Description of the problem

A problem that users have had to face in the past is the following:

Since the weights specified for seismic definition are used only for the lateral load analysis, it used to be necessary to re-specify all or most of these weights once again in load cases for the gravity analysis. So, the MEMBER WEIGHT data had to be re-specified through MEMBER LOADS, JOINT WEIGHTS thru JOINT LOADs, and so on. In other words, all the WEIGHTS in the seismic definition had to be provided once again as LOADS for the gravity analysis through the means of DEAD, LIVE, and other such load cases.

So, the question that naturally comes up is: Since the weights for the seismic definition originate from the same set of loads that are specified under the types Dead, Live, etc., why can't we instruct the program to use the data in those load cases to generate the seismic weights automatically?

Solution

The Reference Load feature of the program can be used to overcome this problem. The load information contained in Reference load types can be used as the feeder data with which to assemble the seismic weights.

To illustrate this method, let us assume that there are four reference load types defined in the manner shown below.

DEFINE REFERENCE LOADS

LOAD R1 LOADTYPE None  TITLE DL1

SELFWEIGHT Y -1 LIST 1 TO 1101

LOAD R2 LOADTYPE None  TITLE DL2 - EQUIPMENT

MEMBER LOAD

42 60 78 92 CON GY -6 3.0

1 TO 31 41 TO 73 UNI GY -0.025

273 282 324 336 349 410 426 548 UNI GY -0.028

JOINT LOAD

44 56 78 90 FY -12

LOAD R3 LOADTYPE None  TITLE LL1 - LIVE LOAD

ONEWAY LOAD

YRANGE 4.69 4.71 ONE -2.5 XRANGE -1 13  ZRANGE -1 11  GY

YRANGE 4.69 4.71 ONE -3.5 XRANGE 12.9 23.1  ZRANGE -1 11  GY

JOINT LOAD

69 78 81 84 95 FX 3.5

LOAD R4 LOADTYPE None  TITLE LL2 - OPERATING LIVE LOAD

ELEMENT LOAD

1029 TO 1277 PR GY -2.1

JOINT LOAD

69 78 81 84 95 FZ 3.5

END DEFINE REFERENCE LOADS

The various loads described under these reference loads can then be used to provide the seismic weights for the seismic definition in the following manner.

DEFINE 1893 LOAD

ZONE 0.36 RF 3 I 1 SS 1 ST 1 DM 0.05

REFERENCE LOAD Y

R1 1.0 R2 1.0 R3 0.25 R4 0.25

 The term "Y" in the expression "REFERENCE LOAD Y" instructs the program that among the various load items that are contained in the reference load cases, only those load items acting along the Y direction should be used for creating the seismic weights. Terms specified along X and Z (such as those present in reference load cases 3 and 4) should not be used.

So, the seismic load case in the model will look like this

LOAD 1 EQ IN X

1893 LOAD X 1.0

and, the gravity load case will look like this

 * COMBINATION LOAD CASE FOR CONCRETE DESIGN

LOAD 7

REFERENCE LOAD

R1 1.2 R2 1.2 R3 1.5 R4 1.5

Thus, the load data has to be specified only once in the model, and can be used for lateral as well as gravity analysis.

A sample model that illustrates this method can be downloaded from

http://communities.bentley.com/products/structural/structural_analysis___design/m/structural_analysis_and_design_gallery/264342.aspx