Handbook on Structures with Steel Portal Frames (Without Cranes)
1987 Edition

Overview of Scope for IS SP Part 40 - Steel Portal Frames

Applicability by Span and Height:

Design Parameters:

• Frame spacing options: 4.5 m or 6.0 m
• Wind zones considered: Zones I, II, III
• Earthquake zones: I through V
• Support conditions: Both fixed and hinged analyzed
• Roof slopes: Ratios of 1:3, 1:4, and 1:5

Design Bases:

• Structural design aligned with IS 800-1962 (compatible with IS 800-1984)
• Wind pressures adopted from IS 875-1964 with normal permeability factor ±0.2
• Portal frame analysis conducted using stiffness method, supported by computer software
• Design includes purlins, girts, frame components, base plates, fasteners, and eaves beams
• Bracing and foundation designs are exemplified, not standardized

Additional Notes:

• Foundation forces incorporate dead load plus an allowance of 30 kg/m² for AC sheeting and girts
• Welded connection details, including haunch stiffener weld sizes and crown stiffener thicknesses, vary with section depth
• Intended for use by qualified structural engineers only

Configuration and Loadings for Portal Frames (IS SP Part 40)

Portal Frame Layout (Clause 1.2)

• Composed of beams and columns designed to resist maximum forces from load combinations
• Column slenderness ratio capped at 250 to account for bending effects
• Section types include rolled I-sections such as ISLB, ISMB, ISWB, ISHB, or built-up sections when necessary

Effective Length Factors (Referencing IS 800-1962, Clause 3.2)

Loading Conditions (Clause 2.1)

• Dead loads excluding columns: range from 40 to 60 kg/m²
• Live loads as per IS 875-1964, adjusted for roof pitch
• Wind loads based on IS 875-1964 for three wind zones
• Wind scenarios analyzed include:
  • Wind perpendicular to ridge with internal suction (WLi)
  • Wind perpendicular to ridge with internal pressure (WL2)
  • Wind parallel to ridge with internal pressure (WL3)
• Drag forces for multi-bay frames are accounted for
• Earthquake loads per IS 1893-1984 are evaluated but usually not critical

Deflection Limits

• Maximum lateral sway of columns limited to Height/325
• Maximum vertical beam deflection limited to Span/325

Approximate Unit Weights of Portal Frames (kg/m²) Extract

Essential Formulas and Tables for Load Evaluation (IS SP Part 40)

Load Combinations for Portal Frame Design (Clause 107.51)

• Combination considered: Dead Load + Live Load (D.L + L.L) = 495 kg/m

Design Force Summary (Table 90)

• MB, Mc: Moments at base and crown respectively
• VA, VE: Vertical reactions
• HA, HE: Horizontal reactions

Load Parameters (Clause 6.0)

• Typical portal span: 18 m
• Column spacing: 6 m
• Column height: 6 m
• Roof slope: 1 in 3 (18.435°)
• Wind pressure: 100 kg/m² (foundation forces scaled accordingly)

Foundation Forces (Table 80)

• Encompass dead, live, and wind loads including moments and horizontal forces
• Add 30 kg/m² for AC sheeting and girts dead load allowance

Important Considerations:

• Apply a 0.75 factor on load combinations involving wind load for design
• Deflections calculated at joint D using the unit load method under dead plus wind load condition (DL + WL2)
• For wind pressures differing from 100 kg/m², scale forces proportionally
• Support conditions (hinged or fixed) influence moment and reaction values

Structural Member Design (IS SP Part 40 & IS 800-1962)

Effective Length Factors (IS 800-1962)

• Strong Axis | 3.0 | 1.5 | | Axial Compression:

• Weak Axis | 0.75 | 0.75 | | Bending Compression (Columns) | 1.0 | 0.75 |

• Maximum slenderness ratio for columns set to 250 to account for bending effects

Deflection Criteria

• Maximum sway deflection for columns: Span/325
• Maximum vertical beam deflection: Span/325

Selection of Sections

• Prismatic rolled I-sections used: ISLB, ISMB, ISWB, ISHB
• If sections not available, adopt next larger size in series
• Built-up sections may be necessary for taller columns (9 m, 12 m heights)

Welded Connection Specifications (Table 84)

• Stiffeners to fit within the beam depth and extend to flange edges

Column Slenderness Ratio Formula:

[ \lambda = \frac{KL}{r} \leq 250 ] Where:

• K = Effective length factor from table
• L = Actual member length
• r = Radius of gyration

flowchart TD
    A[Start: Portal Frame Design] --> B[Assess Loads]
    B --> C[Compute Forces and Moments]
    C --> D[Identify Effective Length Factors]
    D --> E[Evaluate Slenderness Ratio]
    E --> F{Is Slenderness \leq 250?}
    F -- Yes --> G[Proceed with Design]
    F -- No --> H[Modify Section or Bracing]

Key Connection Specifications for Steel Portal Frames (IS SP:40-1987)

Welded Connections (Table 84)

• Crown stiffener weld thickness fixed at 5 mm
• Stiffeners must be accommodated within section depths and extend fully to flange edges

Bolted Connections

• Haunch bolted connections (Table 85):

  • High strength friction grip (HSFG) or high tensile bolts (grade 10k)
  • Bolt diameters vary between 16 to 24 mm depending on section size
  • Pitch and gauge dimensions vary according to section depth (refer to Table 85 and Fig. 8)

• Crown bolted connections (Table 86):

  • Bolt diameters from 16 to 20 mm
  • Bolt pitch and gauge between 30 to 40 mm
  • Detailed geometry shown in Fig. 10

Column Base Details

• Fixed bases (Table 87):

  • Slab base sizes vary from 500x450 mm to 900x650 mm
  • Anchor bolts: 6 in number, diameters between 36 to 56 mm
  • Stiffener plates: thickness 10-16 mm, height 250 mm
  • Base plate connected with 8 mm fillet welds

• Hinged bases (Table 88):

  • Slab base sizes range from 350x300 mm to 650x300 mm
  • Anchor bolts: 4 in number, diameters between 14 to 22 mm
  • Plate thickness between 16 to 20 mm
  • Refer Fig. 11 for base detail layout

Design Remarks

• Anchor bolt lengths are computed as per design example in Clause 6.8
• Connection forces should consider combined dead and wind loads, with proportional adjustments
• Fabrication references available in Figures 5-7 for haunches, Fig. 8 for bolted joints

Specifications and Calculations for Foundation Loads (IS SP Part 40)

Foundation Load Values (Typical for Steel Portal Frames)

• Forces vary depending on column height (6m, 9m, 12m), number of bays, and footing location
• Forces should be scaled proportionally for wind pressures other than 200 kg/m²
• Include additional 30 kg/m² dead load for AC sheeting and girts in foundation force calculations

Soil Bearing and Footing Depth

• Allowable soil bearing pressure: 15,000 kg/m²
• Minimum footing depth below ground level: 2.5 m
• Soil backfill unit weight: 1,500 kg/m³

Load Combinations for Design

• Evaluate both Dead Load + Live Load and Dead Load + Wind Load scenarios
• The governing condition is often Dead Load + Wind Load due to uplift and moment effects

Anchor Bolt Design

• Example tension in anchor bolts: 13,205 kg for 3 bolts
• Use net bolt area as 0.75 times the gross area (e.g., for 36 mm diameter bolts)
• Calculate development length based on tension and base plate design

Design Flow Summary

flowchart TD
    A[Identify Loads] --> B[Compute Foundation Forces]
    B --> C[Select Appropriate Footing Type]
    C --> D[Design Footing and Anchors]

Deflection Computation Guidelines per IS SP:40 (1987)

• Load for Deflection Analysis: Use an amplified load equal to 1.333 times the value from Table B
• Point of Maximum Deflection: Horizontal deflection peaks at joint D
• Calculation Procedure: Employ the unit load method using moment diagrams for actual (M) and unit (m) loads

Deflection Formula:

[ \delta = \frac{\sum (Area\ of\ M \times Ordinate\ of\ m\ at\ centroid\ of\ M)}{EI} ]

Where:

• E = Modulus of elasticity
• I = Moment of inertia of the cross-section

Sample Data from Table 91

Sum of products gives numerator for deflection calculation

Deflection Limits and Load Combinations

• Deflections checked against allowable limits specified in Clause 3.52
• Load combinations DL + LL or DL + WL as per Clause 107.51
• Example maximum sway deflection parameter: 0.774 × 10⁵ cm⁴

Summary

• Use unit load method with moment diagrams
• Calculate integral of M × m via areas and ordinates product
• Divide by EI to find deflection
• Ensure deflections comply with serviceability criteria

flowchart LR
    A[Apply Actual Load] --> B[Construct Moment Diagram (M)]
    B --> C[Apply Unit Load at Deflection Point]
    C --> D[Construct Unit Load Moment Diagram (m)]
    D --> E[Calculate Areas under M]
    E --> F[Determine Ordinates of m at Centroids]
    F --> G[Compute Deflection Using Formula]

Summary of Fabrication and Erection Instructions (IS SP:40-1987)

Welded Connection Guidelines (Table 84)

• Stiffeners must fit within beam depths and extend fully to flange edges
• Crown stiffeners welded with 5 mm fillet welds

Bolted Connection Details

• Haunch and crown bolted connections use HSFG bolts (grade 10k) with diameters mainly between 16 and 20 mm
• Bolt pitch and gauge vary by section type (ISLB, ISMB, ISWB, ISHB), detailed in Tables 85 and 86
• Refer to figures 5 through 10 for geometric details

Column Base Construction

• Fixed bases (Table 87) and hinged bases (Table 88) specify slab dimensions, plate thickness, number and diameter of anchor bolts, and stiffener sizes
• Anchor bolts typically 30-56 mm diameter, quantity 4-6
• Base plate connections made with 8 mm fillet welds
• Anchor bolt length calculated following Clause 6.8 design example

Sag Rod and Purlin Connections

• Sag rods usually 10-12 mm diameter, placed every 7th or 8th panel
• Maximum panel sizes: roof purlins 1400 mm, wall girts 1700 mm
• ISA sections used for struts and sag rods

Foundation Loads

• Forces for fixed and hinged bases provided
• Wind loads based on 200 kg/m²; scale for other pressures as appropriate

Bracing and Sag Rod Details (IS SP Part 40)

Sag Rods for Purlins and Girts (Clause 12.7)

• Typical sag rod diameter: 10 mm to 12 mm (ISRO rods)

Sag Rod Force Calculation (Clause 123.18)

[ F_{sag rod} = \frac{5 \times 123.18 \times \sin 18.435^\circ \times 6 \times 8}{8} = 1169 \text{ kg} ]

• Required net cross-sectional area:

[ A_{net} = \frac{1169}{1500} = 0.78 \text{ cm}^2 ]

• Use 12 mm diameter rod (area approx. 1.13 cm²) for safety margin

Bracing Design (Clause 6.9)

• Recommended bracing type: Type b
• Wind forces perpendicular to ridge resisted primarily by frame action; nominal bracing in gable and side walls
• Diagonal sag rods installed every 8th panel of purlins and at the uppermost panel

Typical Sag Rod Detail (Refer Fig. 4)

graph TD
    Purlin -->|Sag Rod| Frame

Guidelines for Expansion Joints in Steel Portal Frames (IS SP:40-1987)

• Applicability: Expansion joints are generally unnecessary for buildings under 180 m in length. For longer structures, expansion joints should be incorporated to segment the structure into independent units with separate superstructure supports.

• Structural Separation: Wind bracing and other structural elements must be discontinuous across the expansion joint, ensuring independent bracing systems on each side.

• Closure Details: The joint gap should be properly bridged using roof and wall cladding to maintain weatherproofing.

Expansion Joint Summary Table

Additional Recommendations:

• Design joints to accommodate thermal expansion and contraction
• Typical gap width determined by expected movement; consult detailed codes or manufacturer guidelines
• Ensure waterproof sealing to prevent leakage

flowchart LR
    A[Is Building Length > 180m?] -->|No| B[Expansion Joint Not Required]
    A -->|Yes| C[Provide Expansion Joint]
    C --> D[Divide Structure into Independent Segments]
    D --> E[Ensure Discontinuity of Bracing]
    E --> F[Bridge Gap with Cladding and Roof Materials]

Key Parameters and Data from IS SP Part 40 Design Example

Basic Project Parameters (Clause 6.0)

Wind Load Cases (kg/m²)

• Wind drag effects at crown points are included for multi-bay frames

Design and Analysis Highlights

• Portal frame analysis conducted using stiffness method supported by computational tools
• Structural design follows IS 800-1962, compatible with IS 800-1984
• Internal pressure factor from IS 875-1964 for normal permeability (±0.2)
• Design covers purlins, girts, frame members, base plates, fasteners, and eaves beams
• Bracing and foundation designs provided as typical examples, not standardized
• Joint detail illustrations are indicative
• Economical design achieved by optimizing spans, bay numbers, and roof slopes

Typical Design Parameter Table (Clause 4.5)

Summary Points from IS SP:40 (1987)

• Unit weights of portal frames (in kg/m²) include steel frame and purlins but exclude sag rods, base plates, and girts

• Data provided for spans ranging from 9 m to 30 m, various column heights, number of bays, roof slopes (1:3, 1:4, 1:5), support types (fixed and hinged), frame spacings (4.5 m and 6.0 m), under wind pressures of 100 and 150 kg/m²

• Typical unit weight ranges:

  • For 9 m span, fixed support with one bay and 4.5 m column height: approximately 22 to 34 kg/m² depending on roof slope and spacing
  • Hinged supports generally result in heavier frames due to lower restraint

• Design observations:

  • Lateral deflection limit of 1/325 generally governs member design
  • Stiffness method analysis is computer-based
  • Structural design follows IS 800-1962 with minor variations expected under IS 800-1984
  • Internal pressure factored as per IS 875-1964
  • Joint details are illustrative, not comprehensive
  • Bracing and foundation designs vary; typical examples included
  • Design examples provide practical guidance

• Economical design tips:

  • Reducing number of bays and optimizing frame spacing decreases steel weight
  • Fixed supports tend to be more economical compared to hinged
  • Roof slope influences steel quantity moderately

Representative Table Extract (Unit Weight kg/m² for 100 kg/m² Wind Pressure, 4.5 m Spacing)

Key References and Tables from IS SP:40 (1987)

1. Foundation Forces for Portal Frames (Table 79)

   • Data provided for frame spacings 4.5 m and 6.0 m, column heights 6 m, 9 m, and 12 m
   • Forces include dead, live, and wind loads (wind pressure at 200 kg/m²)
   • Downward force considered positive; horizontal force positive to the left
   • For wind pressures of 100 or 150 kg/m², scale forces accordingly
   • Add 30 kg/m² for AC sheeting and girts dead load

2. Welded Connection Details (Table 84)

   Section Depth (mm)	Haunch Weld Size (mm)	Haunch Stiffener Thickness (mm)	Crown Stiffener Thickness (mm)
   200 - 350	8	16	6 to 8
   400 - 600	6	18	8

   • Stiffeners fit within section depth; breadth extends to flange edge
   • Crown stiffeners welded with 5 mm fillet weld

3. Bolted Connection Details

   • Haunch (Table 85) and crown (Table 86) bolted connections specify bolt diameters, spacing (pitch and gauge), and bolt types (HSFG 10k)
   • Refer to figures 5 through 10 for connection geometry

4. Column Base Details

   • Fixed (Table 87) and Hinged (Table 88) bases specify slab sizes, anchor bolt diameters and quantities, and stiffener thickness
   • Anchor bolt length calculated as per design example (Clause 6.8)
   • Base plate connections use 8 mm fillet welds

Important Notes:

• Foundation forces and connection details assume roof slopes of 1:3, 1:4, and 1:5 for conservative design
• Detailed drawings found in figures 1 and 5 to 11
• Adjust forces based on actual wind pressures

flowchart TD
    A[Frame Spacing & Column Height] --> B[Foundation and Connection Design]