This program provides analysis of lateral seismic forces on multi-story buildings according to the 1994 UBC lateral force formulas 28-1, 28-6, 28-8 and the diaphragm force formula 31-1. Also included is a section that will assist the user in determining the overall seismic factor. By entering building dimensions and the number of stories, the basic building period is determined using the UBC formulas for basic building periods. From this value, and user defined Z, I, S, Hn, and Ct factors, C/RW limits are checked and overall seismic factor determined.

 

Force distribution factors for each level is determined using formula 28-7, and the base shear applied to each level for the evaluation of story shears and overturning moments. Also, these forces are used along with formula 31-1 to determine the required diaphragm forces at each level. Any number of stories may be specified, and the additional top force FT is applied when required.

 

 

Basic Usage

This program is used in areas where seismic forces on multi-story buildings will have considerable effect on the design of the lateral resisting system.

 

Enter the values for Z, I, Rw, S, and C based upon your building. The value for C can be calculated in two ways.....either entering the actual building period -or- the simplified UBC method of height and Ct factor.

 

From the entered values the program calculates the overall seismic factor to be used. This is applied to the story weight Wi entered by you.

 

The multi-story force analysis is based on filling out a table, one line for each story. You need to start at the BOTTOM of the table, working upward for each story.

 

Assumptions & Limitations

The analysis is based upon the 1994 UBC static force formulas, and as such, determine lateral forces according to the static model approach.

The seismic factor cannot be varied at each level to account for varying dynamic properties that the user may wish to use to perform alternative studies.

 

 

Example

The data entry for this example is shown in the screen captures that accompany the Data Entry Tabs and Results & Graphics Tabs sections to follow.

 

 

 

Data Entry Tabs

This set of tabs provides entries for all input in this calculation. While you are entering data and switching between these tabs you can view the desired resulting information on the tabs on the right-hand side of the screen (calculated values, sketches, diagrams, etc.). A recalculation is performed after any entry data is changed. After each data entry you can view the results on the right-hand set of tabs.

 

General Tab

This tab allows you to select which code method to use to calculate your seismic factor and also other common values regarding the seismic zone and building.

                       

 

 

Select Governing Building Code

Select the radio button for the code calculation method you wish to use.

 

Seismic Zone

Factor Enter the seismic factor which is based on the seismic risk map area (found in UBC Table 23-I).

 

Seismic Zone Factor

sd

 

Building Period

The building period can be specified by either:

 

UBC Calculations Tab

This tab provides data entry and calculated values according to 1994 UBC criteria. Please see that code for further explanations of the values.

 

 

 

UBC Calculations Tab

This tab provides data entry and calculated values according to 1997 UBC criteria. Please see that code for further explanations of the values.

 

 

Building Forces Tab

This table performs the distribution of total base shear to the various floors, based upon UBC formula 28-5, 28-6, and 28-7. The resulting distribution of forces will resemble a triangular distribution with the maximum intensity at the top of the building.

 

 

Level #

This is the level # above the base. Base shear and overturning moment are given as the Base Totals shown at the bottom of the table. These level numbers are modified during the program's self-modifying process while adjusting to the requested number of levels.

 

Weight Wi

Enter the weight of each level of the structure in this area. This weight will be multiplied by the overall seismic factor ZIC/RW for the calculation if Fx.

 

Height Hi

Enter the height of each level above the base. This represents the vertical distances between each floor mass Wi.

 

Wi * Hi

This is an intermediate calculation for FX.

 

Ft : Top Force

The added top force is calculated only if the building period is greater than 0.7 seconds. This added top force is equal to 0.07 times the calculated period times the total base shear, and is applied to the uppermost level of the structure if applicable, this is the top force Ft applied at the top level.

 

Fx

After all the factors in the equation for Fx are ready, the lateral force to each level of the structure Fx is calculated using formula 28-7 of the UBC. This force is applied to each level for calculation of story shears and overturning moments.

 

Lateral Force

This is simply a restatement of Fx with Ft added if applicable.

 

Story Shear

Using the calculated forces at each level, story shear gives the total shear acting at each level, and is the summation of the lateral forces at each level at and above the current level.

 

Story Moment

Using the calculated Story Shears at each level, story moment gives the total moment acting at each level, and is the summation of the lateral forces times moment arms for all levels at and above the current level.

 

Base Totals

This item lists the total base shear and overturning moment at the bottom of the structure.

 

Diaphragm Force Distribution Tab

This table performs an analysis of the forces to be carried by the floor diaphragm, using UBC formula 12-11.

 

 

Wpx

This is the weight of the floor system at each level, and is the same as  W i  which the user entered in the previous table. This value is automatically transferred down from the earlier input cells.

 

Lat. Force

This is the force that the diaphragm at each level must be designed to adequately carry, and is the result of UBC formula 28-9. To arrive at the final Fpx, the Fp is calculated at each level and compared with the maximum value of 0.75ZI times Wi at each level, and the minimum of the two is used.

 

Diaphragm Fpx

This is the calculated force that must be applied at each level, and is equal to the Lateral Force value calculated in the preceding table.

 

 

Sketch Tab

This tab provides a sketch of the beam with loads and resulting values shown.

 

 

 

Sample Printout