Impact Loads & Wind Loads

Impact Loads. Moving vehicles may bounce or sidesway as they

move over a bridge, and therefore they impart an impact to the deck.The

percentage increase of the live loads due to impact is called the impact

factor, I.This factor is generally obtained from formulas developed from

experimental evidence. For example, for highway bridges the AASHTO

specifications require that

where L is the length of the span in feet that is subjected to the live

load.

In some cases provisions for impact loading on the structure of a

building must also be taken into account. For example, the ASCE 7-10

Standard requires the weight of elevator machinery to be increased by

100%, and the loads on any hangers used to support floors and balconies

to be increased by 33%.

Wind Loads. When structures block the flow of wind, the wind’s

kinetic energy is converted into potential energy of pressure, which

causes a wind loading. The effect of wind on a structure depends upon

the density and velocity of the air, the angle of incidence of the wind, the

shape and stiffness of the structure, and the roughness of its surface. For

design purposes, wind loadings can be treated using either a static or a

dynamic approach.

For the static approach, the fluctuating pressure caused by a constantly

blowing wind is approximated by a mean velocity pressure that acts on

the structure. This pressure q is defined by its kinetic energy,

where is the density of the air and V is its velocity. According to the

ASCE 7-10 Standard, this equation is modified to account for the

importance of the structure, its height, and the terrain in which it is

located. It is represented as

qz = 0.00256Kz Kzt Kd V2 1lb>ft22

qz = 0.613Kz Kzt Kd V2 1N>m22

where

the velocity in mih (m/s) of a 3-second gust of wind measured

33 ft (10 m) above the ground. Specific values depend upon

the “category” of the structure obtained from a wind map.

Hurricane winds caused this damage to a condominium in

Miami, Florida.