Effect of Backstay on Tall Structures with Podium

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 09 Issue: 04 | Apr 2022 www.irjet.net p-ISSN: 2395-0072
© 2022, IRJET | Impact Factor value: 7.529 | ISO 9001:2008 Certified Journal | Page 529
Effect of Backstay on Tall Structures with Podium
Nirav Bhatu1, Prof. Vishal B. Patel2, Prof. Pratiti Bhatt3
1M.Tech. (Civil) Structural Engineering, BVM Engineering College, Vallabh Vidyanagar, Gujarat.
2Assistant Professor, Dept. of Structural Engineering, BVM Engineering College, Vallabh Vidyanagar, Gujarat.
3Assistant Professor, Dept. of Structural Engineering, BVM Engineering College, Vallabh Vidyanagar, Gujarat.
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Abstract - Structural engineering is the art and science of creating structures that are safe, cost-effective, and durable. The
entire exercise necessitates conceptual thinking based on structural engineering principles, as well as acceptable behavior and
performance of the building during normal functional use and during extreme catastrophic events such as earthquakes,
windstorms, and other natural disasters, all while adhering to the most recent codal provisions. The scope of this study is to study
the modeling technique and to understand the realistic behavior of Multiple towers with common podium type structures under
lateral loads considering the backstay effect as per IS:16700 (2017) “Criteria for Structural Safety of Tall Concrete Buildings". In
this study, different models were prepared by changing the height of the podium and no. of towers in structure, andacomparative
analysis is performed on the single tower with podium type ofstructuresand multipletowerswithcommonpodiumtypestructures
by varying the height of Podium and no. of towers in Structure with shear wall and without ShearwallontheperipheryofPodium.
For the seismic analysis, Linear static and Responsespectrumapproachesareconsidered, andobtainedresultsarerepresentedand
compared graphically. In this study, the analysis of Multiple towers with common podium type structures is done using the
structural analysis tool ETABs.
Key Words: Podium, Backstay effect, multiple towers with common podium, seismic analysis, Equivalent static
method, Response spectrum method.
1. INTRODUCTION
Due to rise in population & land scarcity in urban areas, the need for tall structures is increasing day by day. Tall buildings are
an evolving construction practice in developing countries including India. In any city, especially in metro cities, after certain
horizontal development, no more land is available for development. So, to have optimum use of land, multistorey towers
became popular. Due to the complexity of the structures, the most advanced engineering design techniques are needed in tall
structures and to satisfy demand of larger commercial space near road level and make the building compliant with minimum
parking space requirements forsuchmixed-usedevelopmentaccordingtoprevailing bye-laws, ArchitectsandDevelopershave
come up with the unique idea of Podium type Buildings. Many tall structures have an arrangement in which the below few
stories have a larger plan area than the towers above. A podium is the lowest level of tall building construction with a larger
floor plan area and significantly higher seismic force resistance than the tower above. As compared to low and mid-rise
buildings, the design criteria for high-rise buildings are different. Shear walls (lateral systems) have traditionallybeen viewed
as simple cantilever beams fixed at the base. This analogy is reasonably correct for the above-grade structure,butfor(podium
+ tower) type building, a more realistic and justifiable analogy would be a cantilever with a back span to take into account the
effects of the relatively large lateral stiffness of the podium. Backstay effects are the transfer of lateral forces from the tower's
seismic-force-resisting components to additional elements within the podium, usuallyvia oneor morefloordiaphragms.Atall
building's lateral force resistance and force transfer through floor diaphragms at these levelshelpitresistseismicoverturning
forces. Based on its similarity to the back span of a cantilever beam, This component of overturning resistanceisreferred toas
the backstay effect. Sometimes it is also referred to as “Shear Reversal”, because the shear in seismic load resisting elements
can change its direction within the podium levels. BIS released the Code IS:16700 (2017) “Criteria for Structural Safety of Tall
Concrete Buildings" in which various criteria for tall structures and detail regarding the analysis of the podium type tall
structures with consideration of backstay effect is given. To account for the realistic behaviorofthepodiumtypetall structure,
IS:16700(2017) also gives the sensitivity analysis approach. The details regarding modeling and accepted criteria for such a
structural configuration and backstay effect are discussed inPEER/ATC72-1andIndianstandard16700-2017“Criteria forthe
structural safety of tall concrete buildings”. In IS: 16700 (2017), the details regarding sensitivity analysis of such structure by
changing the stiffness parameters for elements within the podium level are given and in PEER/ATC 72-1 the instructions for
the modeling of Podium Diaphragms, Collectors, and Backstay Effects are given.
1.1 Aim of study
To Study the backstay effect and shear reversal that occurs in the multiple tower with common podium type structures.

1.2 Objectives Of Study
Following are the objectives of the study:-
1. To study and understand the conceptofbackstay/Shearreversal observedinmultiple towerswithcommonpodium
type structures by considering the provisions given in the Indian standards (IS 16700: 2017).
2. To study the effect of the podium on the behavior of the tower.
3. To perform comparative Analysis on single tower with podium type structures and multiple towers with common
podium type structures by varying height of Podium and no. of towers in Structure with shear wall and without
Shear wall on the periphery of Podium.
4. To study the effect on shear reversal and backstay effect by changing the height of podium/no. of podium stories.
5. To study the effect on shear reversal and backstay effect by changing the no. of towers in the structure.
6. To study, interpret and compare the analysis results among the following structural systems:-
a. Single tower with podium type of structures
b.Three towers with common podium with Shear wall at the periphery of the podium.
c.Three towers with common podium without Shear wall at the periphery of the Podium.
d. Four towers with common podium with Shear wall at the periphery of the Podium.
e.Four towers with common podium without Shear wall at the periphery of the Podium.
2. LITERATURE REVIEW
By focusing on writing audits from various authors, the following were completed:-
Mehair Yacoubian et al. (2017). Linear and non-linear analyses were done on a tall R.C.C podium-typebuildinginthisstudy.
The structure was also tested by altering the podium height. The tower was also placed at a certain offset for the study. The
diaphragms were modeled as semi-rigid to account for in-plane horizontal deformation. The author concluded that the dual-
wall framing action can be affected by Diaphragm flexibility. At the podium tower interface, the maximum strutting forces in
coupling beams were detected and at the podium tower interface level, shear force reversal was also detected. To reduce the
podium restraint effects on the building, other design approaches such as expansion joints can be considered. It was also
revealed that the conventional in-plane rigid diaphragm assumption leads to unconservative tower wall design.
Babak Rajaee et al. (2009). In this paper, the authors stated that The maximum bending moment (flexural plastic hinge)
occurs above the diaphragms and the shear force reverses below the flexural hinge when a large amount of the overturning
moment in the wall is transferred to the foundation walls by force couples in two or more stiff floor diaphragms. The reverse
shear force below the flexural hinge may be substantially greater than the base shearforceabovetheflexural hinge,depending
on the stiffness of floor diaphragms and the shear and flexural rigidity of high-rise concrete walls. The maximum reversed
shear force is related to the wall's bending moment capacity and inversely proportional to the accompanyingbaseshearforce,
according to nonlinear dynamic studies. And based on this study, they gave several conclusions and recommended design
procedures to deal with reversed shear Forces in high-rise tower walls connected to stiff base structures.
Md Taqiuddin et al. (2019). The in-plane strutting forces and in-plane floor deformation at the tower-podium contact were
the main points of focus for this research. The reactive forces generatedatthetowerpodiuminterfacelevel andtheirimpacton
podium tower-type structures were discussed in this work. This research was conducted on twotypesofpodium buildings:1)
3B+G+50 and 2) 3B+G+9. In CSI ETABS, analysis sets were performed by changing the podium width while keeping the tower
dimensions constant. Flat slabs/Flat plates were considered as diaphragms and they were modeled as semi-rigid. The study
was also carried out by altering the column spacing. Thefindingswerecomparedforthewindloadimpactsonthestructure.On
the basis of the outputs of the ETABS models of parameters such as displacements, drifts, axial forces, and shell stresses, a
comparative study was performed. The assumption of a rigid diaphragm at the podium levels suppresses the in-plane forces
generated at the diaphragm levels, according to this study. When the space between columns is lowered, the strutting forces
produced in diaphragms increase. Tower displacements can be reduced by using podiums and the drift is unaffected by
increasing the podium's size.
Geetha et al. (2019). In this study, buildings with different podium heights were analyzed to observe changes in back-stay
effects on a podium-tower type building. A 36m x 36m tower and a 108m x 108m podium with varying heights were under
consideration. The buildings were analyzed by equivalent static and response spectrum analysis. And results of bending
moments, shear forces, and displacements were observed. The findings of the analysis in terms of parameterslikeshearforce,

bending moments, and top story displacement were compared to corresponding results for various structural configurations.
In the case of the response spectrum approach, the top displacement decreases after incrementing at a certain point and then
remains independent of the podium height. The backstay effect is imposed by the podium at the podium–towerinterfacelevel
and so, the backstay forces at the tower and podium interface rise as the podium stories increase. They also observed that the
behavior of the structure was more critical when the tower was offsetfromthe center, asopposedtowhenthetowerwasinthe
center.
Ankan Kumar Nandi et al. (2020). In this study, the backstay effect was investigated, as well as the usage of a retaining wall
to increase lateral stiffness, as stipulated in the latest tall building code IS:16700-2017 forlowandhighrisestructures.Models
with low to high rise stories and rigid and semi-rigid diaphragms were created for this study. By considering the podiumfloor
diaphragm as a semi-rigid and rigid diaphragm, the influence of diaphragm flexibility on backstay forces at the tower and
podium interface level was investigated. The effect of the placement toweratthecenterandcorneronbackstayforceswasalso
investigated. Two structural cases were selected as 20 and 40-storey framed structures for a comparative study of rigid and
flexible diaphragms situations. Both structural cases were analyzed using thestructural analysissoftwareETABS. Thefindings
were compared using factors such as base shear, story shear, top story displacement, and story drift. In addition, the tower
structure with podium structures was compared to the bare frame structure. When the backstayeffectwastakenintoaccount,
there was a 35% reduction in top displacement in both 20-story and 40-story buildingswhencomparedtothebareframe. The
diaphragm drifted inside the allowed range due to the action of the backstay. The effect ofbackstayincreasesasthestructure's
weight increases, resulting in a corresponding increase in base shear. The authors concluded thatwhena towerwaslocatedat
the center of the plot area, it produces better outcomes than when it was placed in the corner.
Kishan B. Champaneriya et al. (2021). The authors' goal in this research was to comprehend the realistic behavior of such
structures under lateral loads while taking into account the backstay effect, as defined by IS: 16700(2017). To understandthe
variations in the shear force distribution among structural elements when the tower and Podium are modeled together, a
sensitivity analysis was performed asperIS:16700(2017)considerations,takingintoaccountthestiffnessparametersgivenin
the code & the variations in force distribution were comparedto structuresthatdidnothavea backstayeffect.Itwasconcluded
that by increasing podium height, the backstay forces canbeincreased.Itwasalsoconcludedthatbyincreasingthethicknessof
podium diaphragms and area of Podium, the backstay forces can be increased.
Kush Shah et al. (2020). In this paper, the authors’ scope was to study the integrated modeling technique for a real-time
3B+G+20 storey building having a tower and below-grade podium to be abletoforecast its behaviorunder earthquakeloadsin
a realistic manner. The impact of a below-grade podium with a larger area and lateral stiffness than an above-grade tower on
lateral load distribution, behavior, performance, and design philosophy of lateral load resisting systems, such as floor
diaphragms at the intersections of below-grade podium and towers, was studied. When the tower and podium are modeled
together, a set of Backstay Sensitivity analyses was performed on the structure to understand the behavior and changesinthe
force distribution among various structural elements and the impact of the overturning resistance provided by the backstay
effect on the tower's behavior and performance was compared and analyzed against the behavior and performance of the
tower without the effect. It was found that when we analyze and design below Grade Podium type towers considering the
Tower and Podium separately and combined together, the magnitude and direction of the forces generated in the diaphragm,
beams, shear walls, and columns change significantly.
3. CONCLUSIONS
Following are the conclusions from the review study:-
1. when we analyze and design below Grade Podium type towers considering the Tower and Podium separately and
combined together, the magnitude and direction of the forces generated in the diaphragm, beams, shear walls, and
columns change significantly.
2. Some structural engineers' current practice of modeling and designing towersandpodiumsseparatelyorconsidering
one or two bays surrounding the tower may result in several structural elements being over or under-designed,
compromising the overall safety and serviceability of the structure, including the foundation.
3. By increasing podium height, the backstay forces can be increased.
4. With the increase in thickness of the podium, the backstay forces can be increased.

5. The backstay forces can be increased by increasing the Area of the podium.
6. Displacement of structure reduces with an increase in Backstay effects and when a tower was located at the center of
the plot area, it produces better outcomes than when it was placed in the corner.
7. The behavior of the structure was more critical when the tower was offset from the center, as opposed to when the
tower was in the center.
8. When the space between columns is lowered, the strutting forces produced in diaphragms increase and Tower
displacements can be reduced by using podiums.
9. At the podium tower interface, the maximum strutting forces in coupling beams were detected and at the podium
tower interface level, shear force reversal was also detected.
10. The conventional in-plane rigid diaphragm assumption leads to unconservative tower wall design.
REFERENCES
[1] Mehair Yacoubian, Nelson Lam, John L. Wilson, “Effects of podium interference on shear force distribution in tower walls
supporting tall buildings”, (Elsevier, 2017).
[2] Babak Rajaee And Perry Adebar. 2009. “Seismic design of high-riseconcretewalls:reverseshearduetodiaphragmsbelow
flexural hinge.” ASCE / August 2009, DOI :- 10.1061/(ASCE)0733-9445(2009)135:8(916)
[3] Md Taqiuddin, Dr. V Lakshmi Shireen Banu, “Numerical study on behavior of non-tower building attached with tower”,
Volume:6, Issue:9 (IRJET,2019)
[4] Geetha, Kiran Kamath, “Seismic performance of a tall multi storey tower connected by a large podium”, Volume:8, Issue:2
(IJRTE, 2019).
[5] Ankan Kumar Nandi, Jairaj C, “Backstay effect of diaphragm in tall building”, Volume:9, Issue:3 (IJITEE, 2020)
[6] Kishan B. Champaneriya, Vishal B. Patel, Atul N. Desai, “Effect of backstay on tall structurewithpodiumstructure”,Volume
7, Issue 2, (IJARSCT 2021)
[7] Kush Shah, Hiren Desai & Dhara Shah, “Effect of backstay on 3B+G+20 storey RC building”, National Conference on
Structural Engineering , NCRASE – 2020, 21-22 August 2020, National Institute of Technology Jamshedpur, Jharkhand
India.

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