LESSON 8.4 — Steel Structures

A. Standard Map

B. Mechanism in Words

1. Section shape matches dominant force — I for bending (beams), H/CHS for axial compression (columns).
2. Floor loads cascade: deck → secondary beams (joists) → primary beams → girders → columns.
3. Portal frames resist lateral load through rigid beam-column joints; haunch deepens section at eave.
4. Trusses carry load in axial tension/compression — top chord compression, bottom chord tension under gravity.
5. Composite action requires headed stud connectors — without them, steel and concrete slide independently.

C. Core Concept Explanations

C1. Steel Sections — Identification Table

Why Section Shape Matters

The shape of a structural steel cross-section determines how efficiently it carries different types of force. For bending, material placed far from the neutral axis (top and bottom flanges) contributes most to moment of inertia. For axial load (columns), a compact cross-section with equal radii of gyration in both axes resists buckling equally in all directions. For torsion, a closed hollow section is orders of magnitude stiffer than an open section of the same area.

Steel Section Visual Identification Table

Section Use Summary by Force Type

C2. Steel Floor Hierarchy

The Four-Level Chain

Steel-framed floors transfer loads through a defined hierarchy of members, each carrying an increasing cumulative load to the next level below.

Floor Load (DL + LL)
        ↓
  Metal Deck / Composite Slab
  (spans between secondary beams; 1–4 m)
        ↓
  Secondary Beams (Joists)
  (collect deck load over tributary width; span 4–10 m between primaries)
        ↓
  Primary Beams (Girders)
  (collect reactions from secondary beams as point loads; span 6–15 m to columns)
        ↓
  Columns
  (carry cumulative floor loads; transfer to foundations)
        ↓
  Foundations

Each Level — Role and Typical Dimensions

Why Steel Floors Are Used

Steel-framed floors are chosen over RCC when:
– Long spans (>9 m) are needed without heavy sections
– Speed of construction is critical (pre-fabricated members, no curing time)
– Building occupancy changes require future removal or modification of members
– Fire protection is applied as intumescent coating (steel alone has poor fire resistance)
– Composite action with a concrete slab is used to increase stiffness and reduce steel weight

C3. Portal Frame

What Is a Portal Frame

A portal frame is a single-storey rigid frame consisting of two columns rigidly connected at their tops to a pitched or horizontal rafter. The rigid connections at the eaves (column-rafter junction) and optionally at the ridge (apex) mean the frame acts as a continuous loop — loads applied anywhere are shared by all members.

Plan geometry: A building is formed by repeating identical portal frames at regular spacing (6–10 m bay spacing), connected along the length by purlins (horizontal members spanning between rafter pairs) and girts (horizontal members spanning between column pairs) that carry the roof cladding and wall cladding.

Governing code: IS 800:2007 — General Construction in Steel.

Structural Behaviour

Under gravity loading, the rigid eave connections transfer bending moments from the rafter into the columns — the columns must resist both the vertical reaction and the horizontal outward thrust (like the legs of an arch). This horizontal thrust must be resisted at the base:
– Pinned base: Base plate free to rotate; horizontal thrust is carried by the floor slab or a tie rod between column bases at foundation level
– Fixed base: Base plate bolted rigidly to foundation; moment transferred into the footing; requires deeper/larger footing

The most critical feature of the portal frame is the haunch at the eave — the tapered deepening of the rafter section near the column. The haunch:
– Increases the section depth at the location of maximum bending moment (the eave joint)
– Reduces the weight of the rafter midspan (where bending moment is lower)
– Provides a stiff connection region for bolted splices

Connection Types

When Portal Frames Are Used

Portal frames are the dominant structural system for single-storey industrial, warehouse, and commercial buildings in India and globally. They are preferred when:
– Clear span required: 20–60 m without internal columns
– Low-rise: single storey (eave height 5–15 m)
– Economy of steel: the haunched rafter is more material-efficient than a uniform beam over large spans
– Speed: portal frames are fabricated off-site, transported, and erected rapidly
– Flexibility: the column-free interior is maximally flexible for warehousing, manufacturing, or retail

Typical Indian examples: Warehouses along NH corridors, DMIC logistics parks, factory sheds in Pune/Surat industrial zones, large-format retail (IKEA, Amazon fulfillment centers), aircraft hangars.

C4. Trusses

How a Truss Works — Fundamental Principle

A truss converts the bending that a solid beam would experience into axial forces in individual members by triangulating the geometry. Since every triangle is a stable shape — unlike a rectangle, which can deform into a parallelogram — a truss transfers load through axial tension and compression in its members without any member needing to resist bending (under idealised pin-jointed assumptions).

Under gravity loads on a simply supported truss:
– Top chord: Compression (the chord is being shortened, like the top fibre of a beam in bending)
– Bottom chord: Tension (the chord is being stretched, like the bottom fibre of a beam)
– Web members (diagonals and verticals): Alternating tension and compression depending on their orientation and the truss type

This conversion is the efficiency gain — members carry only axial load, which they do more efficiently than bending. A truss uses far less steel than a solid beam of the same span and capacity.

Truss Type Comparison

Force Reversal Under Wind Uplift

Under gravity loads, the top chord is in compression and the bottom chord is in tension. Under wind uplift (net upward pressure on a roof), the forces reverse:
– Top chord → tension
– Bottom chord → compression

This force reversal is critical for design:
– Members designed only for gravity loads may fail under uplift if they are slender compression members not braced for buckling
– Connections (especially at apex and eave) must be checked for both sign combinations
– Lightweight truss roofs in India are particularly vulnerable to cyclone uplift — IS 875 Part 3 governs

C5. Space Frames

What Is a Space Frame

A space frame is a three-dimensional triangulated structure — a truss extended in three dimensions rather than in a single plane. Every member is connected at nodes (typically ball-and-socket or welded hollow-section joints), and loads applied anywhere are distributed through the network by axial forces in all connected members simultaneously.

The critical structural advantage over a conventional two-dimensional truss is redundancy and stiffness in all directions: a space frame of the same span and material weight as a planar truss is stiffer, carries loads more efficiently, and can span in two directions simultaneously without requiring parallel trusses.

Structural Behaviour

• Members are primarily in axial tension or compression (very little bending)
• Loads applied at nodes travel through multiple paths to supports
• Supports can be arranged freely — the space frame is not constrained to support at all four corners; point supports, perimeter supports, and intermediate supports are all possible
• Deflection is very small relative to span due to the three-dimensional stiffness

Geometry and Node Types

The most common space frame geometry uses a double-layer flat grid: two parallel layers of a square or triangular grid offset from each other, connected by diagonal members between layers.

When Space Frames Are Used in India

• Airport terminals — Indira Gandhi International Terminal 3 (Delhi), Chennai airport, Bengaluru Kempegowda Terminal — require column-free spans of 50–100 m for check-in halls and departure areas
• Exhibition and convention halls — Pragati Maidan, HITEX Hyderabad — large unobstructed floor areas
• Sports stadiums and indoor arenas — SAI National Centre roofs; multi-purpose indoor arenas
• Railway station concourses — large canopy spans over platforms

Space frames vs portal frames: A portal frame works in a single plane and requires many repeated frames along the building length; the purlins spanning between frames are the weak link. A space frame covers the full two-dimensional area in a single continuous structural system — far more efficient for large square or irregular plan areas.

C6. Tension Structures

The Logic of Tension as Structure

Cables, nets, and membranes carry load exclusively in tension. Unlike a beam (which must resist both tension and compression through its depth), a cable simply adjusts its geometry — it deforms until the tension in the cable is in equilibrium with the applied load. This means:
– No material is wasted carrying compression
– Very high-strength steel cable (fy ≈ 1500–2000 MPa vs 250–350 MPa for structural steel sections) can be used — most of the material’s capacity is utilised
– Spans achievable far exceed those of any bending or compression system for the same steel tonnage
– The geometry under load is a catenary (for a single span cable) or a complex three-dimensional surface (for a cable net)

The structural price: A pure tension structure has no stiffness unless the cables are pre-tensioned. An unstressed cable has zero lateral stiffness — it flaps freely under dynamic load. Pre-tensioning introduces initial tension that provides stiffness proportional to the tension level. This is analogous to how a guitar string vibrates at a defined frequency only when tuned (tensioned).

Types of Tension Structures

Cable-stayed structure:
A rigid deck (beam, truss, or girder) is supported by cables running diagonally from masts above. The cables are in pure tension; the mast is in compression; the deck resists horizontal thrust from the cable-deck junction. Used for long-span roofs (stadiums, airport canopies) and bridges. The cable pattern can be harp (parallel cables), fan (all cables from mast top), or semi-fan.

Cable net:
A net of crossing cables, typically pre-tensioned in opposite curvature (anticlastic — like a saddle). The two families of cables curve upward and downward respectively; under any load, one family increases tension while the other decreases it, maintaining overall stiffness. Used for large canopy roofs (the Munich Olympic roofs, 1972, by Frei Otto, are the definitive example).

Tensegrity:
A system of isolated compression struts floating within a continuous network of tension cables — no strut touches another strut. The entire structure is stable because the cables prestress the struts. Conceptualised by Buckminster Fuller; developed by Kenneth Snelson. Used architecturally for pavilions and exploratory structures; engineeringly in deployable space structures.

Membrane structure:
A thin, flexible fabric (PTFE-coated glass fibre or ETFE) pre-tensioned to a curved form, supported by cables and masts. Used for stadium canopies, entrance canopies, and environmental enclosures. The membrane carries in-plane tension only — no bending capacity.

Key Properties Compared

C7. Steel-Concrete Composite Construction

What Is Composite Action

When a steel beam is connected to a concrete slab above it through shear connectors (headed stud anchors welded to the top flange of the steel beam), the two materials act as a single structural unit. This is composite action.

Without shear connectors, the steel beam and the concrete slab each bend independently under load — the steel beam deflects downward and the concrete slab tries to slide horizontally relative to the beam top flange. The shear connectors prevent this slip, forcing the beam and slab to deflect together as a single deep section.

Why Composite Action Is Efficient

The transformed section principle: The composite beam has a much larger effective depth than the steel beam alone. For the same load and span:
– A non-composite steel beam needs a certain I (moment of inertia)
– Adding a composite slab above it effectively moves the neutral axis upward and dramatically increases the total I of the combined section
– This means a smaller (lighter, cheaper) steel beam achieves the same span and stiffness as a heavier non-composite beam

Typical result: A composite beam requires 20–40% less steel than an equivalent non-composite beam. For multi-storey office buildings with spans of 9–12 m, this is a significant cost and weight reduction.

Metal Deck (Composite Slab Deck)

The metal deck (profiled steel sheet) performs two functions:
1. Temporary formwork during construction — it supports the wet concrete without propping
2. Tensile reinforcement once the concrete hardens — the ribs of the profiled sheet bond with the concrete, acting as bottom reinforcement for the composite slab

Profile types:
– Trapezoidal (open trough): Ribs are trapezoidal; concrete bonds mechanically with the rib geometry; typical depth 50–80 mm
– Re-entrant: Ribs have undercut profiles that provide additional mechanical interlock with concrete

Composite slab span: 2–4.5 m between supporting beams (secondary beams/joists). The overall floor depth (deck + topping) is typically 100–150 mm.

Shear Connectors

Headed stud shear connectors (Nelson studs) are the standard device. They are:
– Welded to the top flange of the steel beam by arc stud welding (through the deck sheet in most cases)
– Typically 19 mm diameter × 100–125 mm height
– Spaced uniformly along the beam — the number of studs determines the degree of composite action (full composite = 100%, partial composite = 40–80% commonly used in practice)

Full vs partial composite: Full composite provides the maximum stiffness and moment capacity but requires more studs. Partial composite (50–75%) is typically sufficient for deflection control while reducing stud count and cost.

When Composite Construction Is Used

D. Worked Numericals and Parameter Tables

Structural System Selection Table

E. Common Confusions

• Beam and girder are interchangeable in the floor hierarchy — They occupy different levels in the hierarchy. A joist/secondary beam spans between girders. A **girder/primary beam…
• In a Pratt truss, the diagonals are in compression — In a Pratt truss under gravity load, the diagonals are in tension and the verticals are in compression. The Pratt is…
• Portal frame = space frame (both are used for large-span industrial roofs) — A portal frame is a two-dimensional planar rigid frame, repeated at intervals along a building length. A space frame is …
• A cable net is in compression on one axis and tension on the other — A cable net (anticlastic surface) is in tension in both cable families, but the two families curve in opposite direc…
• I-sections and H-sections are the same — both look like a letter I — They are different sections for different roles. An I-section (Universal Beam) has narrow flanges relative to depth — op…
• The haunch in a portal frame is a decorative tapered connection — The haunch is a critical structural element — it deepens the rafter cross-section at the eave joint, where the bending m…

F. Exam Traps

G. Answer-Writing Cues

MSQ (sections):

MCQ (portal frame):

Composite:

H. PYQ Linkage Note

I. Mini-Check — Lesson 8.4

Q. Section Identification / Truss Type

Q1. Which of the following steel sections are most suitable for use as columns (axial compression + biaxial bending)? Select all that apply.

(A) Universal Beam (I-section / UB)
(B) Universal Column (H-section / UC)
(C) Circular Hollow Section (CHS)
(D) Rectangular Hollow Section (RHS)
(E) Angle section (L-section)

Q2. Match the truss type to its defining geometric characteristic:

(A) Warren truss
(B) Pratt truss
(C) Fink (W-pattern) truss
(D) North Light truss
(E) Bowstring truss

(i) Curved top chord in compression acting as an arch; straight horizontal bottom chord in tension
(ii) Asymmetric profile; one steep glazed face for diffuse daylighting
(iii) Alternating diagonal members only; no vertical members between chords
(iv) Vertical members in compression; diagonal members in tension; diagonals slope toward centre from supports
(v) W-shaped web configuration; diagonals subdivide each panel; common in residential roofs

Q. Portal Frame

Q. In a steel portal frame, the haunch at the eave connection serves which primary structural purpose?

(A) It provides a weather-tight junction between roof cladding and wall cladding
(B) It deepens the rafter cross-section at the location of maximum bending moment, reducing the required section elsewhere
(C) It transfers horizontal wind loads from the wall to the roof without a moment connection
(D) It prevents the portal frame from behaving as a pin-jointed mechanism under vertical load

Q. Composite Slab

Q. A composite slab deck (metal deck + concrete) relies on which mechanism to achieve composite action between the steel beam and the concrete slab?

(A) Welded shear connectors (headed studs) that transfer horizontal shear at the steel-concrete interface
(B) High-strength bolts through the top flange of the steel beam into the concrete
(C) The natural bond between the steel beam top flange and the fresh concrete
(D) The weight of the concrete pressing down on the steel deck

MCQ — Truss Type

Under gravity loading, a Pratt truss has:

(A) Diagonals in compression; verticals in tension
(B) Diagonals in tension; verticals in compression
(C) All members in compression
(D) Top chord in tension; bottom chord in compression