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Structural Steel Design

Design of steel joist metal roof

Introduction

Open web steel joists with metal decks is a very simply and economic way for roof construction.  Example are as follows.

Metal roof deck

Metal roof decks are made from light gage cold form metal sheets.  There are wide rib, intermediate rib, and narrow rib as shown below.  The height of rib is normally 1.5 inches.  Other types are also available depended on manufacturer.  Width of deck is normally 36 inches.  The sheet is normally from 18 to 22 gages. Material is normally ASTM A611 with a minimum yield strength of 33 ksi.

Steel Deck Institute (SDI) is the organization that regulates manufacturing of metal deck.

Vulcraft Online catalog: Vulcraft steel roof and floor deck catalog

Section properties are as follows:

Gage

Thick.

(Inch)

Wide Rib

Intermediate Rib

Narrow Rib

I (in4/ft)

Sp

(in3/ft)

Sn

(in3/ft)

I (in4/ft)

Sp

(in3/ft)

Sn

(in3/ft)

I (in4/ft)

Sp

(in3/ft)

Sn

(in3/ft)

22

0.0295

0.169

0.186

0.192

0.121

0.112

0.121

0.112

0.098

0.106

21

0.0325

0.192

0.213

0.221

0.137

0.127

0.135

0.127

0.111

0.119

20

0.0358

0.212

0.234

0.247

0.151

0.139

0.148

0.122

0.122

0.130

19

0.0418

0.0253

0.277

0.289

0.180

0.166

0.172

0.145

0.145

0.152

18

0.0474

0.292

0.318

0.327

0.207

0.190

0.195

0.167

0.167

0.172

Allowable stress of 20 ksi (0.6*Fy) is normally used for design of roof deck.  Live load deflections are normally limited to 1/240. Load tables are available through venders such as Vulcraft.  Examples are shown below:

Wide Rib

Max. SDI const. Span

Allowable total load (Dead + Live) (psf)

No of span

Deck

gage

Span (C. to C. of support)

5’-0”

5’6”

6’-0”

6’-6”

7’-0”

1

22

5’-7”

91

71

57

47

40

20

6’-5”

115

89

71

58

48

18

7’-8”

162

124

98

79

65

2

22

6’-11”

102

85

71

61

52

20

7’-9”

132

109

91

78

67

18

9’-1”

174

144

121

103

89

3

22

6’-11”

128

106

89

76

65

20

7’-9”

165

136

114

97

84

18

9’-1”

218

180

151

129

111

 

Intermediate Rib

Max. SDI const. Span

Allowable total load (Dead + Live) (psf)

No of span

Deck

gage

Span (C. to C. of support)

5’-0”

5’6”

6’-0”

6’-6”

7’-0”

1

22

4’-9”

60

49

41

35

30

20

5’-5”

74

61

51

44

38

18

6’-5”

101

84

70

59

49

2

22

5’-10”

65

53

45

38

33

20

6’-6”

79

65

55

47

40

18

7’-8”

104

86

72

62

53

3

22

5’-10”

81

67

56

48

41

20

6’-6”

99

82

69

58

50

18

7’-8”

130

107

90

77

66

Metal decks are fastened to steel joists with puddle welds or self-tapping screws.  Normal fastener layout patterns are as follows:

The decks are normally overlapped at their sides and are attached together with self-tapping screws. The size and pattern of fasteners to joists and number of side lap fasteners affect the capacity of roof to resist uplift from wind and capacity to transfer wind force through roof diaphragm.

 

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