5.5 Steel Structures

5.5 Steel Structures

Introduction to Steel Design

Steel is a versatile, high-strength, and ductile construction material used extensively in industrial buildings, bridges, towers, and high-rise structures. Its advantages include high strength-to-weight ratio, speed of construction, and recyclability. The design of steel structures involves selecting appropriate rolled sections, analyzing their capacity under load, and designing robust connections (bolted or welded) to assemble members. This unit focuses on standard sections, connection design, and the design of fundamental tension and compression members as per IS 800:2007 (General Construction in Steel) and relevant NBC (Nepal) standards, which follow the Limit State Method.

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1. Standard and Built-up Sections

1.1 Standard Rolled Steel Sections

These are hot-rolled in standard shapes and sizes, readily available for use.

1. I-Sections (Beams):

   • Indian Standard Joist (ISJB, ISLB, ISMB, ISWB, ISHB):

     • ISMB (Indian Standard Medium Weight Beam): Most commonly used for beams and columns of medium spans.

     • ISWB (Indian Standard Wide Flange Beam): Wider flanges, better buckling resistance, used for heavier loads.

     • ISHB (Indian Standard Heavy Beam): Heavy sections for major girders.

   • Designation: e.g., ISMB 250 → '250' denotes the depth of the section in mm.

2. Channel Sections (ISJC, ISLC, ISMC):

   • ISMC (Indian Standard Medium Weight Channel): Asymmetric section, used as purlins, bracings, or in built-up sections.

   • Designation: e.g., ISMC 200.

3. Angle Sections (ISA) - Equal & Unequal:

   • ISA (Indian Standard Angle): L-shaped sections.

     • Equal Angle (ISA 65x65x6): Both legs equal.

     • Unequal Angle (ISA 100x75x8): Legs of different lengths.

   • Used as bracing members, truss elements, and for small supports.

4. Tee Sections (Cut from ISHB/ISMB):

   • Obtained by splitting an I-section longitudinally.

   • Used as connecting elements, purlins, and in built-up sections.

5. Hollow Sections:

   • Square Hollow Section (SHS) and Rectangular Hollow Section (RHS): High torsional stiffness, aesthetic appearance.

   • Circular Hollow Section (CHS): Excellent resistance to compression and torsion.

1.2 Built-up Sections

Fabricated by connecting two or more standard sections to achieve properties not available in a single rolled section.

1. Purpose:

   • To increase load-carrying capacity (strength and stiffness).

   • To achieve a higher radius of gyration for better resistance to buckling.

   • To create sections for very large spans/heavy loads.

2. Common Types:

   • Built-up I-section: Two flange plates welded or bolted to a web plate. Used for plate girders.

   • Compound Columns: Two I-sections (e.g., back-to-back) connected by battens or lacing. Used for heavy columns.

   • Box Section: Four plates welded into a hollow box. High torsional rigidity.

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2. Design of Connections

Connections are critical elements that transfer forces between members. Their failure can be catastrophic.

2.1 Bolted Connections

1. Types of Bolts:

   • Black Bolts: Unfinished, low strength. For light structures.

   • Turned Bolts: Machined shank, better fit.

   • High Strength Friction Grip (HSFG) Bolts: Pre-tensioned to clamp plates together, transferring load by friction between plates. Used where slippage is undesirable (e.g., seismic, fatigue).

   • Countersunk Bolts: For flush surfaces.

2. Failure Modes of Bolted Joints:

   • Shear Failure of Bolt: Bolt shears across its shank.

   • Bearing Failure of Plate: Elongation of bolt hole in the connected plate.

   • Tension Failure of Bolt: Bolt fractures under direct tension.

   • Tearing (Shear-out) of Plate: Plate tears along line of bolts.

   • Block Shear Failure: A block of material tears out from the member.

3. Types of Bolted Joints:

   • Lap Joint: Members overlap. Causes eccentricity.

   • Butt Joint: Members aligned, connected via cover plates. More efficient.

4. Design Strength in Shear ($V_{dsb}$):

   • For bolt in single/double shear.

   • $V_{dsb} = \frac{f_{ub} \times (n_n A_{nb} + n_s A_{sb})}{\sqrt{3} \times \gamma_{mb}}$ where $f_{ub}$ = ultimate tensile strength of bolt, $A_{nb}$ = net shear area, $A_{sb}$ = shank area, $n_n, n_s$ = number of shear planes with/without threads, $\gamma_{mb}$ = partial safety factor for bolt material (1.25).

5. Design Strength in Bearing ($V_{dpb}$):

   • $V_{dpb} = 2.5 \times k_b \times d \times t \times \frac{f_u}{\gamma_{mb}}$ where $k_b$ = factor depending on end distance, pitch, $d$ = nominal bolt diameter, $t$ = thickness of connected plate, $f_u$ = ultimate tensile strength of plate.

6. Design Strength of the Connection: Least of $V_{dsb}$ and $V_{dpb}$, multiplied by number of bolts.

7. Pitch, Gauge, Edge and End Distances: Minimum and maximum spacings are specified in IS 800 to prevent failure and ensure constructability.

2.2 Welded Connections

1. Advantages: Provides rigid, continuous connections; efficient use of material; aesthetically cleaner.

2. Types of Welds:

   • Fillet Weld: Most common. Triangular cross-section, placed at the junction of two surfaces. Designed in shear.

   • Butt Weld: Members are placed edge-to-edge and welded through the thickness. Designed for direct tension/compression.

3. Design of Fillet Welds:

   • Throat Thickness ($t_t$): The minimum distance from the root to the face of the weld. $t_t = k \times s$, where $s$ = leg size, $k = 1/\sqrt{2} \approx 0.7$.

   • Design Strength ($f_{wd}$): $f_{wd} = \frac{f_u / \sqrt{3}}{\gamma_{mw}}$ where $f_u$ = smaller of ultimate stress of weld or parent metal, $\gamma_{mw}$ = partial safety factor for weld (1.25 for shop welds, 1.5 for site welds).

   • Strength per unit length: $p_d = f_{wd} \times t_t \times 1$.

   • Total weld length required: $L_w = \frac{\text{Force to be transferred}}{p_d}$.

4. Specifications: Minimum and maximum weld sizes, effective length, and end returns are specified in the code.

2.3 Choice Between Bolted and Welded

• Bolted: Preferred for site connections (no special equipment, easier inspection/rectification), allows for disassembly.

• Welded: Preferred for shop fabrication (higher strength, rigidity, economy of material), used for rigid moment connections.

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3. Design of Simple Elements

3.1 Tension Members (Ties)

1. Function: Carry axial tensile forces (e.g., truss bottom chord, bracing rods).

2. Types: Single angles, double angles, rods, flats, tubes.

3. Failure Modes:

   • Yielding of Gross Section: Excessive elongation.

   • Rupture (Fracture) of Net Section: Failure at the connection where the cross-section is reduced due to bolt holes.

4. Design Strength ($T_d$) - IS 800 Cl. 6.3:

   • Due to Yielding of Gross Section: $T_{dg} = \frac{A_g f_y}{\gamma_{m0}}$ where $A_g$ = gross area, $f_y$ = yield stress, $\gamma_{m0}$ = partial safety factor for material (1.1).

   • Due to Rupture of Net Section: $T_{dn} = 0.9 \times \frac{A_{nc} f_u}{\gamma_{m1}} + \beta \times \frac{A_{go} f_y}{\gamma_{m0}}$ where $A_{nc}$ = net area of connected legs, $A_{go}$ = gross area of outstanding legs, $f_u$ = ultimate stress, $\beta$ = shear lag factor (function of connection geometry).

   • Design Strength: $T_d$ = Minimum of $T_{dg}$ and $T_{dn}$.

5. Slenderness Ratio ($\lambda$):

   • For tension members, a maximum $\lambda$ of 350 is recommended to prevent excessive sagging and vibration.

3.2 Struts / Axially Loaded Compression Members (Columns)

1. Function: Carry axial compressive forces (e.g., truss top chord, building columns).

2. Primary Failure Mode: Buckling - Elastic or inelastic instability.

3. Slenderness Ratio ($\lambda$):

   • kLr_{min}

   • Effective length ($kL$) depends on end conditions (fixed, pinned, free).

4. Design Compressive Strength ($P_d$) - IS 800 Cl. 7.1:

   • $P_d = A_e f_{cd}$ where $A_e$ = effective sectional area, $f_{cd}$ = design compressive stress.

   • $f_{cd}$ is calculated as: $f_{cd} = \frac{\chi f_y}{\gamma_{m0}}$ $\chi = \frac{1}{\phi + [\phi^2 - \lambda^2]^{0.5}} \leq 1.0$ $\phi = 0.5 [1 + \alpha (\lambda - 0.2) + \lambda^2]$

   • Where:

     • $\lambda$ = non-dimensional slenderness ratio = $\sqrt{\frac{f_y}{f_{cr}}}$.

     • $f_{cr}$ = elastic critical stress = $\frac{\pi^2 E}{(kL/r)^2}$.

     • $\alpha$ = imperfection factor (depends on buckling curve a, b, c, or d).

5. Buckling Class: Sections are classified into curves (a, b, c, d) based on their shape, axis of buckling, and fabrication process, which accounts for residual stresses and geometric imperfections.

6. Effective Area ($A_e$): For angles and other connected members, the gross area may need to be reduced to account for shear lag at connections.

3.3 Column Bases

1. Function: Transfer axial load and moment from the steel column to the concrete foundation.

2. Types:

   • Slab Base: For columns with small axial loads (no moment). A single steel plate distributes the load.

   • Gusseted Base: For columns with large axial loads and/or moments. Additional gusset plates and stiffeners are welded to the column and base plate to provide greater moment resistance and distribute load.

3. Design of Slab Base (Axially Loaded):

   • Area of Base Plate (A): $A \geq \frac{P_u}{0.6 f_{ck}}$, where $P_u$ is factored load, $f_{ck}$ is concrete characteristic strength. This limits bearing pressure on concrete.

   • Thickness of Base Plate (t_p):

     • The plate is designed as a cantilever projecting from the column face, subjected to bearing pressure from concrete.

     • Bending moment per unit width: $m = w \times a^2 / 2$, where $w$ = bearing pressure, $a$ = cantilever projection.

     • Thickness: $t_p = \sqrt{\frac{6 m}{f_y / \gamma_{m0}}}$

4. Holding Down Bolts (Anchor Bolts): Designed to resist uplift (if any) and provide stability during erection. Their design involves calculating tension due to moment and shear transfer.

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4. Relevant Codes and Standards

• IS 800:2007: General Construction in Steel - Code of Practice. The primary code for steel design in India, based on Limit State Method.

• IS 811:1987: Specification for cold-formed light gauge steel structural sections.

• IS 816:1969: Code of Practice for Use of Metal Arc Welding for General Construction in Mild Steel.

• IS 3757:1985: Code of Practice for Use of High Strength Friction Grip Bolts.

• IS 4000:1992: Code of Practice for High Strength Bolts in Steel Structures.

• IS Handbook No. 1: Contains section properties (weight, area, moment of inertia) for all standard steel sections.

• NBC 110:2020 (Nepal): Steel Structure. The governing code for steel design in Nepal.

Conclusion: The design of steel structures involves a logical process: selecting appropriate sections, designing connections to transfer forces efficiently, and analyzing members for strength and stability. Tension members are governed by yielding/rupture, while compression members are dominated by buckling considerations. A thorough understanding of section properties, connection behavior, and the codal provisions of IS 800 is essential for creating safe, economical, and functional steel frameworks.