roof live loads

Metal Building Systems Manual I. Load Application

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1.3.2 Roof Live Load Example

This example demonstrates calculations for a typical roof live load for a given building.

A. Given:

Building Length: 100 feet Bay Spacing: 5 bays @ 20'-0" Frame Type: 4 spans @ 25'-0" multi-span rigid frame Roof Slope: 1:12 Purlin Spacing: 5'-0"

B. Purlins:

Tributary Loaded Area = 5' 20' = 100 sq. ft. Uniform Roof Live Load = 20 5' = 100 plf.

1.) Alternate Span Loading:

Case 1:

Case 2:

100 plf 100 plf 100 plf

End EndInterior

100 plf 100 plf

End End Interior

I. Load Application Metal Building Systems Manual

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2.) Adjacent Span Loading:

Case 1:

Case 2:

Case 3:

Case 4:

100 plf

End EndInterior

100 plf

End End Interior

100 plf

End EndInterior

100 plf

End End Interior


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C. Frames:

Tributary Loaded Area = 25' 20' = 500 sq. ft. < 600 sq. ft. Roof Live Load from Table 1.3(a) = 20 (1.2 - 0.001 500) = 14 psf Uniform Roof Live Load = 14 20' = 280 plf.

2.) Alternate Span Loading:

Case 1:

Case 2:

2.) Adjacent Span Loading:

Case 1:

280 plf 280 plf

End End Interior

280 plf

End EndInterior

280 plf 280 plf

End End Interior


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Case 2:

Case 3:

1.3.3 Minimum Roof Live Loads

Minimum roof live loads are specified in IBC 2000, Section 1607.11.2. Note that Table 1.3(a), in this Manual, provides a summary of the specified roof live loads in a format that is more easily programmed.

Section 1607.11.2.5 specifies a minimum live load on overhanging eaves as follows:

Overhanging Eaves In other than occupancies in Group R-3 (single

family or duplex residences), and except where the overhang framing is a

continuation of the roof framing, overhanging eaves, cornices and other

roof projections shall be designed for a minimum uniformly distributed

live load of 60 psf. Note that this provision in IBC 2000 was deleted as part of the 2000 ICC Code Development Cycle because it is redundant with ASCE 7-98, Section 7.4.5, summarized in Section 1.5.6 of this Manual for Ice Dams and Icicles Along Eaves.

280 plf

End End Interior

280 plf

End End Interior


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Table 1.3(a) Roof Live Loads

Roof Slope,

F:12

Tributary Loaded Area (At) in Square Feet for any

Structural Member

At 200 200 < At < 600 At 600

F 4 20 20(1.2-0.001At) 12

4 < F < 12 20(1.2-0.05F) 20(1.2-0.001At)(1.2-0.05F) 12 12

F 12 12 12 12

1.4 Wind Loads

In this section, the wind load requirements of IBC 2000 are summarized and examples are provided for the application of wind loads on metal buildings. IBC 2000, Section 1609.1.1, requires wind loads to be determined using the provisions of ASCE 7-98, Section 6. Alternately, simplified provisions in Section 1609.6 may be used provided the building meets the criteria used to derive the simplified provisions from ASCE 7-98. For this Manual, the simplified provisions are too restrictive with regard to building configurations, so the provisions of ASCE 7-98 are provided in a form more easily applied to a wider variety of buildings and roof types.

ASCE 7-98 specifies three methods for determining wind loads, (1) Simplified Procedure, (2) Analytical Procedure, or (3) Wind Tunnel Procedure. The simplified procedure in ASCE 7-98 is more restrictive than the simplified procedure in IBC 2000, therefore, the procedures provided in this Manual comply with the analytical procedure.

The procedures summarized in this section are applicable to buildings with gable

roofs up to 45, single sloped roofs up to 30, stepped roofs, multispan gable roofs, and sawtooth roofs. The mean roof height is assumed not to exceed 60 feet and the eave heights must be less than or equal to the building least horizontal dimension. Velocity pressure tables are provided for Exposures B and C. The procedures are intended for completed buildings and may not be appropriate for structures during erection. For any other conditions, refer to ASCE 7-98.

This summary of ASCE 7-98 wind loads also assumes that the building is not subject to topographic effects as defined in ASCE 7-98. It is pointed out in the design


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procedure where a modification would be applied, but the user is referred to ASCE 7-98 for the determination of the appropriate factor.

The minimum design load for the main wind force resisting system is stipulated in Section 6.1.4.1 of ASCE 7-98 as follows:

The wind load to be used in the design of the main wind force resisting system

for an enclosed or partially enclosed building or other structure shall not be

less than 10 psf multiplied by the area of the building or projected onto a

vertical plane normal to the assumed wind direction. The design wind force for

open buildings shall not be less than 10 psf multiplied by the area Af.

The minimum design load for components and cladding is stipulated in Section 6.1.4.2 of ASCE 7-98 as follows:

The design wind pressure for components and cladding of buildings shall be not

less than a net pressure of 10 psf acting in either direction normal to the

surface.

1.4.1 Velocity Pressure

The velocity pressure, qh, used to compute the design wind pressures is calculated according to the following procedure:

1. Select Basic Wind Speed, V, for building location (See ASCE 7-98,

Figure 6-1 or IBC 2000, Figure 1609). [Note: See Section IX of this Manual for a county listing of the basic wind speed.]

2. Select Importance Factor, Iw (See Table 1.1a) 3. Select Exposure Category (A, B, C, or D - See Definitions, Section

1.4.4)4. Compute the Velocity Pressure, qh, based on the mean roof height (or

eave height if 10). See Table 1.4.1(a) and 1.4.1(b) for tabulated values of qh for Exposure B and C, respectively.


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1.4.2 Design Pressure Main Wind Force Resisting System

The design wind pressure used for the main wind force resisting system (MWFRS) is computed as follows:

Select the Enclosure Classification (Enclosed, Partially Enclosed, or Open - See Definitions, Section 1.4.4)

Select the appropriate External Pressure Coefficient GCpf from Figure 6-4 in ASCE 7-98, and the appropriate Internal Pressure Coefficient GCpi from Table 6-7 in ASCE 7-98. Alternately, Tables 1.4.5(a) and 1.4.5(b) in this Manual provide combined external and internal pressure coefficients, [(GCpf) - (GCpi)].

Compute the design pressure using the following equation:

p = qh[(GCpf) - (GCpi)] (Eq. 1.4.2)

where,p = Design wind pressure in pounds per square foot (psf). qh = Velocity pressure in pounds per square foot (psf). GCpf = External pressure coefficient from Figure 6-4, ASCE

7-98.GCpi = Internal pressure coefficient from Table 6-7, ASCE 7-

98

1.4.3 Design Pressure Components and Cladding

The design wind pressure used for components and cladding is computed as follows:

Select the appropriate External Pressure Coefficient GCp from Figures 6-5 through 6-7 in ASCE 7-98, and the appropriate Internal Pressure Coefficient GCpi from Table 6-7 in ASCE 7-98. Alternately, Tables 1.4.6(a) through 1.4.6(h) in this Manual provide convenient equations for the combined external and internal pressure coefficients, [(GCp) - (GCpi)].

Compute the design pressure using the following equation:

p = qh[(GCp) - (GCpi)] (Eq. 1.4.3)

where,p = Design wind pressure in pounds per square foot (psf). qh = Velocity pressure in pounds per square foot (psf). GCp = External pressure coefficient from Figures 6-5

through 6-7, ASCE 7-98. GCpi = Internal pressure coefficient from Table 6-7, ASCE 7-

98

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