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BACKSTAY EFFECT
Published on March 3, 2020
Image: Yacoubin, 2017
Sandeep k 3 articles
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Design Engineer at Walter P Moore
The design of modern tall buildings, which includes buildings height
more than 50 meter, create an series of challenges that need to be
solved through consideration of scientific, engineering, and code of
standard specific to the modelling, analysis, and acceptance criteria
appropriate for these unique structural systems.
Due to large parking requirements, amenities and other building service
requirements, podium structure is become important part of these tall
structure. The base of a tall building is often referred as a podium. The
main components in a podium are roof and floor slab. They act like
diaphragms in shear and flexure, distributing forces to the vertical load
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carrying system in podium. The below grade diaphragms are connected
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to perimeter retaining walls which creates large and laterally stiff box.
Interaction between tower, below grade diaphragms and perimeter
walls causes backstay effect.
What do you mean Backstay Effect?
PEER/ACT 72-1 defines Backstay effects as the transfer of lateral
forces from the seismic-force resisting elements in the tower into
additional elements that present within the podium, typically through
one or more floor diaphragms. The lateral force resistance in the
podium levels, and force transfer through floor diaphragms at these
levels, helps a tall building resist seismic overturning forces. This
component of overturning resistance is referred to as the backstay
effect, based on its similarity to the back-span of a cantilever beam.
Figure 1: Backstay effect (Yacoubin, 2017)
Evaluation of backstay effects requires consideration of two seismic
load paths, both of which helps to resist overturning of the building.
One path provide resistance to overturning by the foundation directly
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below the lateral load resisting elements of the tower (Direct load
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path) and the second path is the backstay resistance provided by in-
plane forces in the lower floor diaphragms and perimeter walls
(Backstay Load Path).
Figure 2: Tall building structural system (ATC 72-1)
Overturning moment and shear force get shared by these two load paths
as shown in figure 2. It is quite easy to understand this phenomena but
it become complex to develop mathematical model to capture accurate
distribution of forces and evaluate backstay effect. The amount of
forces get transferred to backstay load path is depend upon various
parameter such as stiffness of diaphragms, stiffness of perimeter walls,
below and surrounding soil strata of perimeter walls. Modelling these
parameter become challenging. Therefore is it be more conservative to
ignore backstay effect and model tower as direct load path?
Before going further let’s understand this by considering lateral system
as simple cantilever beam fixed at end with loads applied at floor level
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as showing in figure 3a. But this analogy is only valid for tower system
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above grade level. More accurate analogy will include below grade
structure which behaves as back-span to cantilever and this can be
presented as overhanging beam with pin support at grade level as
showing figure 3b.
Figure 3a: Lateral Load Resisting System as Cantilever Model
Figure 3b: LLRS considering below grade as Propped Cantilever
Model
By comparing these two figure, is it fair to conclude that it be more
conservative to ignore backstay effect and model tower as direct load
path? With reference to these beam analogy, it is clear that backstay
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effect can create higher demands in some elements than it is predicted,
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such as shear demand is increased in main LLRS of tower in below
grade and therefore it becomes important to evaluate backstay effect. It
also observed that overturning moments in the core is reduced and
redistributed to perimeter walls. But realistic restraint at grade level is
may vary from very stiff to non-existence. It can be modelled as spring
as shown in figure 3c.
Figure 3c: LLRS considering below grade as spring support Model
Spring represents the combined stiffness of various elements such as:
· Diaphragms to core and Diaphragms to perimeter wall connection
· Diaphragms and perimeter walls stiffness
· Foundation stiffness
· Passive soil resistance against the perimeter walls.
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While considering these stiffness, uncertainty in the stiffness properties
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should be taken into account. Such as reduction in stiffness of concrete
elements due to cracking, openings and bond slip. Also ability of
connections to transfer of backstay shear. The difference in properties
of supporting soils below a foundation and perimeter wall can affect the
backstay effects. Passive soil component is relatively small to other
components and may be ignored in many cases.
Modeling of Structural Elements
Seismic analysis and design for backstay effects includes an assessment
of what portion of the overall building overturning is resisted by each
load path; and a design to provide adequate strength in the structural
elements of each load path.
All structural elements contributing backstay effect should be included
in model, with suitable stiffness properties considering multiple
scenario as given in table below.
Direct load path through the foundation include stiffness of pile or mat.
Backstay load path includes relative stiffness of the diaphragms and the
perimeter walls and vertical in-plane rocking resistance below the
walls, provided by the surrounding soil.
The backstay diaphragms should be modeled as semi-rigid diaphragm.
Any large opening should be modeled and appropriate meshing should
be done for accurate results.
Upper-bound and lower-bound stiffness properties should be
considered for each critical element. To determine governing design
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forces for the elements in each load path, assumptions can be grouped
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into two overall cases:
Case 1 – Upper bound backstay effect. A set of stiffness assumptions
that provides an upper-bound estimate of forces in the backstay load
path and a lower bound estimate of forces in direct load path. This case
will govern the design forces for the podium floor diaphragms and
perimeter walls, and the associated connections.
Case 2 – Lower bound backstay effect. A set of stiffness assumptions
that provides a lower-bound estimate of forces in the backstay load path
and an upper-bound estimate of forces in the foundation below the
tower. This case will govern the design forces for the tower foundation
elements.
Table: Stiffness Assumptions for Structural Elements
Backstay effects are not limited to tall buildings with podium at
grade. Similar effects can occur at any location over the height of a
building where lateral elements are discontinued or reduced in stiffness,
such as at building setbacks or stepbacks.
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There are few exceptions where backstay effect can be ignored, such as
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buildings without significantly increased seismic-force-resistance at the
base. Also by providing structural separation between the tower and
podium which include seismic deformation without transfer of lateral
forces of tower to podium, can avoid backstay effect.
Very tall structure with core wall structural system may have more
significant backstay effect than low rise and well distributed structural
system. Ignoring at or below grade structural system in analysis may
underestimate demand in few elements. Quick assessment will help
whether backstay effect should include in analysis or not. If so, in depth
study should be made in accordance with code of standard such as
PEER/ATC 72-1 or IS 16700-2017.
Reference:
“Modeling and acceptance criteria for seismic design and analysis of
tall buildings”, PEER/ATC 72-1:2010
“Criteria for structural safety of tall concrete buildings”, IS
16700:2017.
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Sandeep k 3 articles Following
Design Engineer at Walter P Moore
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Published • 2y
Modeling of tall building including below grade structural elements create series of
challenges, one of these challenge is backstay effect. This article will briefly explain backstay
effect, and its modeling aspect.
#tallbuildings #highrise #seismic #backstay #podium #structuralengineering
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Abhijeet Kulkarni •
You 2y
Country Director - Structures
Well written and explained.
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Sandeep k •
1st 2y
Design Engineer at Walter P Moore
Thank you Sir Abhijeet Kulkarni
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Chirag Padamwar •
1st 2y
Structural Engineer at WME Consultant
Sandeep k Do you have any case study where backstay effect considered in
analysis ??
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