ASCE 7-05 defined the base for seismic design as:
“The level at which the horizontal seismic ground motions are considered to be imparted to the structure”.
Most of the times this is misinterpreted or misunderstood that where the base is considered when determining the height of building for seismic calculations specially when the site is sloped or if the basement is open from one side.
Read the following article published in December 2009 issue of Structure magazine, this article will answer lots of questions.
NOTE: This discussion applies only to SAP2000. ETABS automatically includes the accidental eccentricity specified in the response-spectrum load case.
To consider accidental torsion in a response-spectrum case, eccentricity should be added as a separate static load case. The response-spectrum (SPEC) and eccentric (ECC) load cases should then be combined as SPEC + ECC and SPEC – ECC load combinations. Each level should have a rigid diaphragm, otherwise accidental eccentricity is not of significance.
Accidental torsion is applied through the following process:
- Define a load case named ECC (or other) of Type = Quake, Self-weight multiplier = 0, and Auto = None.
- Select any point on the diaphragm at each level.
- Select Assign > Joint/Point Loads > Force > Moment Global ZZ, then specify the appropriate value for torsional moment.
- Evaluate SPEC + ECC and SPEC – ECC load combinations. Note that the software already considers SPEC in both directions.
Accidental torsion may be considered without rigid diaphragms, though loading should be more distributed. Torsional moment is calculated at joint locations within a flexible diaphragm by first resolving the total lateral force in a given direction at each story level. This is done by summing the auto-seismic loads applied to joints within each story level. This force is then multiplied by the eccentricity to generate torsional moment (T). This torsion is then distributed to all joints at the story level in proportion to the joint mass as follows:
- Torsion at a given joint at a story level = T * (Mass at the joint / Total mass of all joints at the story level)
It may be convenient to use an Excel spreadsheet to calculate torsion at joint locations. Using interactive database editing, data may then be entered into SAP2000
Per ACI 318-08 section 188.8.131.52 – “Grade beams designed to act as horizontal ties between pile caps or footings shall be proportioned such that the smallest cross-sectional dimension shall be equal to or greater than the clear spacing between connected columns divided by 20 but need not be greater than 18 inches. Closed ties shall be provided at a spacing not to exceed the lesser of one half the smallest orthogonal cross-sectional dimension and 12 inches.”
But ACI is not telling us that what force these tie beams should be designed for, so here is the IBC 2009 section to tell us that:
IBC2009 section 1809.13 for shallow foundations and 1810.3.13 for deep foundations:
for SDC C, D, E or F, ties shall be capable of carrying, in tension or compression, a force equal to the lesser of the product of the larger pile cap or column design gravity load times the seismic coefficient, Sds,divided by 10, and 25 percent of the smaller pile or column design gravity load. (there is more in this section so please read the code before applying this to your case)
In simple words Tie beam force; FT = Larger of ( Pu_large x Sds /10 , Pu_small x 25%)
A very well written tips which explain the behavior and design of Earthquake was provided by IIT India. I have uploaded all the tips documents below:
A very informative technical guidline for Seismic Design of Cast-in-Place Concrete Diaphragms Chords, and Collectors was recently published by NEHRP. it can be downloaded from the following link