• Version
• 247 Downloads
• 121.19 MB File Size
• 1 File Count
• March 5, 2017 Create Date
• March 5, 2017 Last Updated

The Shock and Vibration Bulletin - Part 5; Dynamic Analysis, Modal Test and Analysis

INTRODUCTION 
Most building codes require that a 
structure be designed to resist earth— 
quakes. The method of analysis usually 
specified is based on static analysis 
procedures . An equation, which is a 
function Of the total weight Of the 
structure, is used to determine the 
total horizontal earthquake force to be 
resisted by the supports of the struc— 
ture. This force is tnen divided into 
increments acting along the height of 
the s tructure. The method used in 
distributing the horizontal force resul— 
ts in having more force near the top Of 
the s . The Structure is then 
analyzed, treating the earthquake loads 
as constant static forces and without 
considering the structure's response Eo 
the dynamic nature of this earthquake 
loading. A Static method of earthquake 
analysis was a good practical method to 
use prior to the development of the high 
speed computers and the various sophis— 
ticated programs capable Of analyzing 
very large and complex Structures A 
better ana and design can result 
by using a dynamic earthquake input at 
the base of a structure and then 
determine the earthquake loads resulting 
from the actual response Of a structure 
to the dynamic load. Using such a 
method the load distribution becomes a 
function of the structures flexibility

and it changes if the flexibility Of 
certain members is revised. This type 
of approach is more realistic and it 
identif more adequately the structur— 
e •s behavior during an earthquake. In 
addition, the designer develops more 
confidence in the adequacy of the struc— 
ture to resist an earthquake when he 
observes its response and understands 
its structural behavior when subjected 
to a dynamic force. 
The studied in this paper is 
shown in F igure 1. The actual structure 
of boiler is represented in the computer 
by a mathematical model. 
The accuracy 
of the results will depend on the 
accuracy of the model However, in 
large complex structures it is impossi— 
ble to model all structural details. 
When time and economics allow , the 
designer can develop more conf idence in 
the mathematical model by observing the 
changes in its response when certain key 
members are modeled differently . This 
way, he can determine the sensitivity Of 
certain members and how detailed their 
modeling has to be. For this reason the 
mathematical model used in this paper is 
cornpared to a similar but a more detail— 
ed model The results Of the analysis 
using two models are tabulated and 
compared. Reconunendations are made as