WHAT ARE SERVICE LOADS IN STRUCTURAL ENGINEERING? BASIC CONCEPTS

In designing structural members, designers should use whichever is larger of the following:

1. Loadings specified in the local or state building code.

2. Probable maximum loads, based not only on current site conditions and original usage of proposed building spaces but also on possible future events.

Loads that are of uncertain magnitude and that may be treated as statistical variables should be selected in accordance with a specific probability that the chosen magnitudes will not be exceeded during the life of the building or in accordance with the corresponding mean recurrence interval.

The mean recurrence interval generally used for ordinary permanent buildings is 50 years. The interval, however, may be set at 25 years for structures with no occupants or offering negligible risk to life, or at 100 years for permanent buildings with a high degree of sensitivity to the loads and an unusually high degree of hazard to life and property in case of failure.

In the absence of a local or state building code, designers can be guided by loads specified in a national model building code or by the following data:

Loads applied to structural members may consist of the following, alone or in combination: dead, live, impact, earth pressure, hydrostatic pressure, snow, ice, rain, wind, or earthquake loads; constraining forces, such as those resulting from restriction of thermal, shrinkage, or moisture-change movements; or forces caused by displacements or deformations of members, such as those caused by creep, plastic flow, differential settlement, or sideways (drift).

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.