Vertical force of earthquake according to clause 3-3-9 of standard 2800 and its application in ETABS software
When the building is designed for the forces caused by the horizontal components of the earthquake, it is usually responsible for the vertical forces of the earthquake, and also the safety factor that is considered for the design of gravity loads creates additional resistance against the vertical forces of the earthquake, which ensures safety against the forces Earthquakes occur. The magnitude of the vertical component of an earthquake is about one-third of its horizontal components, however, in some cases, the magnitude of the vertical component can reach two-thirds of the horizontal component, hence the standard 2800 states in paragraph 3-3-9-1 that the vertical force caused by the earthquake, which is the effect of the vertical component of the acceleration of the earthquake in the building, should be included in the calculations in the following cases.
A) For buildings that are located in seismic zoning with a very high relative risk (A=0.35), the vertical forces of the earthquake should be considered in the entire structure (in the entire structure).
b) Special beams
Beams whose span is more than fifteen meters, along with their supporting columns and walls.
Beams carrying a concentrated vertical load that is significant compared to other loads transferred to the beam, together with their columns and supporting walls.
Significant concentrated vertical load:
If the concentrated load is at least half of the total load on the beam, that load is considered significant.
Calculation of vertical earthquake force for the entire structure in areas with very high relative risk (A=0.35):
The relationship of the vertical force of the earthquake for the whole building is as follows:
Fv=0.6AIWp
Wp: dead load (when the earthquake force is calculated for the whole building).
How to apply the vertical force of the earthquake in the software:
Pay attention to the two load combinations in the figure below:
The term 0.2Sds*D is the vertical earthquake force, which is equal to 0.6AIWp
0.2Sds*D=0.6AIWp
In the above relation, D is Wp and A=0.35, so we have:
0.2Sde=0.6*0.35*I
Sds=1.05*I
I: Coefficient of importance of the structure
Now that Sds has been determined, it can be done by running the command
Design menu > {Design Type} > View/Revise Preferences
in the episode
Design System Sds
Enter the value of I*1.05, in this way, if you use the default load combination of the software, the software will correct the dead load coefficient by applying the Sds factor to the dead load. This dead load coefficient in residential buildings (I=1.0) and for the case that the earthquake is vertical downwards, changes from 1.2 to 1.41, and if the earthquake is vertical, the coefficient changes from 0.9 to 0.69.
Attention, if the structure is not very high in seismic risk zoning (A<0.35), then the Sds coefficient will be entered as zero (0). Pay attention to the following figures. —- Send a question (about the ambiguities of the 2800 standard): https://telegram.me/morsal_mottaghi Subscribe to the channel: Standard No. 2800 (4th Edition) morsal mottaghi https://telegram.me/standardNo2800 This post is written by Morsal_mottaghi