WIND LOADS ON ROOFS DESIGN AND CALCULATIONS BASIC AND TUTORIALS

WIND LOADS ON ROOFS DESIGN AND CALCULATIONS BASIC INFORMATION
Wind Load Calculations On Roofs For Design


Wind Loads Calculation
Wind loads are randomly applied dynamic loads. The intensity of the wind pressure on the surface of a structure depends on wind velocity, air density, orientation of the structure, area of contact surface, and shape of the structure.

Because of the complexity involved in defining both the dynamic wind load and the behavior of an indeterminate steel structure when subjected to wind loads, the design criteria adopted by building codes and standards have been based on the application of an equivalent static wind pressure.

This equivalent static design wind pressure p (psf) is defined in a general sense by
p = qGCp (9.136)

where q velocity pressure, psf
G gust response factor to account for fluctuations in wind speed
Cp pressure coefficient or shape factor that reflects the influence of the wind on the various parts of a structure

Velocity pressure is computed from
qz = 0.00256 KzKztKdV^2I (9.137)


where Kz velocity exposure coefficient evaluated at height z
Kzt topographic factor
Kd wind directionality factor
I importance factor
V basic wind speed corresponding to a 3-s gust speed at 33 ft above
the ground in exposure C


Velocity pressures due to wind to be used in building design vary with type of terrain, distance above ground level, importance of building, likelihood of hurricanes, and basic wind speed recorded near the building site.

The wind pressures are assumed to act horizontally on the building area projected on a vertical plane normal to the wind direction.

ASCE 7 permits the use of either Method I or Method II to define the design wind loads. Method I is a simplified procedure and may be used for enclosed or partially enclosed buildings.

ASCE 7 Method II is a rigorous computation procedure that accounts for the external, and internal pressure variation as well as gust effects. The following is the general equation for computing the design wind pressure, p:
p = qGCp qi(GCpt) (9.138)

where q and qi velocity pressure as given by ASCE 7
G gust effect factor as given by ASCE 7
Cp external pressure coefficient as given by ASCE 7
GCpt internal pressure coefficient as given by ASCE 7

Codes and standards may present the gust factors and pressure coefficients in different formats. Coefficients from different codes and standards should not be mixed.

SNOW LOADS ON ROOFS DESIGN AND CALCULATIONS BASIC AND TUTORIALS

SNOW LOADS ON ROOFS DESIGN AND CALCULATIONS BASIC INFORMATION
What Are Snow Loads On Roofs Design And Calculations


Snow Loads Calculation
Determination of designing snow loads for roofs is often based on the maximum ground snow load in 50-year mean recurrence period (2% probability of being exceeded in any year).

This load or data for computing it from an extreme-value statistical analysis of weather records of snow on the ground may be obtained from the local building code or the National Weather Service.

Photo courtesy of Ask The Builder

Some building codes and ASCE 7-95 specify an equation that takes into account the consequences of a structural failure in view of the end use of the building to be constructed and the wind exposure of the roof:

pf = 0.7CeCtIpg (9.134)

where Ce wind exposure factor (range 0.8 to 1.3)
Ct thermal effects factor (range 1.0 to 1.2)
I importance factor for end use (range 0.8 to 1.2)
pf roof snow load, lb per ft2
pg ground snow load for 50-year recurrence period, lb per ft2

The “Low-Rise Building systems Manual,” Metal Building Manufacturers Association, Cleveland, Ohio, based on a modified form of ASCE 7, recommends that the design of roof snow load be determined from

pf = IsCpg(9.135)
where Is is an importance factor and C reflects the roof type.