IS 733 : 1983Wrought Aluminium and Aluminium Alloy Plate, Sheet and Strip for General Engineering Purposes - Specification

IS 733:1983 specifies wrought aluminium and aluminium alloy bars, rods and sections (extruded) for general engineering purposes — the extruded aluminium stock behind window/door framing, curtain-wall mullions, louvres, handrails, partitions and architectural metalwork. It defines the alloy designations, tempers and mechanical properties you select an extrusion by.

*(Note: the code-database title for this entry reads 'Plate, Sheet and Strip'; IS 733's established scope is bars, rods and (extruded) sections — flat-rolled aluminium plate/sheet/strip is covered by separate standards such as IS 737. The discrepancy has been flagged for correction; this note addresses extruded bars/rods/sections.)*

It is read with the architectural-metal stack:

• IS 1948 / IS 1949 — aluminium windows / doors (the products made from these sections)
• IS 737 — wrought aluminium alloy sheet and strip · IS 740 — castings
• IS 1868 — anodic coatings on aluminium (durability/finish)

Aluminium structural/architectural extrusions are specified by alloy + temper, which together fix strength, corrosion resistance and extrudability:

• The workhorse architectural alloy is the 6xxx series (Al-Mg-Si, e.g. 63400 / 6063, 64430 / 6061-type) — good extrudability, corrosion resistance and anodising response
• Temper designations (T5, T6, etc.) define the heat-treatment/work-hardening state and hence the 0.2% proof stress, UTS and elongation — a T6 temper is significantly stronger than T4/T5
• Key properties from IS 733: proof and ultimate strength, elongation, and the section's dimensional tolerances
• Aluminium's modulus is ~⅓ of steel (~70 GPa) — so deflection, not strength, very often governs slender mullions and spans; and its thermal expansion is ~2× steel, demanding movement joints

A correct specification states alloy *and* temper (e.g. '63400-T6'), the section, and the finish (anodised/powder-coated) — not just 'aluminium section'.

Brief: an aluminium mullion spanning floor-to-floor (~3.6 m) carrying wind on the glazed bay.

Step 1 — wind load: design wind pressure to IS 875 Part 3 × the mullion's tributary width → a line load on the mullion.

Step 2 — alloy/temper: choose a 6xxx architectural alloy in a T6 temper (e.g. 63400-T6) for adequate proof strength and good anodising.

Step 3 — deflection governs: size the section so mid-span deflection under wind ≤ the serviceability limit (commonly ~ span/175 or a fixed cap for glazed systems) — because E ≈ 70 GPa, this almost always governs over the strength check.

Step 4 — strength check: bending stress ≤ the allowable for the alloy/temper from IS 733 (with the relevant safety factor).

Step 5 — movement & isolation: detail thermal-movement provision (≈2× steel expansion) and isolate aluminium from steel/concrete to prevent bimetallic/alkaline corrosion. Select the lightest section that passes the deflection limit.

1. Specifying 'aluminium section' without alloy + temper. Strength varies several-fold between tempers; an unspecified extrusion can be a soft T4 where a T6 was assumed.

2. Designing for strength, not deflection. With E ≈ ⅓ of steel, slender mullions/spans are almost always deflection-governed — a strength-only check gives a flimsy, oil-canning façade.

3. Ignoring thermal movement. Aluminium expands ~2× steel; no movement joints → buckling, sealant failure and glass stress.

4. Bimetallic / alkaline contact. Aluminium in contact with steel, or cast into concrete/mortar, corrodes — isolate with gaskets/coatings.

5. No finish specified. Bare aluminium pits in coastal/polluted air — specify the anodising class (IS 1868) or powder-coat system; finish is a durability requirement, not cosmetics.

• IS 1948 / IS 1949 — aluminium windows / doors (products from these sections)
• IS 737 — wrought aluminium and aluminium alloy sheet and strip
• IS 740 — wrought aluminium alloy castings · IS 5 — colours (finish)
• IS 1868 — anodic coatings on aluminium · IS 1079/relevant — material handling
• IS 875 Part 3 — wind loads on glazing/cladding · IS 800 — structural backup steel
• IS 4351 / IS 1948 — door/window framing context

IS 733 is reaffirmed and remains the procurement reference for extruded architectural aluminium, with the alloy/temper system broadly aligned to the international 6xxx designations system suppliers quote. The page/database title for this entry appears mis-described (it lists plate/sheet/strip) — for flat product use IS 737; this note covers IS 733's true subject, extruded bars, rods and sections, and the discrepancy is flagged for correction.

The engineering message for façade and architectural work is consistent: aluminium design is governed by stiffness and movement, not yield strength. Specify alloy + temper + finish explicitly, size sections on the deflection limit for the glazing system, detail generous thermal movement, and electrically/chemically isolate aluminium from steel, concrete and mortar. Façade failures attributed to 'weak sections' are almost always deflection, thermal movement or galvanic-corrosion detailing errors — not the alloy.