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aditya ...
Joined: 05 Apr 2008 Posts: 188
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Posted: Mon May 21, 2018 4:33 pm Post subject: Design for Vertical Earthquake Effects as per IS 1893 Part1: 2016 |
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Dear Sefians,
I need to carrry out seismic analysis and design of a multi-storeyed building in Zone V. As per clause 6.3.3.1 of IS 1893: Part 1-2016, we need to consider vertical earthquake effects. I need clarifications on the following points regarding the application of clause:
1. Design vertical seismic coefficient=(2/3)x design horizontal seismic coefficient. Now shall we calculate base shear=design vertical seismic coefficient x seismic weight?
2. base shear calculated in point 1 above shall be applied in +/- vertical direction (i.e. down and up?)?
3. How shall we carry out storey-wise distribution of this "vertical?"base shear?
4. What will be the point of application of storey-wise vertical seismic forces? Centre of mass of each storey?
5. Shall we consider separate load cases EQz+ and EQz-?
6. Load combinations shall be as per clause 6.3.4.1?
Can somebody help in this regard with example?
with best regards,
Aditya |
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u.mukesh General Sponsor
Joined: 26 Jan 2003 Posts: 99 Location: Delhi
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Posted: Thu May 24, 2018 7:30 am Post subject: Design for Vertical Earthquake Effects as per IS 1893 Part1: 2016 |
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Hello dear
Vertical earthquake will increase (when downward) the moments in beams;
and all these reactions from beams (at diff floors) will increase the axial force in columns.
You should make proper combinations and you will get the forces.
There is no logic of vertical base shear..........
as vertical loads will create additional axial forces in columns and foundations.
Regards
Mukesh Upadhyay
On Mon, May 21, 2018 at 10:03 PM, aditya <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote: | Dear Sefians,
I need to carrry out seismic analysis and design of a multi-storeyed building in Zone V. As per clause 6.3.3.1 of IS 1893: Part 1-2016, we need to consider vertical earthquake effects. I need clarifications on the following points regarding the application of clause:
1. Design vertical seismic coefficient=(2/3)x design horizontal seismic coefficient. Now shall we calculate base shear=design vertical seismic coefficient x seismic weight?
2. base shear calculated in point 1 above shall be applied in +/- vertical direction (i.e. down and up?)?
3. How shall we carry out storey-wise distribution of this "vertical?"base shear?
4. What will be the point of application of storey-wise vertical seismic forces? Centre of mass of each storey?
5. Shall we consider separate load cases EQz+ and EQz-?
6. Load combinations shall be as per clause 6.3.4.1?
Can somebody help in this regard with example?
with best regards,
Aditya
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aditya ...
Joined: 05 Apr 2008 Posts: 188
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Posted: Fri May 25, 2018 2:10 am Post subject: |
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Dear Er. Mukesh Upadhyay Sir,
Thanks a lot for your suggestions. I am looking after how to calculate the earthquake forces to simulate the effect of vertical seismic forces in a building and how to apply it as per codal provisions. It would be great if somebody would suggest on how to model it in seismic coefficient method through Staad.pro/SAP2000/ETABS software or even through manual calculation.
with best regards and thanks,
Aditya |
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aditya ...
Joined: 05 Apr 2008 Posts: 188
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Posted: Fri Feb 14, 2020 1:33 pm Post subject: |
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Dear Sefians,
Can somebody guide me on how to apply vertical seismic forces in different storeys as per IS 1893 Part 1: 2016? Will its distribution similar to horizontal seismic forces in different storeys? Code seems silent on this topic.
with regards,
Aditya |
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mallan SEFI Regulars
Joined: 17 Mar 2016 Posts: 21
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Posted: Sun Feb 16, 2020 8:43 am Post subject: |
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I agree with Er. Mukesh's comment above.
At any point of time, each and every mass on the building is continuously excited at acceleration = g. However, during an earthquake event, the building gets excited similar to a person standing in an elevator. So, if i call the acceleration produced by vertical earthquake as z, the weight of each and every mass on the structure becomes m*(g+z) or m*(g-z). So, just adding or subtracting a load equal to a fraction of the seismic weight should be sufficient (value of fraction depends on the Av).
For example, lets say Av as per IS1893 = 5%, i.e. EQZ = 0.05(DL+0.5LL)
I would apply it by increasing/decreasing gravity loads as -
1.5DL+1.5EQX+(1.5*0.3)EQZ = 1.5225DL+0.01125LL+1.5EQX
0.9DL+1.5EQX -(1.5*0.3)EQZ = 0.8775DL -0.01125LL+1.5EQX
Let me know what you think of this.
-Akshay Mallan |
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aditya ...
Joined: 05 Apr 2008 Posts: 188
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Posted: Sun Feb 16, 2020 1:32 pm Post subject: |
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Dear Akshaya Mallan Sir,
Thanks a lot for responding to my query. The solution suggested by you will work for design in the case of Equivalent Static Method. The problem I am facing is how to distribute the total base shear at each storey so that I can apply the load in the analytical model and use the results later in response spectrum method for scaling of the base shear as per codal provision. Is there any work around?
with best regards,
Aditya |
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Dr. N. Subramanian General Sponsor
Joined: 21 Feb 2008 Posts: 5538 Location: Gaithersburg, MD, U.S.A.
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Posted: Tue Feb 08, 2022 7:09 pm Post subject: |
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Dear Er Aditya
I saw your posting only today. The vertical EQ load (2/3 of the horizontal shear) has to be applied in the same nodes where you are applying the horizontal loads in vertical upward and downward directions(2 cases).
As per Er Hemal Mistry:
"IS 1893-1 requires to combine upward (-ve) and downward (+ve) vertical EQ loads in all EQ load combinations which includes DL, LL and horizontal EQ loads with different Partial load factors for loads, resulting in very large no. of load combinations. However, no. of load combinations can be significantly reduced by combining +ve and -ve vertical EQ effects with other loads wisely and logically. e.g. +ve (downward) vertical EQ may be combined with 1.5DL & 1.2DL+1.2LL only and -ve (upward) vertical EQ may be combined with 0.9DL only.
Upward vertical EQ may be critical in high seismic zone for large cantilevers/large spans for load combination 0.9DL-1.5EQv(+/-)1.5EQh"
Prof. Ashok Jain opinioned
"In my limited experience, 1.5DL + 1.5 EQ governs in most cases. PLF 1.2 has never governed, nor wind"
Warm regards
Subramanian
aditya wrote: | Dear Akshaya Mallan Sir,
Thanks a lot for responding to my query. The solution suggested by you will work for design in the case of Equivalent Static Method. The problem I am facing is how to distribute the total base shear at each storey so that I can apply the load in the analytical model and use the results later in response spectrum method for scaling of the base shear as per codal provision. Is there any work around?
with best regards,
Aditya |
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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3835
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Posted: Wed Feb 09, 2022 5:45 pm Post subject: Re: Design for Vertical Earthquake Effects as per IS 1893 Part1: 2016 |
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Just some thought on very brainstorming query from Er Aditya , appreciate his depth in the subject- experts may kindly correct
1. I think Vertical load plus/minus due to vertical seismic component is an equivalent inertia effect . Hence not to be considered in base shear calcs.
2 . As above
3 since it is vertical effect causing plus/minus equivalent axial force , storey wise distribution is directly propotional to cumulative seismic weight at respective storey.
4. Yes center of mass
5. Yes
6. As per Code
aditya wrote: | Dear Sefians,
I need to carrry out seismic analysis and design of a multi-storeyed building in Zone V. As per clause 6.3.3.1 of IS 1893: Part 1-2016, we need to consider vertical earthquake effects. I need clarifications on the following points regarding the application of clause:
1. Design vertical seismic coefficient=(2/3)x design horizontal seismic coefficient. Now shall we calculate base shear=design vertical seismic coefficient x seismic weight?
2. base shear calculated in point 1 above shall be applied in +/- vertical direction (i.e. down and up?)?
3. How shall we carry out storey-wise distribution of this "vertical?"base shear?
4. What will be the point of application of storey-wise vertical seismic forces? Centre of mass of each storey?
5. Shall we consider separate load cases EQz+ and EQz-?
6. Load combinations shall be as per clause 6.3.4.1?
Can somebody help in this regard with example?
with best regards,
Aditya |
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nrk SEFI Regulars
Joined: 20 Apr 2008 Posts: 20
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Posted: Thu Feb 10, 2022 7:32 am Post subject: |
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Dr. N. Subramanian wrote: | Upward vertical EQ may be critical in high seismic zone for large cantilevers/large spans for load combination 0.9DL-1.5EQv(+/-)1.5EQh"
Prof. Ashok Jain opinioned
"In my limited experience, 1.5DL + 1.5 EQ governs in most cases. PLF 1.2 has never governed, nor wind"
Warm regards
Subramanian |
The following is my correspondence with a client on the issue of effect of vertical EQ on horizontal cantilever projections in low seismic zones.
The important load combinations that need to be considered for limit state design are the following.
1.5 DL + 1.5 LL
1.5 DL + 1.5 EL
0.9 DL + 1.5 EL
1.2 DL + 1.2 LL + 1.2 EL
When Equivalent Static Method is used for Earthquake Analysis in ETABS, it does the analysis for Ah, not Av. The contribution of Horizontal Earthquake forces (Ah) to bending moments in Horizontal Cantilever Projections is zero. Hence, the above combinations will reduce to:
1.5 DL + 1.5 LL
1.5 DL
0.9 DL
1.2 DL + 1.2 LL
Hence, 1.5 DL + 1.5 LL is the governing load combination for the horizontal cantilever projections as per ETABS analysis. However, we need to consider the effect of vertical accelerations too in the case of horizontal cantilever projections, i.e. 5 times Av.
Considering 3 times Av, since we are in Zone II, we have, 3Av = 3x2/3 Ah = 2Ah, where Ah=ZI(Sa/g)/(2R)=0.10x1.2x2.5/(2x3)=0.05. Hence 3Av=2x0.05=0.1
This 3Av will act upon the vertical seismic weight in the horizontal cantilever projections, which for practical purposes can be taken as the DL of the horizontal cantilever projections, neglecting LL.
For this case, the load combinations will transform to:
1.5 DL + 1.5 LL
1.5 DL + 1.5 (0.1 DL) = 1.65 DL
0.9 DL + 1.5 (0.1 DL) = 1.05 DL
1.2 DL + 1.2 LL + 1.2 (0.1 DL) = 1.32 DL + 1.2 LL
Either 1.5 DL + 1.5 LL or 1.65 DL should govern the design. It is highly likely that 1.5 DL + 1.5 LL will govern the design.
Adding the 25% LL component in the vertical seismic load will transform the above load combinations to:
1.5 DL + 1.5 LL
1.5 DL + 1.5 (0.1 (DL + 0.25 LL)) = 1.65 DL + 0.0375 LL
0.9 DL + 1.5 (0.1 (DL + 0.25 LL)) = 1.05 DL + 0.0375 LL
1.2 DL + 1.2 LL + 1.2 (0.1 (DL + 0.25 LL)) = 1.32 DL + 1.23 LL
Now, either 1.5 DL + 1.5 LL or 1.65 DL + 0.0375 LL should govern the design. Again, it is highly likely that 1.5 DL + 1.5 LL will govern the design.
The same can be extended to high seismic zones as well, as follows.
Considering 5 times Av, assuming Zone V, we have, 5Av = 5x2/3 Ah = 10Ah/3, where Ah=ZI(Sa/g)/(2R)=0.36x1.2x2.5/(2x5)=0.108. Hence 5Av=10x0.108/3=0.36
This 5Av will act upon the vertical seismic weight in the horizontal cantilever projections, which for practical purposes can be taken as the DL of the horizontal cantilever projections, neglecting LL.
For this case, the load combinations will transform to:
1.5 DL + 1.5 LL
1.5 DL + 1.5 (0.36 DL) = 2.04 DL
0.9 DL + 1.5 (0.36 DL) = 1.44 DL
1.2 DL + 1.2 LL + 1.2 (0.36 DL) = 1.632 DL + 1.2 LL
Either 1.5 DL + 1.5 LL or 1.632 DL + 1.2 LL should govern the design.
Adding the 25% LL component in the vertical seismic load will transform the above load combinations to:
1.5 DL + 1.5 LL
1.5 DL + 1.5 (0.36 (DL + 0.25 LL)) = 2.04 DL + 0.135 LL
0.9 DL + 1.5 (0.36 (DL + 0.25 LL)) = 1.44 DL + 0.135 LL
1.2 DL + 1.2 LL + 1.2 (0.36 (DL + 0.25 LL)) = 1.632 DL + 1.308 LL
Now, either 1.5 DL + 1.5 LL or 1.632 DL + 1.308 LL should govern the design.
In conclusion:
- In seismic zone II, the design of horizontal cantilever projections is governed by either 1.5 DL + 1.5 L or 1.5 DL + 1.5 EL
- In seismic zone V, the design of horizontal cantilever projections is governed by either 1.5 DL + 1.5 L or 1.2 DL + 1.2 LL + 1.2 EL
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