Criteria for design and construction of precast trusses and purlins

IS 3201 Scope - Key Points & Specifications

• Scope: Covers design, fabrication, handling, and erection of precast reinforced and prestressed concrete trusses.

• Definitions (Clause 2.0 & 3.1):

  • E = Modulus of elasticity of concrete
  • Eg = Modulus of elasticity of steel
  • Fou = Ultimate cube strength of concrete at 28 days
  • IAB = Moment of inertia of member AB
  • LAB = Length of member AB
  • MFAB = Fixed end moment at end A of member AB
  • δAB = Relative deflection perpendicular to member axis

• Dimensions (Clause 6.4):

  • Principal dimensions of truss members must ensure gravity axes intersect.
  • Typical fixing details for purlins and trusses provided (e.g., dowels, screed concrete thickness 30 mm, concrete grade M20).

• Handling & Grouting (Clause 7.6):

  • All post-tensioned members shall be grouted as per designer’s specification.

Typical Parameters Summary

Conceptual Illustration of Truss Member Parameters

graph LR
A[End A] -- Length LAB --> B[End B]
A -- Fixed End Moment MFAB --> A
B -- Fixed End Moment MFB --> B
Member AB -- Moment of Inertia IAB --> Member AB
Member AB -- Deflection δAB --> Perpendicular Direction

For detailed design, refer to IS 3201 clauses 2.0, 3.1, 6.4, and 7.6 along with associated codes for loads and concrete design.

IS 3201 - Definitions & Key Symbols (Clause 3.1)

For precast and prestressed concrete trusses, the following symbols are fundamental:

Additional Notes:

• All dimensions are in millimeters.
• Gravity axes of truss members must intersect (Clause 6.4).
• Post-tensioned members require grouting as per designer's specs (Clause 7.6).

Typical Usage Formulae:

• Flexural stiffness:
  [ EI = E \times I_{AB} ]

• Deflection relation:
  [ \delta_{AB} = \text{relative deflection perpendicular to member axis} ]

• Fixed end moment (example):
  Depends on loading and boundary conditions; refer to structural analysis tables for trusses.

graph LR
A[Member AB] -->|Length = LAB| B[Supports]
A -->|Moment of Inertia = IAB| C[Cross Section]
A -->|Deflection = δAB| D[Load Effects]

This summary sets the foundation for understanding and applying IS 3201 definitions in truss design and detailing.

IS 3201: Symbols and Notations (Clause 3.1)

Key Notes:

• All dimensions are in millimeters (mm).
• Modulus of elasticity values are essential for stress-strain calculations.
• Ultimate cube strength (Fou) is used for concrete capacity.
• Moment of inertia (IAB) and length (LAB) are critical for member stiffness and deflection.
• Fixed end moments (MFAB) are used in frame and truss analysis.
• Deflection (δAB) helps in serviceability checks.

Typical Usage Formulae:

• Stress due to bending:
  [ \sigma = \frac{M y}{I} ] where ( M ) = bending moment, ( y ) = distance from neutral axis, ( I ) = moment of inertia.

• Deflection of member:
  [ \delta = \frac{P L^3}{48 E I} ] for simply supported beam with central load ( P ).

graph LR
A[Member AB] -->|Length LAB| B[Geometry]
A -->|Moment of Inertia IAB| C[Stiffness]
A -->|Modulus of Elasticity E or Eg| D[Material Property]
A -->|Ultimate Strength Fou| E[Concrete Strength]
A -->|Fixed End Moment MFAB| F[Load Effects]
A -->|Deflection δAB| G[Serviceability]

This summary of symbols and their meanings aids in structural analysis and design per IS 3201.

IS 3201 - Key Formulas & Specifications for Materials

1. Modulus of Elasticity

• Steel (Ea):
  • Mild steel: As per IS 456-1978
  • High tensile steel: As per IS 1343-1980
• Concrete (E): Modulus of elasticity of concrete denoted as E (Clause 3.1).

2. Material Strengths & Symbols (Clause 3.1)

3. Concrete & Grouting (Clause 7.6)

• All post-tensioned members must be grouted.
• Grout specifications as per designer's instructions.
• Typical concrete grades used: M20 for screed and fill.

4. Dead Load Weights (Clause 6.1.1.1)

• Use actual weights for roofing sheets/ceiling.
• If unavailable, refer IS 875 (Part 1)-1987 for unit weights.

Summary Table: Modulus of Elasticity

graph LR
A[Steel] -->|Ea| B[Modulus of Elasticity]
C[Concrete] -->|E| B
D[Grouting] --> E[Post-tensioned members]
F[Dead Loads] --> G[Roofing sheets & Ceiling]

Note: Refer IS 456 and IS 1343 for detailed material properties and IS 875 for unit weights of materials.

IS 3201: Structural Analysis & Design Key Points

1. Design of Members (Clause 6.6)

• Members are designed based on forces from analysis.
• Use rigid joint analysis or pin-jointed truss assumption for triangulated trusses up to 30 m span (Clause 5.5).
• Secondary stresses due to joint rigidity and prestress typically compensate each other.

2. Dimensions (Clause 6.4)

• Principal dimensions per Clauses 6.4.1 to 6.4.4.
• Gravity axes of members must intersect at joints ensuring equilibrium.

3. Load Considerations

• Follow separate IS codes for:
  • Dead Loads: IS 875 (Part 1)
  • Imposed Loads: IS 875 (Part 2)
  • Wind Loads: IS 875 (Part 3)
  • Earthquake Loads: IS 1893 (4th Revision)

4. Material Specifications

• Reinforcement: IS 432 (Mild & Medium Tensile Steel), IS 1786 (High Strength Deformed Bars)
• Prestressing Steel: IS 6003, IS 1343

5. Grouting (Clause 7.6)

• Post-tensioned members must be grouted as per designer’s specification.

Typical Structural Analysis Approach

flowchart TD
    A[Load Application] --> B[Structural Model]
    B --> C{Joint Type?}
    C -->|Pin-jointed| D[Truss Analysis]
    C -->|Rigid-jointed| E[Rigid Frame Analysis]
    D --> F[Member Forces]
    E --> F
    F --> G[Design Member Sections]
    G --> H[Check Against IS Code]

Summary Table: Truss Analysis Assumptions

For detailed formulas on member forces, prestress losses, and design checks, refer to:

• IS 1343 (Prestressed Concrete)
• IS 456 (Plain & Reinforced

IS 3201 - Design of Members (Clause 6.6)

1. Reinforced Concrete Compression Members (6.6.2)

• Design as per IS 456:1978 for compression members.
• Assume pin-jointed ends for effective length.
• Consider combined direct load + bending moment.
• Use permissible stresses for bending × reduction factor (for long columns).
• Account for lateral support by purlins or stiffeners.

2. Reinforced Concrete Tension Members (6.6.3)

• Provide sufficient reinforcement to resist tension at permissible tensile stress of steel.
• Ensure the steel area ( A_s ) satisfies:
  [ T = A_s \times f_{st} ] where
  ( T ) = tension force,
  ( f_{st} ) = permissible tensile stress in steel.

3. Prestressed Concrete Tie Members (6.6.5)

• Design considering prestressing force to counteract tensile stresses.
• Ensure no tensile stresses in concrete under service loads.

Key Formula Summary

Effective Length Assumption (Compression Members)

graph LR
A[Member Ends] -->|Pin-jointed| B(Effective Length \(L_{eff}\))
B --> C{Use in design per IS 456}

For detailed permissible stresses and reduction factors, refer to IS 456:1978 tables and clauses on columns and tension members.

IS 3201: Construction, Handling, and Erection of Precast Prestressed Concrete Trusses

1. Construction

• Loads to consider (Clause 6.1):
  • Dead loads
  • Live loads
  • Wind loads
  • Seismic loads
  • Handling and erection loads
• Material specifications:
  • Use M20 grade concrete for screed and concrete fill.
  • All post-tensioned members must be grouted as per designer's specification (Clause 7.6).
• Connections:
  • Use dowels and bent bars (e.g., 10 mm bars bent down after erection) for purlin fixing.
  • Proper detailing of laps, welds, sleeves, nuts, especially in tension members (Clause 6.10).

2. Handling and Hoisting (Clause 7.2 & Table 7.2)

• Trusses must be handled carefully to avoid damage.
• Use appropriate lifting points as per design.
• Hoisting equipment should be capable of handling design erection loads.
• Ensure temporary supports during erection to prevent overstressing.

3. Typical Fixing Details

4. Key Notes

• Account for shrinkage and temperature effects in truss seatings.
• Grouting of post-tensioned members must be done immediately after tensioning.
• Follow detailed drawings (e.g., FIG. 3, 4, 5) for purlin and truss connections.

flowchart TD
    A[Design Phase] --> B[Consider Loads: Dead, Live, Wind, Seismic, Handling]
    B --> C[Material Specification: M20 Concrete, Grouting]
    C --> D[Fabrication: Post-tensioning & Grouting]
    D --> E[Handling: Use Proper Lifting Points]
    E --> F[Erection: Temporary Supports & Fixing Details]
    F --> G[Final Fixing: Purlins & Screed Concrete

IS 3201 Key Points on Bracing and Stability

1. Bracing Requirements (Clause 7.3)

• Top and bottom chords of trusses must be braced to:
  • Transfer horizontal forces perpendicular to the truss plane.
  • Prevent buckling of compression members.
• Bracing transfers forces longitudinally along the structure.
• Typical bracing details include insert plates and welds (e.g., 150 mm long weld at 750 mm c/c).
• Refer to Fig. 2 for steel bracing details and insert plate locations (P1, P2).

2. Stability Considerations

• Trusses are generally assumed pin-jointed for analysis (Clause 5.5), valid for spans ≤ 30 m.
• Rigidity effects compensate secondary stresses, so pin-jointed assumption is accurate.
• For longer spans or special cases, full rigid-joint analysis is recommended.

3. Handling & Hoisting (Clause 7.6)

• Post-tensioned members must be grouted as per designer’s specs.
• Proper bracing during erection ensures stability.

Typical Bracing Detail Summary (Fig. 2)

Stability Check Formula (Compression Member Buckling)

[ P_{cr} = \frac{\pi^2 E I}{(K L)^2} ]

• (P_{cr}): Critical buckling load
• (E): Modulus of elasticity
• (I): Moment of inertia of member cross-section
• (K): Effective length factor (depends on end conditions)
• (L): Length of member

graph LR
A[Top Chord] -- Brace --> B[Insert Plate P1]
B -- Welded --> C[Brace Rod]
C -- Welded --> D[Insert Plate P2]
D -- Brace --> E[Bottom Chord]

Summary:

• Always brace top and bottom chords to prevent lateral buckling.
• Use insert plates and welds as per Fig. 2.
• Analyze tr

IS 3201 Key Points on Purlins: Design & Fixing

1. Types of Purlins (Clause 7.4)

• Structural steel sections (see SP 6 (1)-1964)
• Cold formed steel sections (IS 811-1965)
• Reinforced concrete (solid, angle, channel, trussed)
• Prestressed concrete (IS 1343-1980 applies for prestressed)

2. Design Considerations (Clause 6.7 & 7.4.1)

• Use balanced cantilever or pretensioned purlins to reduce self-weight.
• Prestressed purlins must follow IS 1343 provisions.
• Cross-section shapes vary (refer to Fig. 5 in IS 3201 for typical sections).

3. Spacing of Purlins (Clause 6.4.4)

• Governed by roofing sheet widths or other roofing materials.
• Must conform to relevant IS standards for roofing.

4. Fixing Details (Clause 7.4)

• Typical fixing shown in Fig. 3 (purlin to truss connection).
• Ribbed slab purlin details in Fig. 4.

Typical Design Formula for Purlin Bending:

[ M_u = w \times \frac{l^2}{8} ]

Where:

• (M_u) = Ultimate bending moment
• (w) = Uniform load on purlin (including self-weight + roofing load)
• (l) = Span length between supports

Design bending stress:

[ \sigma = \frac{M_u}{Z} ]

Where (Z) = Section modulus of the purlin cross-section.

Summary Table: Purlin Material & Code Reference

Grouting of Post-tensioned Members (IS 3201 Highlights)

• Clause 7.6: All post-tensioned precast prestressed truss members shall be grouted.
• Grout Specification: As per designer's instructions (typically cement-sand-water mix with additives for flow and strength).

Key Specifications for Grouting:

Important Notes:

• Grouting fills ducts around tendons to protect against corrosion and bond prestressing steel to concrete.
• Grout must be free from voids; proper venting and injection methods are essential.
• Use non-shrink additives if specified.
• Follow IS 1343 for prestressed concrete detailing and grouting practice.

Typical Grouting Process Flow:

flowchart TD
    A[Prepare Grout Mix] --> B[Flush Duct with Water/Air]
    B --> C[Inject Grout at Low Pressure]
    C --> D[Ensure Full Duct Filling]
    D --> E[Seal Duct Ends]
    E --> F[Cure Grout for 7 Days]

For detailed design and execution, refer to IS 1343 (Code of Practice for Prestressed Concrete) and project-specific specs.

IS 3201: Loadings and Load Combinations for Precast Concrete Trusses

Key Loads to Consider (Clause 6.1)

• Dead Loads: Self-weight of the truss and permanent fixtures.
• Imposed Loads: Live loads as per IS 875 (Part 2).
• Wind Loads: As per IS 875 (Part 3).
• Seismic Loads: According to IS 1893 (Fourth Revision).
• Handling & Erection Loads: Temporary loads during transport and installation.
• Shrinkage & Temperature Effects: Consider for truss seatings.

Load Combination Guidelines

Refer to IS 456 and IS 1893 for load combinations. Typical factored load combinations for ultimate limit state design:

Where:

• DL = Dead Load
• IL = Imposed (Live) Load
• WL = Wind Load
• EL = Earthquake Load

References for Loads

• Dead & Imposed Loads: IS 875 (Parts 1 & 2)
• Wind Loads: IS 875 (Part 3)
• Seismic Loads: IS 1893 (Fourth Revision)

Summary Diagram of Load Application

graph LR
    DL[Dead Load]
    IL[Imposed Load]
    WL[Wind Load]
    EL[Earthquake Load]
    HL[Handling & Erection Load]
    SE[Shrinkage & Temperature Effects]

    DL --> Design
    IL --> Design
    WL --> Design
    EL --> Design
    HL --> Design
    SE --> Design

Note: Ensure the gravity axes of truss members intersect (Clause 6.4) for proper load transfer.

IS 3201: Dimensions and Geometry of Precast Concrete Trusses

Key Points from Clause 6.4:

• Principal dimensions of precast reinforced and prestressed concrete trusses are specified in Clauses 6.4.1 to 6.4.4.
• Gravity axes of different members must intersect properly to ensure structural stability.

Important Symbols (Clause 3.1):

Thickness (Clause 6.6.1):

• Thickness of members to be as per design requirements ensuring durability and strength.

Handling & Hoisting (Clause 7.6):

• All post-tensioned members must be grouted according to designer specifications.

Typical Dimensions & Fixing Details (From Figures 3, 4, 5):

• Purlin fixing: Dowels from purlin to truss, 10 mm bars bent after erection.
• Concrete screed: 30 mm thick M20 concrete screed on top chord.
• Ribbed slab purlin details: Shown in typical cross-sections.

Summary Table: Typical Member Dimensions (Indicative)

Gravity Axis Alignment:

• Ensure gravity axes of chords and webs intersect at joints to avoid eccentric loading.

flowchart TD
    A[Top Chord] --> B[Joint]
    C[Web Member] --> B
    D[Bottom Chord] --> B
    B --> E[Gravity Axis Intersection]
``

IS 3201: Reinforcement Requirements Summary

1. Minimum Reinforcement (Clause 6.6.4)

• Thick members: Minimum 4 bars of 6 mm diameter at corners.
• Thin members (< 75 mm thick): Minimum 2 bars of 6 mm diameter.
• Applies regardless of force magnitude.

2. Transverse Reinforcement (Clause 6.8)

• Pitch spacing must follow IS 456:1978.
• Transverse reinforcement ensures shear resistance and confinement.

3. Steel Specifications (Clause 4.3.1)

• Use:
  • Mild/medium tensile steel bars per IS 432 (Part 1)-1982.
  • High strength deformed bars per IS 1786-1985.

Key Reference: Pitch of Transverse Reinforcement (IS 456:1978)

Minimum Reinforcement Visual:

graph LR
A[Member Thickness] --> B{≥ 75 mm?}
B -- Yes --> C[4 bars of 6 mm diameter at corners]
B -- No --> D[2 bars of 6 mm diameter]

Summary:

• Always provide minimum corner bars even if forces are low.
• Follow IS 456 for spacing of stirrups or ties.
• Use IS standard steel bars for reinforcement.

IS 3201 Key Points on Camber and Deflection

Camber (Clause 6.5.1)

• For precast reinforced concrete trusses, provide camber ≥ calculated deflection due to dead + applied loads.
• For prestressed precast trusses, camber is not essential.

Deflection Symbols (Clause 3.1)

Fixed End Moment due to Deflection (Clause 5.3.1)

[ M_{FAB} = + \left(\delta_{AB}\right)^2 ]

• For preliminary design, use: [ M_{prelim} = 0.7 \times M_{FAB} ]

Design Tips

• Use flatter diagonals in trusses to reduce net moments.
• Camber should compensate for dead + applied load deflections to maintain serviceability.

Summary Table: Camber & Deflection

flowchart LR
    A[Load Applied] --> B[Deflection δ_AB]
    B --> C[Calculate Fixed End Moment M_FAB]
    C --> D[Preliminary Moment = 0.7 M_FAB]
    D --> E{Camber Required?}
    E -- Precast RC --> F[Provide Camber ≥ δ_AB]
    E -- Prestressed --> G[Camber Not Essential]

This ensures truss serviceability and structural integrity per IS 3201.

IS 3201: References to Other Standards - Key Points

• Clause 6.6.5.4: Compliance with IS 1343-1980 (Code of Practice for Prestressed Concrete) is mandatory for all related requirements.

• Symbols (Clause 3.1):

  • E = Modulus of elasticity of concrete
  • Eg = Modulus of elasticity of steel
  • Fou = Ultimate cube strength of concrete at 28 days
  • IAB = Moment of inertia of member AB
  • LAB = Length of member AB
  • MFAB = Fixed end moment at end A of member AB
  • δAB = Relative deflection perpendicular to member axis

• Handling & Hoisting (Clause 7.6 and Table 7.2):

  • All post-tensioned precast truss members must be grouted as per designer's specification.

• Typical Fixing Details:

  • Purlin connections with trusses (end, middle, ribbed slab) are dimensioned in millimeters.
  • Use M20 concrete for screed and fill.
  • Dowels and bent bars ensure structural integrity after erection.

Summary Table: Related IS Codes

Practical Note:

Always cross-reference IS 3201 with IS 1343-1980 for prestressed concrete design and detailing to ensure full compliance.

graph LR
A[IS 3201] --> B[IS 1343-1980]
A --> C[Handling & Hoisting]
A --> D[Fixing Details]
B --> E[Prestressed Concrete Design]
C --> F[Grouting Specifications]
D --> G[Purlin Connections]

For detailed design, refer to IS 1343-1980 for prestressing parameters and IS 3201 for truss-specific details.