Member Stability [ver 1.1 1098]

On this page: [Overview] [Lateral Stability] [Major Axis Stability]

Member Stability

Overview

Determining bending and axial section capacities requires information about major axis and minor axis support conditions. Major axis support conditions consist of distance between major axis supports and the end support conditions. These two factors are used to determine the effective length values used in the manual and shown in the table in this section. Minor axis stability can be addressed using either through-fastened provisions or by use of angle, strapping or other members which provide discrete weak axis brace points. More information about member stability is provided in this section, in the example calculations and in the capacities tables.

Lateral Stability

Through-Fastened
Requirements
The through-fastened provisions include both section requirements and applications requirements. (Reference AISI C.3.1.3) Pages I-B-2 and I-B-3 contain tables and indicators for the section requirements. Additional requirements include:

Minimum distance from the center of support to either end of the lap shall not be less than 1.5 times the section depth
Member span length no greater than 33 feet (10 m)
For continuous span systems, the longest member span shall not be more than 20% greater than the shortest span
Both flanges are prevented from moving laterally at the supports
Roof or wall panels shall be steel sheets, minimum of 0.019 in. (0.48 mm) coated thickness, having a minimum rib depth of 1 in. (25.4 mm), spaced a maximum of 12 in. (305 mm) on centers and attached in a manner to effectively inhibit relative movement between the panel and purlin flange
Insulation shall be glass fiber blanket 0 to 6 inches (152 mm) thick compressed between the member and panel in a manner consistent with the fastener being used
Fastener type: minimum No. 12 self-drilling or self-tapping sheet metal screws or 3/16 – in. (4.76 mm) rivets, washers 1/3 in. (12.7 mm) diameter
Fasteners shall not be standoff type screws
Fasteners shall be spaced not greater than 12 in. (305 mm) on centers and placed near the center of the beam flange

When requirements for through-fastened are met, lateral bracing of the inside flange may be omitted if the value for allowable total load (Load from the capacities tables) is reduced as specified:

Simple Span Capacities for C-Sections (pages II-I-1 through II-I-10)

Multiply value for Load by 0.4

Simple Span Capacities for Z-Sections (pages III-I-1 through III-I-7)

Multiply value for Load by 0.5

Continuously Braced Capacities for Z-Sections (pages III-J-1 through III-L-54)

Multiply value for Load by 0.7

I-B-1

15 October 1998

Through-Fastened Condition
The following tables list C-sections and Z-sections and indicate whether each meets the conditions of section requirements for the through-fastened provisions. (Reference AISI C3.1.3) The T-F indicator is defined as follows:

Y	Yes:	Meets section requirements
P	Partial:	At least one requirement is not met but is within 5 percent
N	No:	Does not meet section requirements

C-Sections

Section

T-F

Section

T-F

Section

T-F

12 X 4.0 C 12 G

P

9 X 2.5 C 12 G

Y

7 X 2.5 C 12 G

Y

12 X 4.0 C 13 G

P

9 X 2.5 C 13 G

Y

7 X 2.5 C 13 G

Y

12 X 4.0 C 14 G

N

9 X 2.5 C 14 G

Y

7 X 2.5 C 14 G

Y

9 X 2.5 C 15 G

Y

7 X 2.5 C 15 G

Y

12 X 3.5 C 12 G

P

9 X 2.5 C 16 G

Y

7 X 2.5 C 16 G

Y

12 X 3.5 C 13 G

P

12 X 3.5 C 14 G

P

8 X 4.0 C 12 G

N

6 X 4.0 C 12 G

N

8 X 4.0 C 13 G

N

6 X 4.0 C 13 G

N

12 X 2.5 C 12 G

N

8 X 4.0 C 14 G

N

6 X 4.0 C 14 G

N

12 X 2.5 C 13 G

N

8 X 4.0 C 15 G

N

6 X 4.0 C 15 G

N

12 X 2.5 C 14 G

N

8 X 4.0 C 16 G

N

6 X 4.0 C 16 G

N

10 X 4.0 C 12 G

N

8 X 3.5 C 12 G

N

6 X 2.5 C 12 G

N

10 X 4.0 C 13 G

N

8 X 3.5 C 13 G

N

6 X 2.5 C 13 G

N

10 X 4.0 C 14 G

N

8 X 3.5 C 14 G

N

6 X 2.5 C 14 G

N

10 X 4.0 C 15 G

N

8 X 3.5 C 15 G

N

6 X 2.5 C 15 G

N

8 X 3.5 C 16 G

N

6 X 2.5 C 16 G

N

10 X 3.5 C 12 G

P

10 X 3.5 C 13 G

P

8 X 2.5 C 12 G

Y

4 X 4.0 C 12 G

N

10 X 3.5 C 14 G

P

8 X 2.5 C 13 G

Y

4 X 4.0 C 13 G

N

10 X 3.5 C 15 G

N

8 X 2.5 C 14 G

Y

4 X 4.0 C 14 G

N

8 X 2.5 C 15 G

Y

4 X 4.0 C 15 G

N

10 X 2.5 C 12 G

Y

8 X 2.5 C 16 G

Y

4 X 4.0 C 16 G

N

10 X 2.5 C 13 G

Y

10 X 2.5 C 14 G

Y

8 X 2.0 C 12 G

N

4 X 2.5 C 12 G

N

10 X 2.5 C 15 G

Y

8 X 2.0 C 13 G

Y

4 X 2.5 C 13 G

N

8 X 2.0 C 14 G

Y

4 X 2.5 C 14 G

N

10 X 2.0 C 12G

N

8 X 2.0 C 15 G

Y

4 X 2.5 C 15 G

N

10 X 2.0 C 13G

N

8 X 2.0 C 16 G

Y

4 X 2.5 C 16 G

N

10 X 2.0 C 14G

N

10 X 2.0 C 15G

N

7 X 4.0 C 12 G

N

4 X 2.0 C 12 G

N

7 X 4.0 C 13 G

N

4 X 2.0 C 13 G

N

7 X 4.0 C 14 G

N

4 X 2.0 C 14 G

N

7 X 4.0 C 15 G

N

4 X 2.0 C 15 G

N

7 X 4.0 C 16 G

N

4 X 2.0 C 16 G

N

I-B-2

15 October 1998

Z-Sections

Section

T-F

Section

T-F

Section

T-F

12 X 3.5 Z 12 G

P

10 X 2.5 Z 12 G

P

8 X 2.5 Z 12 G

P

12 X 3.5 Z 13 G

P

10 X 2.5 Z 13 G

Y

8 X 2.5 Z 13 G

Y

12 X 3.5 Z 14 G

P

10 X 2.5 Z 14 G

Y

8 X 2.5 Z 14 G

Y

10 X 2.5 Z 15 G

Y

8 X 2.5 Z 15 G

Y

12 X 3.0 Z 12 G

P

10 X 2.5 Z 16 G

Y

8 X 2.5 Z 16 G

Y

12 X 3.0 Z 13 G

P

12 X 3.0 Z 14 G

P

9 X 2.5 Z 12 G

P

7 X 2.5 Z 12 G

P

9 X 2.5 Z 13 G

Y

7 X 2.5 Z 13 G

Y

12 X 2.5 Z 12 G

N

9 X 2.5 Z 14 G

Y

7 X 2.5 Z 14 G

Y

12 X 2.5 Z 13 G

N

9 X 2.5 Z 15 G

Y

7 X 2.5 Z 15 G

Y

12 X 2.5 Z 14 G

N

9 X 2.5 Z 16 G

Y

7 X 2.5 Z 16 G

Y

10 X 3.5 Z 12 G

Y

8 X 3.5 Z 12 G

N

6 X 2.5 Z 12 G

P

10 X 3.5 Z 13 G

Y

8 X 3.5 Z 13 G

N

6 X 2.5 Z 13 G

P

10 X 3.5 Z 14 G

Y

8 X 3.5 Z 14 G

N

6 X 2.5 Z 14 G

P

10 X 3.5 Z 15 G

Y

8 X 3.5 Z 15 G

N

6 X 2.5 Z 15 G

P

10 X 3.5 Z 16 G

N

8 X 3.5 Z 16 G

N

6 X 2.5 Z 16 G

P

10 X 3.0 Z 12 G

Y

8 X 3.0 Z 12 G

Y

4 X 2.5 Z 14 G

N

10 X 3.0 Z 13 G

Y

8 X 3.0 Z 13 G

Y

4 X 2.5 Z 15 G

N

10 X 3.0 Z 14 G

Y

8 X 3.0 Z 14 G

Y

4 X 2.5 Z 16 G

N

10 X 3.0 Z 15 G

Y

8 X 3.0 Z 15 G

Y

10 X 3.0 Z 16 G

Y

8 X 3.0 Z 16 G

Y

3.5 X 1.5 Z 14 G

N

3.5 X 1.5 Z 15 G

N

3.5 X 1.5 Z 16 G

N

I-B-3

15 October 1998

Lateral Brace Locations
Section capacities are sensitive to the distance between lateral braces (i.e., braces at the top and bottom flanges). For some panel configurations, fasteners through the panel may act as lateral braces for the flange to which they are attached, and may partially brace the opposite flange. (Reference Table of Sections Qualifying for Through-Fastened and AISI C3.1.3) Panels which attach to C-section or Z-section supporting members using clips (e.g., standing seam roof panels) should not be considered as providing lateral bracing to the supporting sections. For such panel configurations, discrete lateral braces must be provided.

The discrete lateral braces may be angle, strapping or similar members. They should be designed as specified in AISI specifications D3.2. The Lateral Braced Capacities tables provide the capacities of sections spanning multiple bays and having discrete lateral braces. The Continuously Braced Capacities and Simple Span Capacities tables assume the compression flange is continuously supported. This typically requires lateral supports as shown below.

Section	Lateral Support Distance (ft.)
12 X 3.5 Z	5.1
12 X 3.0 Z	4.4
12 X 2.5 Z	3.4
10 X 3.5 Z	5.3
10 X 3.0 Z	4.5
10 X 2.5 Z	3.7
9 X 2.5 Z	3.8
8 X 3.5 Z	5.5
8 X 3.0 Z	4.7
8 X 2.5 Z	3.9
7 X 2.5 Z	4.0
6 X 2.5 Z	4.2

I-B-4

15 October 1998

Brace Points
When determining the Laterally Braced Capacities for Z-sections, the inflection point is used as a brace point. The inflection points and associated Ly values are reported in the capacities tables. The following illustrates a case where the number of brace locations is 3. Arrows on the sketches indicate the locations of braces. The reference line for the moment curve is the bottom of the section. Ly is equal to SPAN divided by the number of brace locations (# Brace Locs). The lateral support distances for each region are computed as shown.

Region(s)	Lateral Support Distance
A,B,C,D	Minimum of DIST and Ly
E	Minimum of INFL PT R and Ly

Region(s)	Lateral Support Distance
A	Minimum of INFL PT L and Ly
B, C, D, E	Minimum of DIST and Ly
F	Minimum of INFL PT R and Ly

I-B-5

15 October 1998

Major Axis Stability

Effective Lengths
Reduced properties for the sections are calculated based on the following effective major axis buckling lengths (KLx).

Section Depth (in.)

KLx
(ft.)

12

20

10

20

9

18

8

18

7

18

6

16

4

12

3.5

12

1.5

8

1.25

8

1.125

7

1.0

7

.875

6

.75

5

.625

4

I-B-6

15 October 1998

Section	T-F	Section	T-F	Section	T-F

12 X 4.0 C 12 G	P	9 X 2.5 C 12 G	Y	7 X 2.5 C 12 G	Y
12 X 4.0 C 13 G	P	9 X 2.5 C 13 G	Y	7 X 2.5 C 13 G	Y
12 X 4.0 C 14 G	N	9 X 2.5 C 14 G	Y	7 X 2.5 C 14 G	Y
		9 X 2.5 C 15 G	Y	7 X 2.5 C 15 G	Y
12 X 3.5 C 12 G	P	9 X 2.5 C 16 G	Y	7 X 2.5 C 16 G	Y
12 X 3.5 C 13 G	P
12 X 3.5 C 14 G	P	8 X 4.0 C 12 G	N	6 X 4.0 C 12 G	N
		8 X 4.0 C 13 G	N	6 X 4.0 C 13 G	N
12 X 2.5 C 12 G	N	8 X 4.0 C 14 G	N	6 X 4.0 C 14 G	N
12 X 2.5 C 13 G	N	8 X 4.0 C 15 G	N	6 X 4.0 C 15 G	N
12 X 2.5 C 14 G	N	8 X 4.0 C 16 G	N	6 X 4.0 C 16 G	N

10 X 4.0 C 12 G	N	8 X 3.5 C 12 G	N	6 X 2.5 C 12 G	N
10 X 4.0 C 13 G	N	8 X 3.5 C 13 G	N	6 X 2.5 C 13 G	N
10 X 4.0 C 14 G	N	8 X 3.5 C 14 G	N	6 X 2.5 C 14 G	N
10 X 4.0 C 15 G	N	8 X 3.5 C 15 G	N	6 X 2.5 C 15 G	N
		8 X 3.5 C 16 G	N	6 X 2.5 C 16 G	N
10 X 3.5 C 12 G	P
10 X 3.5 C 13 G	P	8 X 2.5 C 12 G	Y	4 X 4.0 C 12 G	N
10 X 3.5 C 14 G	P	8 X 2.5 C 13 G	Y	4 X 4.0 C 13 G	N
10 X 3.5 C 15 G	N	8 X 2.5 C 14 G	Y	4 X 4.0 C 14 G	N
		8 X 2.5 C 15 G	Y	4 X 4.0 C 15 G	N
10 X 2.5 C 12 G	Y	8 X 2.5 C 16 G	Y	4 X 4.0 C 16 G	N
10 X 2.5 C 13 G	Y
10 X 2.5 C 14 G	Y	8 X 2.0 C 12 G	N	4 X 2.5 C 12 G	N
10 X 2.5 C 15 G	Y	8 X 2.0 C 13 G	Y	4 X 2.5 C 13 G	N
		8 X 2.0 C 14 G	Y	4 X 2.5 C 14 G	N
10 X 2.0 C 12G	N	8 X 2.0 C 15 G	Y	4 X 2.5 C 15 G	N
10 X 2.0 C 13G	N	8 X 2.0 C 16 G	Y	4 X 2.5 C 16 G	N
10 X 2.0 C 14G	N
10 X 2.0 C 15G	N	7 X 4.0 C 12 G	N	4 X 2.0 C 12 G	N
		7 X 4.0 C 13 G	N	4 X 2.0 C 13 G	N
		7 X 4.0 C 14 G	N	4 X 2.0 C 14 G	N
		7 X 4.0 C 15 G	N	4 X 2.0 C 15 G	N
		7 X 4.0 C 16 G	N	4 X 2.0 C 16 G	N

Section Depth (in.)	KLx (ft.)
12	20
10	20
9	18
8	18
7	18
6	16
4	12
3.5	12
1.5	8
1.25	8
1.125	7
1.0	7
.875	6
.75	5
.625	4