Venting and Drainage Hole Sizing Alternatives for Rectangular and Square Hollow Structural Sections

The Hollow Section Coating Challenge

Rectangular and square hollow structural sections (HSS) present unique challenges for hot-dip galvanizing due to their enclosed geometry creating internal volumes requiring complete chemical cleaning solution access, thorough rinsing, and molten zinc penetration for uniform interior coating coverage. Unlike open structural shapes where all surfaces remain directly accessible to processing chemicals and zinc bath, hollow sections depend entirely on adequate venting and drainage openings enabling fluid entry, circulation, and exit. Insufficient venting or drainage creates trapped air pockets preventing zinc access to interior surfaces, entraps acidic cleaning solutions causing post-galvanizing corrosion, and generates dangerous steam pressure buildup risking explosive failure during zinc bath immersion—creating quality defects, structural damage, and severe safety hazards for galvanizing personnel.

ASTM A385 establishes comprehensive venting and drainage requirements for hollow structural products, but standard specifications primarily address horizontal end plates requiring substantial open area through multiple large holes or extensive corner clips. For vertical HSS members, columns, or applications where designers prefer minimizing end plate penetrations, alternative venting approaches using strategically positioned smaller holes can achieve equivalent drainage effectiveness while preserving structural integrity and aesthetic preferences.

Why Venting and Drainage Matter

Process Chemistry Requirements

Hot-dip galvanizing involves sequential immersion in multiple liquid baths:

Caustic Cleaning (Optional): Alkaline degreasing removes oils, greases, and organic contaminants at 140-180°F

Acid Pickling: Hydrochloric or sulfuric acid dissolves mill scale and rust at ambient to 140°F

• Typical concentration: 10-18% HCl or 5-10% H₂SO₄
• Immersion time: 15-60 minutes depending on scale thickness

Rinsing: Clean water removes residual acid and dissolved iron salts

Fluxing: Zinc ammonium chloride solution coats steel protecting from oxidation and promoting zinc wetting at 140-160°F

Galvanizing: Molten zinc bath immersion at 820-860°F creating metallurgical zinc-iron bond

Each Step Requires:

• Complete solution entry into hollow sections
• Circulation contacting all interior surfaces
• Drainage preventing solution entrapment
• Air displacement allowing liquid penetration

Safety Considerations

Steam Explosion Hazard:

Trapped water or cleaning solution inside sealed hollow sections creates catastrophic failure risk:

Mechanism:

1. Water trapped in sealed cavity
2. Article immersed in 840-860°F molten zinc
3. Trapped water rapidly converts to steam
4. Pressure buildup exceeds structural strength
5. Violent rupture expels molten zinc

Consequence:

• Severe burn injuries to personnel
• Zinc bath contamination and spillage
• Equipment damage
• Facility evacuation

Prevention: Adequate drainage holes ensure all liquids exit before zinc immersion.

Air Entrapment:

Sealed cavities trap air preventing zinc entry:

• Air volume cannot escape as zinc enters
• Pressure equalizes preventing full zinc penetration
• Interior surfaces remain uncoated

Solution: Vent holes at high points allow air release as zinc enters from below.

Coating Quality Requirements

Interior Surface Protection:

While exterior HSS surfaces experience normal atmospheric exposure, interior surfaces face:

• Condensation accumulation in humid climates
• Water infiltration through connection penetrations
• Corrosive environments in some applications (chemical plants, marine structures)

Inadequate Interior Coating:

Produces:

• Internal rust development
• Premature structural failure
• Costly repairs requiring cutting access holes

Complete Interior Coating:

Achieved only when:

• Venting allows air displacement
• Drainage prevents solution entrapment
• Zinc fully penetrates all internal spaces

ASTM A385 Standard Requirements

Standard Overview

ASTM A385: "Standard Practice for Providing High-Quality Zinc Coatings (Hot-Dip)"

Scope: Comprehensive guidance on design, fabrication, and galvanizing practices for optimal coating quality including extensive venting and drainage provisions.

Paragraph 12.4: Specifically addresses hollow structural sections with end plates.

Basic Requirement: Fully Open Ends

Optimal Design:

Fully open HSS ends provide:

• Unrestricted solution flow
• Complete interior access
• Minimal fluid resistance
• Maximum drainage effectiveness
• No air entrapment risk

Application:

Suitable when structural requirements permit:

• Columns with base plates below open tube end
• Beams with open ends at connections
• Cantilever applications

When Not Feasible:

Many applications require closed end plates:

• Finished appearance requirements
• Connection detail requirements
• Structural load transfer
• Environmental sealing
• Debris exclusion

Solution: Incorporate adequate vent/drain holes in end plates.

ASTM A385 Standard Hole Configurations

Box Sections with End Plates (Table 1 Requirements)

For square or nearly-square box sections, ASTM A385 Figure 9 specifies:

Hole Diameter and Corner Clip Options:

Interpretation:

"H+W" Dimension: Height plus width of box section

• 12×12" box: H+W = 24"
• 10×8" box: H+W = 18"

Hole OR Clip: Designer chooses either:

• Circular holes of specified diameter (typically two diagonal corners)
• Clipped corners of specified length (typically all four corners)

Rationale:

Large openings provide:

• Substantial drainage area (typically 30-50% of cross-section)
• Rapid solution exchange
• Reliable air venting
• Suitable for horizontal end plate orientation

Rectangular Tube Trusses (Table 2 Requirements)

For rectangular tubular products with horizontal end plates, ASTM A385 Figure 11 specifies area-based requirements:

Open Area as Percentage of Cross-Section:

Example Calculation:

10×6" rectangular tube:

• H+W = 16"
• Falls in 16" ≤ H+W < 24" range
• Required area: 0.30 × 10 × 6 = 18 in²

Design Options:

• One 5" diameter hole (19.6 in²) ✓
• Two 3.5" diameter holes (19.2 in²) ✓
• Four 2.4" diameter holes (18.1 in²) ✓
• Corner clips totaling 18+ in² ✓

Limitations of Standard Requirements for Vertical Members

Horizontal vs. Vertical Orientation

ASTM A385 Tables Assume: Horizontal end plate orientation during galvanizing:

• One end elevated (vent)
• Opposite end lowered (drain)
• Gravity-driven drainage flow

Large Hole/Clip Requirements:

Appropriate for horizontal mounting but create concerns for vertical applications:

Structural Concerns:

• Large holes (4-8" diameter) significantly reduce end plate effective area
• Load transfer capacity reduced
• Connection bolting patterns compromised

Aesthetic Concerns:

• Large visible holes on architectural columns
• Appearance unacceptable for exposed applications

Design Preference:

Vertical HSS column applications often prefer:

• Smaller, less conspicuous holes
• Multiple small holes versus single large opening
• Corner positioning minimizing visual impact

Alternative Venting Configurations

Minimum Single Opening Requirement

ASTM A385 Paragraph 12.4 Alternative:

"A minimum single vent/drain opening of 25% to 30% of the cross-sectional area, in a corner location."

Application:

For vertical members where single opening is acceptable:

• One hole at top corner (vent)
• One hole at bottom diagonal corner (drain)
• Each hole = 25-30% of cross-section area

Example:

6×6" square HSS:

• Cross-section: 36 in²
• Single hole requirement: 0.25 × 36 = 9 in²
• Hole diameter: √(9 × 4/π) = 3.4" minimum

Two-Hole Alternative Sizing

For applications preferring two holes (top vent, bottom drain):

Square HSS Two-Hole Table:

Rectangular HSS Two-Hole Table:

Placement:

• One hole at top corner (vent function)
• One hole at bottom diagonal corner (drain function)

Four-Hole Alternative Sizing

For applications preferring distributed smaller holes:

Square HSS Four-Hole Table:

Rectangular HSS Four-Hole Table:

Placement:

• Two holes at top corners (vent function)
• Two holes at bottom corners (drain function)

Advantage:

Four smaller holes:

• Less structural impact than two larger holes
• Better distributed around perimeter
• Improved appearance (smaller openings)
• Maintain adequate total vent/drain area

Hole Placement Critical Factors

Orientation During Galvanizing

Immersion Angle:

Articles enter zinc bath at angle (typically 15-45° from horizontal) enabling:

• Air escape from high point as zinc enters
• Zinc flows to low point
• Drainage from low point during withdrawal

High Point = Vent Location:

Air trapped at highest point requires vent hole enabling escape:

• Without vent: Air pressure prevents zinc entry
• With vent: Air releases, zinc penetrates completely

Low Point = Drain Location:

Liquid accumulates at lowest point requiring drain hole:

• Without drain: Solutions trapped after withdrawal
• With drain: Gravity drainage empties cavity

Communication with Galvanizer

Critical Pre-Galvanizing Discussion:

Designer must communicate with galvanizer regarding:

Lifting Orientation: How will fabrication be rigged for immersion?

• Vertical (end plates horizontal)
• Horizontal (end plates vertical)
• Angled

High/Low Point Identification:

Based on lifting orientation:

• Which corners become high points (vent holes needed)
• Which corners become low points (drain holes needed)

Example:

10×10" square HSS column:

Vertical Lifting (End Plates Horizontal):

• Top end plate: Vent holes at two diagonal corners
• Bottom end plate: Drain holes at two diagonal corners

Horizontal Lifting (End Plates Vertical):

• Both end plates: Two holes each (one high corner = vent, one low corner = drain)

Design Coordination:

Hole locations specified by designer must align with actual galvanizing orientation, requiring galvanizer input during design phase.

Internal Stiffeners and Baffles

Compartmentalization Concerns

Internal Stiffeners:

HSS members with internal stiffener plates create separate compartments:

• Each compartment requires independent venting and drainage
• Single end plate holes insufficient if internal baffles block flow

Example:

12" square HSS with internal cruciform stiffener:

• Creates four separate triangular compartments
• Each compartment needs vent and drain access
• Requires either:
  
  • Holes in stiffener plates enabling inter-compartment flow
  • Four vent holes and four drain holes in end plates aligning with each compartment

Solution:

Stiffener Perforation: Cut 2-3" diameter holes in internal stiffeners enabling solution circulation between compartments

Alternative: Additional end plate holes ensuring each compartment has direct vent/drain access

Special Considerations

Length Effects

Extended Length Members:

Long HSS sections (>20 feet) may benefit from:

• Larger hole sizes than tables specify
• Intermediate vent/drain holes along length
• Pre-heating to reduce thermal shock

Rationale:

Longer flow paths increase:

• Solution entry/exit time
• Risk of incomplete drainage
• Trapped air pockets

Larger or additional holes compensate.

Connection Details

Base Plate Applications:

HSS columns with base plates:

Open-Ended Design Preferred:

• Base plate welded below HSS tube end
• Tube remains fully open above base plate
• Eliminates need for base plate holes

Closed End Design: If tube end must be capped:

• Apply hole sizing tables to cap plate
• Position drain holes at expected low points

Post-Galvanizing Hole Sealing

Aesthetic Requirements:

Some applications require hole sealing after galvanizing:

Methods:

• Threaded plugs (steel or aluminum)
• Pressed-fit plugs (zinc or aluminum)
• Welded patch plates with zinc-rich paint touch-up

Consideration:

Specify plugging in design documents preventing field coordination issues.

Quality Verification

Pre-Galvanizing Inspection

Fabricator Verification:

Before shipping to galvanizer:

• Verify all specified holes drilled
• Confirm hole sizes match specifications
• Check hole positioning relative to planned lifting orientation

Galvanizer Pre-Immersion Check:

• Inspect for adequate venting/drainage
• Identify any sealed cavities requiring additional holes
• Confirm lifting orientation aligns with hole placement

Post-Galvanizing Assessment

Interior Coating Quality:

Evaluate:

• Coating thickness on interior surfaces (through holes or cut sections)
• Uniformity of coverage
• Absence of bare spots indicating incomplete zinc penetration

Drainage Verification:

Confirm:

• No liquid retention audible when tipping article
• No staining indicating trapped acid
• Dry interior surfaces

Specification Language

Drawing Notes

Recommended Specification:

"Venting and drainage holes for hollow structural sections shall comply with ASTM A385 or approved alternatives. Hole sizes per [specify: Table 3 for SHS / Table 4 for RHS]. Hole locations shall be coordinated with galvanizer based on planned lifting orientation to ensure high-point venting and low-point drainage."

Design-Build Coordination

Alternative Approach:

"HSS venting and drainage details shall be coordinated with galvanizer and documented on shop drawings. Minimum vent/drain hole area shall be 25-30% of HSS cross-sectional area positioned to enable air release and liquid drainage based on galvanizing orientation."

Rectangular and square hollow structural sections with end plates require adequate venting and drainage openings ensuring complete interior surface access to chemical cleaning solutions and molten zinc while preventing dangerous trapped liquid conditions that create steam explosion hazards during galvanizing immersion. ASTM A385 standard requirements specify large holes (3-8" diameter) or extensive corner clips appropriate for horizontal end plate orientations but often impractical for vertical HSS columns where structural load transfer, connection details, and aesthetic concerns favor smaller, less conspicuous openings. Alternative two-hole and four-hole sizing tables provide equivalent drainage effectiveness using strategically positioned smaller holes ranging from 3/8" to 4" diameter depending on HSS size, with two-hole configurations placing one vent at top corner and one drain at bottom diagonal corner, while four-hole configurations distribute two vents at top corners and two drains at bottom corners providing improved structural performance and appearance. Critical success factors include pre-galvanizing coordination with galvanizers confirming lifting orientation and resulting high/low point locations, proper hole placement aligning with actual immersion geometry rather than theoretical orientation, verification that internal stiffeners don't create isolated compartments lacking vent/drain access, and documentation through clear specification language establishing either prescriptive hole requirements or performance-based coordination protocols. The fundamental principle governing all venting approaches is ensuring each enclosed volume has unobstructed air escape path at highest point during immersion and gravity drainage path at lowest point during withdrawal, with hole area totaling minimum 25-30% of cross-sectional area providing reliable performance across diverse HSS sizes and galvanizing orientations. Read the original AGA resource on this topic here.