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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