F + 0.75(W or 0.7E) + 0.75L +

0.75(Lr or R)

7. 0.6D + W + H

8. 0.6D + 0.7E + H

1. 1.4(D + F)

2. 1.2(D + F + T) + 1.6(L + H) + 0.5(Lr or R)

3. 1.2D + 1.6(Lr or R) + (L or 0.8W)

4. 1.2D + 1.6W + L + 0.5(Lr or R)

5. 1.2D + 1.0E + 1.0L

6. 0.9D + 1.6W + 1.6H

7. 0.9D + 1.0E + 1.6H

Connection Types

Simple Connection: A simple connection transmits a negligible momentacross the connection. A simple connection shall have sufficient rotationcapacity. Inelastic rotation of the connection is permitted.

Moment Connection: A moment connection transmits moment acrossthe connection.

• Fully Restrained (FR): A fully-restrained (FR) moment connection transfersmoment with a negligible relative rotation between the connected members.

• Partially-restrained (PR) moment connections transfer moments, but the relativerotation between connected members is not negligible. In the analysis of thestructure, the force-deformation response characteristics of the connection shallbe included.



Connection Types

Simple connection

FR connection

PR connection

Unstiffened and Stiffened Elements

Unstiffened Element: Supported along only one edge parallel to the direction of the compression force.

Stiffened Element: Supported along two edges parallel to the direction of the compression force.



Local Buckling



Classification of Sections

Compact Section: A compact section reaches its cross-sectional material strength, or capacity, before local buckling occurs.

Noncompact Section: In a non-compact section, only a portion of the cross-section reaches its yield strength before local buckling occurs.

Slender Element Section: In a slender section, the cross-section does not yield and the strength of the member is governed by local buckling.



Stability Design Requirements

Second Order Effects

Flexural, Shear and Axial Deformations

Geometric Imperfections

Member Stiffness Reduction due to Residual Stresses

Second Order Effects

Source: AISC



Residual Stress

Residual stresses are stresses that remain in a member after it has been formed into a finished product.

Sources of residual stresses:• Uneven cooling which occurs after hot rolling of

structural shapes• Cold bending or cambering during fabrication• Punching of holes and cutting operations during

fabrication• Welding

Residual Stress



Methods of Analysis

Analysis Method

Elastic Analysis

Effective Length

Method

First-Order Analysis

Second-Order Analysis

Direct Analysis Method

Inelastic Analysis

Validity of First-Order Analysis

b)

c) Non-sway amplification (B1) of beam-column moments is considered.

1.0 (LRFD)

1.6 (ASD)

Required strength

Yield strength

Additional lateral load at each level independently in two orthogonal directions

First order deflection

Story height

Gravity load (LRFD)

1.6 x Gravity load (ASD)



Second-Order Analysis by Amplified First-Order Elastic Analysis

Required flexural strength

amplifier No translation moment

amplifier Lateral translation moment

Required axial strength No translation

axial forceLateral translation axial force

Applicability of Second-Order Analysis

else Direct Analysis Method (DM)

For ASD: Load x 1.6 and Mr/1.6, Pr/1.6

Minimum lateral load: 0.002Yi independently at each orthogonal direction

If then K = 1.0 else side sway buckling analysis



Design for Tension

Tensile yielding in the gross section:

Tensile rupture in the net section:

Shear Lag Factor



Tensile Strength of Pin Connected Members

Design for Compression

Compressive strength for flexural buckling of non-slender elements:



Slenderness Ratio vs. Compressive Strength

Different Types of Buckling

Flexural Buckling: This type of buckling can occur in any compression member thatexperiences a deflection caused by bending or flexure. Flexural buckling occurs about theaxis with the largest slenderness ratio, and the smallest radius of gyration.

Torsional Buckling: This type of buckling only occurs in compression members that aredoubly-symmetric and have very slender cross-sectional elements. It is caused by aturning about the longitudinal axis. Torsional buckling occurs mostly in built-up sections,and almost never in rolled sections.

Flexural Torsional Buckling: This type of buckling only occurs in compression membersthat have unsymmetrical cross-section with one axis of symmetry. Flexural-torsionalbuckling is the simultaneous bending and twisting of a member. This mostly occurs inchannels, structural tees, double-angle shapes, and equal-leg single angles.



Torsional and Flexural-Torsional Buckling without slender elements

For double-angle and tee-shaped compression members:

Y is the axis of symmetry and Fcry is evaluated for flexural buckling and,

For all other cases:

Flexural Torsional Buckling



Compressive Strength of Members with Slender Elements

Design for Flexure



Limit States

Flexural Strength of Laterally Supported Beam



Beam Behaviour

Lateral Torsional Buckling



Phases of Unsupported Beams

39

Moment Capacity of Unsupported Beams

40



Lateral Bracing Requirement

Un-braced length Lp to achieve just Mp : In-elastic LTB of

compact sections:

Un-braced length Lr to achieve Mr = 0.7Fy Sx : LTB of

compact:

41

Design for Shear

For rolled I shape:With,

For others,



Transverse Stiffeners

Not required if

where

Inelastic Capacity in Shear



Tension Field Action

Applicability of Tension Field Action



BNBC 2020 Requirements for Tension Field Action

Consideration of tension field action is not permitted for:

Nominal Shear Strength Considering Tension Field Action

Requirements for transverse stiffeners:



Questions?Thank you



S13 Steel Structures 1

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