Create the Rigid Diaphragms: Select all, then assign shell rigid diaphragm.
Each node of the slab is now joined to the center of geometry of the entire floor slab.
The ASCE 7-98 Code Load Combinations (changed with 7-02):
•1.2DL + 1.6LL
•1.2DL + 0.8W
•1.2DL + 1.6W + 0.5L
Apply the Loading to the Slab Diaphragm:
Consider the case of wind along the y-axis. That is denoted by the case WINDY.
ETABS prepares a table, shown below.
The shaded box at left, for FY, is a copy and paste from column R of the Excel spreadsheet of MWFRS calculations.
Pay attention to units.
This portion of spreadsheet is part of the analysis performed for Homework #3, for the wind loads upon a forty (40) story building.
Column R is copied and pasted into the ETABS wind loading case WINDY, that is, wind loading along the y-axis.
The WINDxx groups above, follows from the previous page’s combinations. For example, W47 means it is the wind load case 4, combination 7. The ENVE at left, is the envelope that has all the 65 loads combinations.
In the load combination box, upper left, DDWL means the Design Dead load + Wind load + Live load.
Note that the WIND Combo at the above right box, has all 32 combinations. The SERVICE load combination is used to check for lateral deflections.
Notice that the y-displacement of Point Object 4 at the top of the 40th story is 4.95 inches. This corresponds to a horizontal drift = 4.95 in / (379.5)(12) in = 1 / 920 << 1 / 400 criterion.
The building appears to be about 2.3 times stiffer than required. The designer now can choose to,
reduce the size of the outer perimeter columns,
reduce the size of all the columns,
reduce the length of the shear walls, or
a judicious combination of all of the above, to reach 1 / 400 for the total drift.