# Square hollow sections

Structural square tubing: product standards, nominal and design wall, rounded corners, dimensional acceptance and connection limits.

Reviewed: 2026-11-01

Tags: materials, stock-forms, structural-steel, hollow-sections

## Identify more than the square outline

Atlas's **A500 HSS 5 × 5 × ⅜** lists **0.375 in nominal wall**, **0.349 in design wall** and **3.313 in workable flat**. The design wall follows **0.93 × 0.375 = 0.34875 in**, rounded. Check actual corner profile and workable face before laying out attachments. [Atlas A500 square-section table](https://www.atlastube.com/wp-content/uploads/2018/04/A500-Square-Current.pdf)

Square hollow sections (SHS), also called square HSS in North American structural practice, are hollow members with nominally equal outside side dimensions. The outline does not establish grade, manufacturing route, toughness, dimensional tolerances or fitness for a particular connection.

AISC identifies ASTM A500/A500M Grade C as a common structural-HSS specification and distinguishes A1085 from it. Those structural products should not be silently replaced by general mechanical tubing merely because the outside size matches. [AISC HSS guidance](https://www.aisc.org/aisc/solutions-center/hss/)

Tata Steel's **Celsius** is a different, named product: fully normalized hot-finished structural hollow sections to EN 10210. Keep the product route, grade and applicable specification together. An EN hot-finished profile and an ASTM cold-formed profile require comparison of their actual requirements before substitution. [Tata Celsius product](https://www.tatasteeluk.com/construction/products/tube/structural-hollow-sections/celsius)

A useful stock description contains outside width×height, nominal wall, units, length, grade/specification and supplied condition. See [Stock forms and dimensions](index.md).

## Nominal, design and measured wall are different

Steel Tube Institute's **HSS Connex User Notes**, dated November 12, 2024, instruct users checking a custom A500 shape to enter **0.93 times nominal wall** as design thickness. Its A1085 shapes use full nominal thickness. These are stated design conventions, not measurements of delivered stock and not a universal rule for every international hollow-section product. [STI design-wall notes](https://steeltubeinstitute.org/wp-content/uploads/2025/06/HSS_Connex_User_Notes_v2.1.pdf)

## Corners and usable flat faces

An SHS is not an ideal sharp-cornered square with a sharp-cornered hole removed. STI distinguishes corner assumptions used for local-slenderness calculations, workable flat dimensions and other section properties. Its technical reference gives **1.5 times design wall** for the local-buckling corner assumption, **2.25 times** for workable-flat calculations, and **2 times** for other A500/A1085 section properties. These analytical conventions should not be turned into three conflicting shop-inspection radii. [STI geometry discussion, June 2024](https://steeltubeinstitute.org/resources/filling-the-void-a-central-resource-for-designing-concrete-filled-hss-columns/)

For a plate or bracket joining a face, use the appropriate workable flat and actual connection detail. Outside width alone does not establish how wide a flat mating plate can sit before encountering the rounded corners. Internal fit needs its own clearance assessment, including corner profiles, wall variation and any weld seam condition; subtracting twice nominal wall does not guarantee a freely usable square bore.

## Bounded Atlas section example

Atlas's A500 table lists **HSS 5×5×3/8** as follows:

| Field | Published value |
| --- | --- |
| Outside height and width | 5×5 inches |
| Nominal wall | 0.375 inch |
| Design wall | 0.349 inch |
| Nominal weight | 22.37 lb/ft |
| Area | 6.18 in² |
| Second moment of area | 21.7 in⁴ |
| Elastic section modulus | 8.68 in³ |
| Radius of gyration | 1.87 inches |
| Workable flat | 3.313 inches |

These values identify a particular table row and its calculation basis; they are not a member load rating or a dimensional certificate. [Atlas A500 square-section table](https://www.atlastube.com/wp-content/uploads/2018/04/A500-Square-Current.pdf)

**Original checks:** `0.93 × 0.375 = 0.34875 inch`, rounding to the listed 0.349-inch design wall. Also, `sqrt(21.7 / 6.18) = 1.874 inches`, consistent with the rounded radius of gyration. A 12-foot length has nominal mass `12 × 22.37 = 268.44 lb`, excluding fittings and coatings. These checks reconcile the fields; they do not independently validate buckling strength or lifting arrangements.

## Tolerances need a measurement basis

Tata's Celsius square/rectangular table gives outside dimension **±1%, with ±0.5 mm minimum**, wall under-tolerance **−10%** with positive deviation constrained by mass tolerance, and side squareness **90°±1°**. It separately lists concavity/convexity, twist, straightness and mass requirements. Its external corner-profile limit is **2T maximum unless otherwise shown**, a producer-specific statement tighter than the EN 10210 limit noted on the page. [Celsius dimensional tolerances](https://www.tatasteeluk.com/construction/products/tube/structural-hollow-sections/celsius)

Keep those checks separate. A section can satisfy outside-size tolerance while requiring a separate concavity or twist check. Where a fixture needs a tighter fit than the structural stock standard provides, agree additional requirements before ordering. Do not assume cutting a long member into short blanks creates precision-machined faces or guarantees squareness at the cut ends.

## Connections and finishing can govern selection

STI's Connex notes explicitly impose connection-formula applicability limits involving HSS wall slenderness and account for existing axial/moment effects in the connecting member through a chord-stress interaction parameter. Selecting a member by area or elastic modulus alone does not check a local attachment. [STI connection limits](https://steeltubeinstitute.org/wp-content/uploads/2025/06/HSS_Connex_User_Notes_v2.1.pdf)

Resolve load introduction, connection type, weld or bolt access and any required reinforcement with the design. An apparently symmetric square profile does not make every attachment orientation equivalent: the seam, connection geometry, loading and restraint still matter. Keep member checks separate from connection checks and confirm the code edition behind any software output.

For hot-dip galvanizing, STI requires provisions for filling, venting and drainage and warns against leaving HSS sealed because of bursting risk. It also discusses corner cracking associated with cold work. Coordinate the galvanizing detail and inspection with the fabricator and galvanizer; this reference supplies no universal vent-hole layout or coating-life guarantee. [STI galvanizing guidance](https://steeltubeinstitute.org/resources/galvanizing-hss/)

## Source editions and purchasing record

The cited Connex notes print **November 12, 2024** and describe a mixed basis: AISC 360-2016/15th Edition Manual with identified later provisions, including AISC 360-22 for the chord-stress factor. The geometry reference identifies a **June 2024** technical article. Atlas's square-properties PDF and Tata's live product page show no technical revision date; upload paths and website copyright years are not editions.

Order the full section and material designation, nominal size, length/end condition, applicable tolerances, finish and inspection documents. Preserve identification when cutting stock, and confirm current availability for the exact size/wall/grade combination. Neither a square shape nor a supplier application list establishes pressure-service suitability, fatigue life, corrosion resistance or a complete structural approval.