Design of Steel Structures (CE-801) - Unit 3 Short Notes

Unit 3: Design of Steel Structures

I. Introduction and Design Philosophies

A. Properties of Steel vs Cast Iron and Wrought Iron

Why Steel is Superior: High strength-to-weight ratio, ductility (absorbs energy), ease of fabrication (welding, bolting), predictable behavior, and recyclability make it the premier structural material.

B. Limit State Design (LSD) vs Working Stress Design (WSD)

Key LSD Limit States:

1. Ultimate Limit State (ULS): Strength, stability, fatigue (fracture, buckling, rupture).

2. Serviceability Limit State (SLS): Deflection, vibration, durability.

C. Relevant IS Codes

• IS 800:2007: General construction in steel - Code of practice.

• IS 808: Rolled steel beams & channels.

• IS 875: Loads on structures (Dead, Live, Wind, Snow, Seismic).

• IS 1893: Criteria for earthquake resistant design.

• IS 883: Timber structures (for reference).

II. Connections (Very High Frequency)

A. Bolted Connections 1. Types

2. Design Procedure for Bearing-Type Connections (IS 800) Step 1: Bolt Strength

• Shear Strength (Single bolt): $$\displaystyle V_{sb} = \frac{f_u}{\sqrt{3} \gamma_{mb}} \cdot n \cdot A_{sb} \cdot \frac{A_{sb} \cdot A_{g}}{A_{g} + n \cdot A_{sb}} $$ (for threads in shear plane)

  • $$\displaystyle f_u $$ = Ultimate tensile stress of bolt

  • $$\displaystyle \gamma_{mb} $$ = Partial safety factor for bolt material (1.25)

  • $n$ = Number of shear planes (1 for single cover, 2 for double)

  • $$\displaystyle A_{sb} $$ = Nominal shank area

  • $$\displaystyle A_g $$ = Gross area of bolt

  • Simplified (threads in shear plane): $$\displaystyle V_{sb} = \frac{0.6 f_u A_{sb}}{\gamma_{mb}} $$

• Tensile Strength: $$\displaystyle T_{sb} = \frac{0.9 f_u A_{sb}}{\gamma_{mb}} $$ (if threads in tension plane)

• Combined Tension & Shear: $$\displaystyle \sqrt{\left(\frac{V_{sb}}{V_{sb,design}}\right)^2 + \left(\frac{T_{sb}}{T_{sb,design}}\right)^2} \leq 1.0 $$

Step 2: Plate Strength

• Bearing Strength: $$\displaystyle F_b = \frac{2.5 k_b f_u}{\gamma_{mb}} \cdot t \cdot d $$ (per bolt)

  • $$\displaystyle k_b = \min\left(\frac{e}{d_0}, \frac{p}{d_0}, 1.0\right) $$ (bearing coefficient)

  • $e$ = Edge distance, $p$ = Pitch, $$\displaystyle d_0 $$ = Hole diameter

  • $t$ = Plate thickness, $d$ = Bolt diameter

• Net Section Rupture: $$\displaystyle T_{nd} = \frac{0.9 f_u A_{net}}{\gamma_{m0}} $$

  • $$\displaystyle A_{net} = \left( b - (n_g - 1) d_0 - d_{hole} \right) \cdot t $$ (for tension member)

  • $$\displaystyle n_g $$ = Number of bolt holes in the critical section.

Step 3: Geometry Checks

• Minimum Edge Distance: $$\displaystyle e_{min} = 1.7 d_0 $$ (IS 800 Table 13.2)

• Minimum Pitch: $$\displaystyle p_{min} = 2.5 d_0 $$

• Maximum Pitch: $$\displaystyle p_{max} = \min(16t, 200 \text{ mm}) $$

Step 4: Efficiency of Joint $$\displaystyle \eta = \frac{\text{Strength of joint}}{\text{Strength of un-pierced plate}} \times 100\% $$

• For shear: $$\displaystyle \eta = \frac{n \cdot V_{sb}}{F_b \text{ or } V_{sb} \text{ (whichever governs)}} $$

• For tension: $$\displaystyle \eta = \frac{A_{net}}{A_g} \cdot \frac{f_u}{f_y} $$ (approx.)

3. Failure Modes of Bolted Joints (Frequent: Jun 2025, Nov 2023, May 2022)

4. Bearing Type vs Friction Type (Slip-Critical) Connections

5. Advantages of Bolted over Riveted/Welded

• vs Riveted: Faster installation, no skilled riveters, no hot work, easier inspection & dismantling.

• vs Welded: No fire hazard, no distortion/warping, works in all weather, easier to inspect, allows for slight misalignment, reusable.

B. Welded Connections (Very High Frequency) 1. Types

• Fillet Weld: Triangular cross-section. Most common (lap, tee, corner joints).

• Groove Weld: For butt, corner, edge joints. Types: Square, V-groove, U-groove, J-groove.

2. Shop vs Site Welding

• Shop: Controlled environment, better quality, faster, cheaper. Preferable.

• Site: Uncontrolled, requires skilled welders, more inspection. Used for assembly.

3. Design of Fillet Welds (IS 800)

• Throat Thickness: $$\displaystyle t_t = k \cdot s $$ (where $s$ = size of weld, $$\displaystyle k = 0.7 $$ for convex fillet)

• Design Strength (per unit length):

  • Longitudinal: $$\displaystyle f_w = \frac{f_u}{\sqrt{3} \gamma_{mw}} $$ (N/mm)

  • Transverse: $$\displaystyle f_w = \frac{f_u}{\gamma_{mw}} $$ (N/mm)

  • $$\displaystyle \gamma_{mw} $$ = Partial safety factor for weld (1.25 or 1.5 for site weld)

• Design Procedure:

  1. Determine weld size $s \geq \min(t, 3\text{mm})$.

  2. Calculate throat thickness $$\displaystyle t_t = 0.7s $$.

  3. Compute required length $$\displaystyle L = \frac{\text{Factored force}}{f_w} $$.

  4. Check max length: $$\displaystyle L_{max} = \min( 100s, 200\text{mm} ) $$ for intermittent welds.

4. Design of Groove Welds

• Butt Weld: Strength = $$\displaystyle f_y \cdot t_w $$ (where $$\displaystyle t_w $$ = thickness of thinner plate). Full penetration weld develops full tensile strength of base metal.

• Partial Penetration: Strength reduced; design based on effective throat.

5. Advantages & Disadvantages

C. Specific Connection Types 1. Seat Connections

• Purpose: Provide simple support for beam on column.

• Components: Seat angle (welded to column flange), holding angle (welded to beam bottom flange).

• Design: Check bearing of beam on seat angle, strength of weld, and shear in holding angle bolts.

• DiagramCANVAS: Beam resting on a horizontal seat angle welded to column flange, with a small vertical holding angle at the beam's end.

2. Framed Connections

• Purpose: Connect beam web to column flange, transferring shear.

• Components: Connecting angle (welded to beam web, bolted to column flange).

• Design: Check shear in bolts, bearing on angle & column, weld strength.

• DiagramCANVAS: Beam web connected via a single angle to the column flange. Bolts pass through angle and column flange.

3. Pin Connections

• Purpose: Allow rotation, transmit shear & axial force.

• Types: Pin plates (with bushings), direct pin in member holes.

• Applications: Truss joints, bracing, machinery bases.

• Design: Check pin shear & bearing, plate thickness, hole clearance.

4. Lug Angles (Frequent: Jun 2025, Nov 2023)

• Purpose: Provide additional bolts for tension members when main member has limited edge distance or gauge.

• Design: Lug angle size & length chosen to develop required strength. Bolts designed for shear & bearing.

• Placement: Lug on one side (unequal legs) or both sides.

• Sketch:

  DiagramCANVAS: Tension member (e.g., angle) connected to gusset plate. A smaller lug angle is welded to the back of the main member and bolted to the gusset, providing extra bolt rows.

III. Tension Members (Very High Frequency)

A. Design Strength of Tension Members 1. Yielding of Gross Section (Limit State of Yielding)

$$T_{dg} = \frac{f_y A_g}{\gamma_{m0}}$$

• $$\displaystyle f_y $$ = Yield stress, $$\displaystyle A_g $$ = Gross area, $$\displaystyle \gamma_{m0} = 1.1 $$

2. Rupture of Net Section (Limit State of Fracture)

$$T_{dn} = \frac{0.9 f_u A_{net}}{\gamma_{m1}}$$

• $$\displaystyle f_u $$ = Ultimate stress, $$\displaystyle A_{net} $$ = Net area at critical section, $$\displaystyle \gamma_{m1} = 1.25 $$

• Net Area Calculation:

  • For single row: $$\displaystyle A_{net} = (b - d_0) \cdot t $$

  • For multiple rows: $$\displaystyle A_{net} = \left( b - (n_g - 1) d_0 - d_{hole} \right) \cdot t $$ ($$\displaystyle d_{hole} = 2.5 $$ mm extra)

3. Block Shear Failure (Frequent: Jun 2025, Nov 2023, May 2022)

• Mechanism: Rupture along a "block" (combination of tensile failure on net section & shear failure along bolt line).

• Design Strength:

$$T_{db} = \frac{1}{\gamma_{m0}} \left[ \frac{f_u A_{nt}}{\gamma_{m1}} + \frac{f_u A_{nv}}{\sqrt{3} \gamma_{m0}} \right] \quad \text{or} \quad \frac{1}{\gamma_{m1}} \left[ \frac{f_y A_{tg}}{\gamma_{m0}} + \frac{f_u A_{nv}}{\sqrt{3} \gamma_{m0}} \right]$$

*   Use the **smaller** value.

*   $$\displaystyle A_{nt} $$ = Net tensile area, $$\displaystyle A_{tg} $$ = Gross tensile area, $$\displaystyle A_{nv} $$ = Net shear area.

• Critical Path: Path with minimum resistance (tension or shear).

B. Types of Tension Members

• Single Angle: Used in roof trusses (top/bottom chords). Design strength based on single angle properties from IS 800 Table 10(a).

• Double Angle (Back-to-Back): Higher capacity, used for longer members. Connected with gusset plates or tack bolts. Effective net area depends on connection pattern (opposite/same side of gusset).

• Channel Sections with Gusset Plates: Common in trusses. Weld design critical.

• I-Sections & Tubulars: I-sections for heavy loads; tubulars for aesthetics, torsion resistance.

C. Connections for Tension Members 1. Welded Connections to Gusset Plates

• Overlap: Minimum overlap = $2 \times \text{max weld size} + 5\text{mm}$.

• Weld Design: Weld must develop full tensile strength of member.

  • Force per unit length $$\displaystyle f_w = \frac{T_{design}}{L_{eff}} $$ (where $$\displaystyle L_{eff} $$ = total effective weld length).

  • Check weld size $s \geq \min(\text{plate thickness}, 3\text{mm})$.

• Sketch:

  DiagramCANVAS: Angle section welded to gusset plate on 2-3 sides (usually two legs and one back).

2. Bolted Connections

• With Lug Angles: Used when member is thick or edge distance is small.

• Without Lug Angles: Direct bolting of member to gusset. Check net section of member at bolt holes.

D. Special Considerations 1. Reversed Stresses (Wind Effects)

• Problem: Wind on roof trusses causes top chord compression & bottom chord tension (and vice versa under reverse wind).

• Solution: Design member for maximum compressive stress (slenderness effect) and maximum tensile stress (yielding/rupture). Single angle sections are prone to lateral-torsional buckling in compression; use double angles or provide bracing.

2. Shear Lag and Effective Net Area

• Cause: In connections where not all elements of a cross-section are connected directly (e.g., angles to gusset, flanges of I-section to web), the stress distribution is non-uniform. Outer elements lag behind.

• Effective Net Area: $$\displaystyle A_{net,eff} = \beta \cdot A_{net} $$

  • $\beta$ = Reduction factor (IS 800 Table 4.3).

  • For single angle connected by one leg: $$\displaystyle \beta = 0.9 $$ for $$\displaystyle l > 2.5d $$, $$\displaystyle \beta = 0.7 $$ for $l \leq 1.5d$ ($l$ = length of end connection, $d$ = leg length).

  • For I-section with flange connected only at web: $$\displaystyle \beta = 0.9 $$ for $$\displaystyle l > 2.5d $$, $$\displaystyle \beta = 0.7 $$ for $l \leq 1.5d$.

3. Factors Affecting Strength

• Slenderness Ratio ($\lambda$): $$\displaystyle \lambda = \frac{l_e}{r_{min}} $$ (affects compressive strength).

• End Connection Rigidity: Affects effective length $$\displaystyle l_e $$.

• Material Ductility: Affects block shear & net section behavior.

• Hole Quality & Bolt Tightening: Affects bearing & slip.

IV. Beams (Very High Frequency)

A. Laterally Restrained vs Unrestrained Beams

B. Design of Laterally Supported Beams Steps (IS 800):

1. Calculate Design Bending Moment ($$\displaystyle M_d $$) & Shear Force ($$\displaystyle V_d $$).

2. Select Trial Section: Based on $$\displaystyle M_d \approx \frac{f_y Z_e}{\gamma_{m0}} $$ (approx).

3. Check Section Classification: (Plastic, Compact, Semi-compact) based on width-thickness ratios (Table 2, IS 800). Plastic/Compact sections preferred for higher moment capacity.

4. Check Bending Strength: $$\displaystyle M_d \leq M_{d,allow} = \frac{Z_e f_y}{\gamma_{m0}} $$ (for plastic/compact) or $$\displaystyle \frac{Z_e f_y}{\gamma_{m0}} $$ (for semi-compact, use elastic modulus $$\displaystyle Z_e $$).

5. Check Shear Strength: $$\displaystyle V_d \leq V_{d,allow} = \frac{A_w f_y}{\sqrt{3} \gamma_{m0}} $$ (where $$\displaystyle A_w $$ = web area).

6. Check Deflection (SLS): $$\displaystyle \delta_{max} \leq \frac{L}{250} $$ (for general beams) or as per IS 800.

C. Purlin Design (Frequent: Jun 2025, May 2024, Nov 2023) 1. Load Determination (on sloping roof):

• Dead Load (DL): Weight of sheet + purlin self-weight.

• Live Load (LL): As per IS 875 Part 2.

• Wind Load (WL): As per IS 875 Part 3 (suction/pressure). Critical for purlins.

• Load Combination (ULS): $1.5(DL + LL + WL)$ or as per IS 875.

• Load on Purlin (per meter): $$\displaystyle w = \text{load intensity} \times \text{spacing of purlins} \times \sec(\theta) $$ ($\theta$ = roof slope).

2. Spacing & Span: Span = truss spacing. Purlin spacing typically 1.2m to 1.8m.

3. Section Selection:

• I-sections (ISMB/ISLB): Most common.

• Angles: For lighter loads, but check lateral stability.

4. Design Example (Industrial Building):

• Span = truss spacing (e.g., 6m).

• Simply supported on rafters.

• Design for maximum moment ($$\displaystyle wL^2/8 $$) and shear ($wL/2$).

• Check deflection: $$\displaystyle \delta_{max} = \frac{5wL^4}{384EI} \leq \frac{L}{200} $$ (for purlins).

D. Plate Girders (Frequent: May 2024, Nov 2023) 1. Elements:

• Web: Carries shear, prone to buckling.

• Flanges: Carries bending moment.

• Stiffeners: Vertical (prevent web buckling), Horizontal (prevent flange buckling/distortion).

2. Design Procedure:

• Web:

  • Thickness: $$\displaystyle t_w \geq \frac{d_w}{270 \epsilon} $$ (to avoid shear buckling, or provide stiffeners).

  • Shear strength: $$\displaystyle V_{dw} = \frac{A_w f_y}{\sqrt{3} \gamma_{m0}} $$.

• Flanges:

  • Area: $$\displaystyle A_f = \frac{M_d}{f_y (d - t_f)} \cdot \frac{\gamma_{m0}}{1} $$ (approx).

  • Width-thickness ratio check (to avoid local buckling).

• Stiffeners:

  • Vertical: Required if $$\displaystyle d_w / t_w > 67 \epsilon / \sqrt{\tau_r} $$ (where $$\displaystyle \tau_r = V / (d_w t_w) $$). Design as compression member.

  • Horizontal: At points of concentrated loads & supports. Prevent flange buckling.

E. Gantry Girders (Frequent: May 2023, May 2024)

• Profile: Typically I-section with unequal flange areas (top flange larger for stability under crane loads) or box section. Often laterally unrestrained.

• Design Considerations:

  • Crane Loads: Vertical (wheel loads) & Horizontal (braking, skew).

  • Deflection Limits (IS 800):

    • Vertical deflection under crane load: $$\displaystyle \delta_v \leq \frac{L}{500} $$ to $$\displaystyle \frac{L}{1000} $$.

    • Horizontal deflection at top: $$\displaystyle \delta_h \leq \frac{L}{2000} $$.

  • Check for fatigue due to repeated crane loads.

F. Deflection Criteria (General)

• For beams supporting plastered ceilings: $$\displaystyle \delta \leq \frac{L}{250} $$ (total load), $$\displaystyle \frac{L}{350} $$ (live load only).

• For roof purlins: $$\displaystyle \delta \leq \frac{L}{200} $$.

• For gantry girders: See above.

• Check: $$\displaystyle \delta_{max} = \frac{5wL^4}{384EI} $$ (simply supported, UDL).

V. Columns (High Frequency)

A. Single Section Columns

• Design Strength (Axial Compression): $$\displaystyle P_d = \frac{A_g f_y}{\gamma_{m0}} \cdot \frac{1}{\phi + \sqrt{\phi^2 - \lambda^2}} $$ (or use IS 800 Table 6, 7).

  • $$\displaystyle \phi = 0.5 \left[ 1 + \alpha (\lambda - 0.2) + \lambda^2 \right] $$

  • $$\displaystyle \lambda = \frac{l_e}{r} \sqrt{\frac{f_y}{250}} $$ (non-dimensional slenderness)

  • $\alpha$ = Imperfection factor (0.21 for welded, 0.49 for rolled).

• Effective Length ($$\displaystyle l_e $$): Depends on end conditions (pinned, fixed, free). $$\displaystyle l_e = K \cdot l $$ (K from Table 11, IS 800).

• Tubular Sections: High torsional rigidity, uniform strength. Used in trusses, towers. Design similar to I-sections but use $$\displaystyle r_{min} $$.

B. Built-up Compression Members 1. Laced Columns (Frequent: Jun 2025, May 2023, Nov 2023)

• Purpose: Economical for heavy loads; provides bracing between main members.

• Components: Two main members (channels/I-sections) + lacing bars (angles/channels).

• Design of Lacing:

  • Compressive Lacing: Design as column ($$\displaystyle P_{dl} \leq P_{d,lacing} $$).

  • Tensile Lacing: Design for tension ($$\displaystyle P_{dl} \leq \frac{0.9 f_u A_{nl}}{\gamma_{m1}} $$).

  • Angle of Lacing: $\tan \theta \geq 0.7$ (to avoid excessive shear).

  • Spacing ($s$): $$\displaystyle s \leq 60 \cdot r_{min,main} $$ (to prevent local buckling of main member).

• Connections: Welded or bolted at crossings. Single lacing: Bolted at crossings. Double lacing: Welded or bolted alternately.

• Effective Length of Lacing: $$\displaystyle l_{e,lacing} = \text{length between connections} $$.

2. Battened Columns

• Batten Plates: Carry shear from column to column. Design for shear + bending moment from eccentric load.

  • $$\displaystyle M = \frac{P \cdot s}{2} $$ (for intermediate batten).

  • Check combined stress: $$\displaystyle \frac{M}{Z_b} + \frac{V}{A_w} \leq f_y / \gamma_{m0} $$.

• Spacing ($s$): $$\displaystyle s \leq \frac{l_e}{10} $$ (to prevent local buckling of main member).

• Connections: Welded or bolted to main members.

3. Double Angle Struts (Discontinuous) (Frequent: May 2024)

• Connection to Gusset:

  • Opposite Side of Gusset: Net area $$\displaystyle A_{net} = 2 \cdot (b - d_0) \cdot t $$ (if bolts in line). More efficient.

  • Same Side of Gusset: Net area $$\displaystyle A_{net} = 2 \cdot (b - 2d_0) \cdot t $$ (if staggered). Less efficient due to shear lag.

• Design: Check for compression (considering slenderness of single angle) and connection strength.

C. Base Plates (Frequent: Jun 2025, May 2022, Nov 2023) 1. Types: Slab, Gusseted, Ribbed, Embedded. 2. Slab Base Design (Frequent: Nov 2023, May 2022)

• Assumption: Base plate is rigid, pressure uniform under compression.

• Bearing Pressure: $$\displaystyle p = \frac{P}{A_p} \leq f_b $$ (allowable bearing pressure on concrete).

  • $$\displaystyle f_b = 0.45 f_{ck} $$ (for concrete pedestal) or as per IS 800.

• Base Plate Thickness ($$\displaystyle t_p $$): $$\displaystyle t_p = \sqrt{\frac{3P}{f_{bd} \cdot N \cdot B}} $$ (for square/rectangular plate)

  • $$\displaystyle f_{bd} $$ = design bearing stress of base plate ($$\displaystyle f_y / \gamma_{m0} $$).

  • $N, B$ = dimensions of loaded area (column size).

• Weld Design: Weld between column & base plate must transfer full load $P$.

  • Throat thickness $$\displaystyle t_t = \frac{P}{L_{weld} \cdot f_w} $$.

  • Check weld size.

3. Gusseted Base Design (Frequent: Jun 2025)

• Components: Base plate, gusset plates, anchor bolts.

• Design:

  1. Anchor Bolts: Designed for tension ($$\displaystyle T_{b} = \frac{P}{n} $$) and shear.

  2. Gusset Plates: Design as tension member (if bolts in tension) or compression (if in bearing).

  3. Base Plate Thickness: Check for bearing under gusset.

  4. Concrete Pedestal: Size from bearing pressure $$\displaystyle p = \frac{P}{A_{ped}} \leq 0.45 f_{ck} $$.

D. Column Splices (Frequent: Nov 2023)

• Purpose: Connect two column sections.

• Welded Splices: Full penetration butt weld. Simple, rigid. Check weld strength.

• Bolted Splices: Flange plates & web plates. Designed for compression/tension & shear. Used for site connections.

E. Comparison of Lacing and Battens

VI. Trusses and Roof Systems (High Frequency)

A. Types of Roof Trusses

• Based on Slope: King post, Queen post, Fan, Fink, Howe, Pratt, Warren, Gang-Nail.

• Based on Span & Load: Roof trusses (light), Bridge trusses (heavy).

• Common for Industrial Sheds: Pratt (diagonals in tension for gravity loads), Warren (no verticals, economical for longer spans).

B. Components of Steel Roof Trusses (Frequent: Jun 2025, May 2023, Nov 2023)

C. Purlins

• Function: Secondary members supporting roof sheeting, transferring loads to truss rafters.

• Design: As simply supported beams (see IV.C).

• Support: Rest on top flange of rafters. Provide lateral support to rafter compression flange.

D. Gantry Girders

• Function: Support crane runway in industrial buildings.

• Profile: Often I-section with larger top flange for stability. Sometimes box section.

• Design: For combined bending & shear from crane wheel loads & self-weight. Check deflection limits (see IV.E).

VII. Foundations for Steel Structures (High Frequency)

A. Grillage Foundations (Frequent: Jun 2025)

• Purpose: Spread load from column to soil for heavy loads where isolated footing is too large.

• Components: Top plate (under base plate), bottom plate (on concrete), transverse & longitudinal I-sections (grillage beams).

• Design Steps:

  1. Assume size of top/bottom plates (based on bearing pressure).

  2. Design Grill Beams (I-sections): As simply supported beams under strip load ($P / \text{width}$). Check bending & shear.

  3. Check Bearing Pressure on Soil: $$\displaystyle p = \frac{P}{A_{base}} \leq \text{allowable bearing capacity} $$.

  4. Check Thickness of Plates: Under column base (compressive stress) and under grill beams (bearing).

  5. Design Welds: Between plates and grill beams.

VIII. Special Topics (Medium/Low Frequency but examinable)

A. Block Shear Failure (Detailed in III.A.3)

• Key Point: Occurs when a tensile rupture along a net section and a shear rupture along a bolt line happen simultaneously. Governs for connections with long shear lines and short tensile paths.

B. Shear Lag Concept (Frequent: Nov 2023)

• Explanation: In tension members, the stress in elements not directly connected (e.g., outer leg of angle, flange of I-section) is lower than in directly connected elements due to non-uniform stress distribution. The "lag" causes reduction in effective area.

• Effect: Reduces design strength. Accounted by reduction factor $\beta$ in net area calculation.

C. Timber Grouping as per IS 883:1994

• Based on: Modulus of Elasticity (E) and Extreme Fiber Stress in Bending ($$\displaystyle \sigma_b $$).

• Groups: Grade I (high E, high $$\displaystyle \sigma_b $$) to Grade V (low E, low $$\displaystyle \sigma_b $$).

• Use: Selects timber for structural members based on required strength & stiffness.

D. Advantages of Wide Flange Beams over Narrow ISMB Beams

• Higher Moment of Inertia (I) & Section Modulus (Z) for same weight → more efficient.

• Better Lateral Stability: Wider flanges provide more resistance to lateral-torsional buckling.

• Ease of Connection: Wide flanges allow easier bolting/welding of flanges.

• Architectural Preference: Aesthetically pleasing, flat surfaces.

E. Difference between Lacing and Battens (See V.E)

F. Types of Groove Welds and Comparison with Fillet Welds

• Groove Weld Types: Square, Single-V, Double-V, U, J, Bevel.

• Comparison:

  | | Fillet Weld | Groove Weld | | :--- | :--- | :--- | | Joint Type | Lap, Tee, Corner | Butt, Edge, Corner | | Stress | Primarily shear | Primarily tension/compression | | Strength | Lower (throat thickness) | Can develop full base metal strength (full penetration) | | Preparation | Minimal (clean surfaces) | Requires edge preparation (beveling) | | Application | Secondary connections, frames | Primary load-bearing connections, pressure vessels |

G. Steel vs Cast Iron and Wrought Iron (See I.A)

H. Limit State Design vs Working Stress Design (See I.B)

Exam Tips & Common Pitfalls:

1. Bolted Connections: Always check edge distance & pitch minimums. For lap joints, efficiency is often asked.

2. Block Shear: Identify the critical failure path correctly. Calculate $$\displaystyle A_{nt} $$, $$\displaystyle A_{tg} $$, $$\displaystyle A_{nv} $$ accurately.

3. Tension Members with Reversed Stress: Design for compression (slenderness) not just tension.

4. Laced Columns: Check both main members (compression) and lacing (tension/compression). Effective length of lacing is between connections.

5. Base Plates: For slab base, assume uniform pressure. For gusseted base, anchor bolts are critical.

6. Beams: Distinguish laterally supported (no LTB check) vs unsupported (LTB check). Use correct $$\displaystyle l_{LT} $$ for LTB.

7. Purlin Design: Wind load is critical on sloping roofs. Resolve loads normal to purlin.

8. Shear Lag: Remember $\beta$ factors for single angles and I-sections with partial connectivity.

9. Weld Design: For fillet welds, use throat thickness $$\displaystyle t_t = 0.7s $$. For groove welds, full penetration = full strength.

10. Always state IS 800:2007 clause references where possible (e.g., Table 2 for section classification, Table 6 for column curves).