Applies To
	Product(s):	STAAD.Pro
	Version(s):	ALL
	Environment:	ALL
	Area:	Analysis
	Subarea:	Dynamic Analysis
	Original Author:	Phil Riegel, Bentley Technical Support Group

 

How do I model mass for a dynamic analysis?

 

In STAAD the procedure for modeling mass is the same for all types of dynamic analyses (response spectrum loading, time history loading, modal calculation, etc.) and involves using loads are used to represent the mass.  Typically the selfweight load is specified to capture the mass of the structural entities (members, plates, etc.) but other loads such as nodal loads or member loads can be used as well if you wish to account for additional mass (equipment, for example). These masses are "lumped" at the nodes during the dynamic analysis. 

When entering loads to represent mass there are a few key points to keep in mind:

• The loads should be added to the load case containing the dynamic load.
• The loads should be specified in all three global directions:  X, Y, and Z.  Please note, this is true regardless of the direction of the loading.   For example, suppose you have a response spectrum load in the X direction--specifying the mass in the X direction only is not correct, the Y and Z must be included as well. 
• The loads should be positive in sign.     

Consider the example below showing the command syntax for a seismic load case containing a response spectrum load, including mass due to selfweight and joint loads:

LOAD 1 LOADTYPE Seismic-H TITLE Seismic X
SELFWEIGHT X 1
SELFWEIGHT Y 1
SELFWEIGHT Z 1
JOINT LOAD
17 TO 48 FX 2.5 FY 2.5 FZ 2.5
49 TO 64 FX 1.25 FY 1.25 FZ 1.25
SPECTRUM SRSS IBC 2018 X 1 ACC DAMP 0.05 LIN
ZIP 92887 SITE CLASS A FA 0.800 FV 0.800 TL 8.000

As you can see, the selfweight loads are defined in the X, Y, and Z directions and are specified with a factor of positive one.  The joint loads are similarly defined in all three global directions and have positive values.