Structural sections

Hollow Sections

Basic venting and draining rules for hollow sections

Size of holes

  • Holes shall be appropriately sized for the size of the section to be galvanized. See Table 1 to Table 3 for the minimum recommendations for standard hollow sections.
  • Vent holes shall be at least 10mm in diameter or the same thickness as the steel section.
  • The length of the hollow section should also be taken into consideration for the required hole size. The hollow vessel rule (Table 4) may need to be applied for some longer lengths or larger volumes.
  • Large hollow vessels require a vent and drain hole for every 0.5m3 of enclosed volume, each being a minimum of 50mm in diameter. See Table 4.
  • Hollow sections (pipe/CHS, RHS and SHS) require vent and drain holes, each with a diameter equivalent to at least 25% of their diagonal cross section length or multiple holes (for both venting and draining) of equivalent minimum cross sectional area. 
  • The preferred design option (Open Ends Preferred A in Figure 5) is to leave the ends of hollow sections completely open. This will improve the aesthetic quality of the fabrication.
  • Where open ends cannot be provided, the provision of at least two holes positioned opposite each other. (as shown in Figure 7 and the Preferred B option in Figure 8)
  • Holes located in the centre of a hollow section, as shown in Figure 7 and 8, are unsatisfactory.

Location of holes

  • Vent and drain holes shall be located as close as possible to the high and low points of the hollow section when hung to prevent air locks, entrapment of pre-treatment solutions and zinc pooling as well as being oriented in the same plane as the fabrication (Figure 7).
  • Holes must not be located in the centre of end plates and connections. This will cause cleaning fluids to be trapped and result in uncoated surfaces inside the plate or connection as well as potential ‘blowouts’ where the cleaning fluids are expelled from the hole under pressure creating bare spots on the finished article. On withdrawal from the galvanizing bath, centre located holes will trap zinc (Figure 7).
  • Hollow sections connected together require external vent and drain holes as close to the connection as possible. Internal venting may also be used to ensure pre-treatment solutions and zinc can flow freely through the sections and steam generated from any liquids remaining inside the sections can be efficiently vented. For more information on internal venting, see ‘Using Hollow Section’.
Hole size to be a minimum of 25% of the internal diagonal

Figure 6 – Diagonal Cross Sections

Figure 7 – Hollow Section Ends – Inside View

Figure 8 – End Plates for Hollow Sections

External Venting

External venting is one method of detailing that eliminates the need for internal vent holes, which can complicate fabrication and be difficult to verify by galvanizers.

External venting also allows the main chord member to remain unaltered and can
speed up dip times, thus improve coating quality.

This form of venting requires vent holes to be placed as close as possible to the joint between hollow sections. Vent holes should be placed on the opposite end and opposing face to avoid air pockets (Figure DN3). 

Holes should be located close to the corners and not placed on faces where excess zinc will drain out onto other surfaces leaving zinc runs and drips that lower the quality of the coating appearance (Figure DN4). 

The example in Figure 5 shows two drain holes on the outside face at the lowest point on the perpendicular steel member. More than one hole may be required for larger cross section areas (Figure DN6).
The total area of these holes must equal to the area of a hole with a diameter of at least 25% of the diagonal cross-section of that joint.

External Notch Venting

Notches and cut-outs provide the advantage of quicker fabrication time, especially with laser or plasma cut sections. They also provide improved zinc draining and are particularly useful for CHS joints as they completely avoid zinc traps and air pockets that may occur on the internal edges of the chord member intersections (Figure 8).

This form of detailing also eliminates the need for internal venting and allows for easy verification of drainage by galvanizers. 

To attain a quality coating finish and avoid zinc drips and runs, notches are encouraged on the external face of the hollow members and not in the same plane as chord members (Figure 10). Consult with your galvanizer about adequate notch sizing for best coating finish.

Internal Venting

Some galvanizers try to avoid internal venting for safety reasons, as it is not always possible to verify that it has been completed, as well as being time consuming. 

However, if required, the best practice for internal venting is the complete cut out of the internal section of the joint. Inspection holes of at least 10 mm in diameter are required when it is otherwise not possible to verify that the internal venting is in place (Figure 13).

If it is not possible to cut out the complete internal cross section, venting in the form of multiple vent holes is adequate as long as the total area of those holes amounts to the area of a circle with a diameter of 50% of the diagonal cross-section of the joint Figure 12.

For practical purposes, it is best to round up the size of the vent hole to the nearest mm relative to the fabrication tool you are using.

A list of pre-calculated internal vent hole size requirements for common hollow section sizes can be found below.

Figure DN1: Opposing holes at each end are required to allow zinc and air to flow in and out of the section.
Figure DN2: External vent holes should be fabricated close to the joint.
Figure DN4: Zinc slowly draining out of this small hole has solidified on the adjacent surface resulting in a rough lumpy surface.
Figure DN6: Many external vents are used due to the large cross section area of the joint.
Figure DN10: External venting in the same plane as the chord member is discouraged and best practice is to vent on an external face so that zinc will flow away from the fabrication.

Figure DN3: Zinc drain holes on the lower most point of the hollow section connection must have equivalent air vent holes on the opposing end of that same hollow section.

Note: Sharp notches and cut outs should be avoided as they can lead to higher stress concentrations.

Figure DN9: Notch venting allows the complete evacuation of galvanizing pre-treatment fluids and zinc, thus eliminating zinc traps which are common on externally vented CHS joints.

Figure DN14: An internal vent hole as seen through an inspection hole.

Figure DN12: Cross section view of internal venting. 

Figure DN15: Inspection hole to allow checking of internal venting.

Figure DN16: Internal Vent Holes – Drawings to clearly show items are “Internally Vented”.

Table 1: Standard Holes Sizes for CHS/Pipe

NB

Outside Diameter
(mm)

1 Hole
Ø (mm)

2 Holes
Ø (mm)

4 Holes
Ø (mm)

20

26.9

10

10

10

25

33.7

10

10

10

32

42.4

11

10

10

40

48.3

12

10

10

50

60.3

15

11

10

65

76.1

19

13

10

80

88.9

22

16

11

90

101.6

25

18

13

100

114.4

30

20

14

125

139.7

35

25

17

150

165.1

45

30

22

 

168.3

45

30

22

 

219.1

55

40

30

 

273.1

70

50

35

 

323.9

85

60

40

 

355.6

90

65

45

 

406.4

105

75

55

 

457

115

85

60

 

508

130

90

65

Table 2: Standard Holes Sizes for SHS

A x B
(mm)

1 Hole
Ø (mm)

2 Holes
Ø (mm)

4 Holes
Ø (mm)

20 x 20

10

10

10

25 x 25

10

10

10

30 x 30

11

10

10

35 x 35

12

10

10

40 x 40

14

10

10

50 x 50

18

13

10

65 x 65

25

16

11

75 x 75

25

19

13

89 x 89

35

22

16

90 x 90

35

25

16

100 x 100

35

25

18

125 x 125

45

35

22

150 x 150

55

40

30

200 x 200

75

50

35

250 x 250

90

65

45

300 x 300

110

75

55

350 x 350

125

90

65

400 x 400

145

100

75

Table 3: Standard Holes Sizes for RHS

A x B
(mm)

1 Hole
Ø (mm)

2 Holes
Ø (mm)

4 Holes
Ø (mm)

50 x 25

14

10

10

65 x 35

18

13

10

75 x 25

20

14

10

75 x 50

25

16

11

100 x 50

30

20

14

125 x 75

40

30

18

150 x 50

40

30

20

150 x 100

45

35

25

200 x 100

60

40

30

250 x 150

75

55

40

300 x 200

90

65

45

350 x 250

110

80

55

400 x 200

115

80

60

400 x 300

125

90

65

Note: ‘1 hole’, ‘2 holes’ and ‘4 holes’ means the number of holes in each otherwise unopen end. 

Hot Rolled Sections

  • End plates, gussets and stiffeners all restrict drainage. Vent and drain holes or openings (e.g. snipes) need to be provided in the corners of connected structural member (Figure 9 and Figure 10). 
  • The corners of end plates, gussets and stiffeners can also be cropped (referred to in industry as ‘snipes’) to allow for venting and draining (Figure 9 and Figure 10). 
  • See the Snipe Guide (Figure 11) for the recommended snipe sizes for various structural members. 

Figure 9 – Hot Rolled Profiles – Typical Designs

Figure 10 – End Plate Options

Figure 11 – Snipe Guide

Figure 12 – Snipe Gussets

Figure 13 – Design Orientation Options