UNIT IV
Seismic Design Requirements and Methods. RC Buildings – IS Code provision for
Vertical Irregularities – Mass Irregularity Torsional Irregularity- Design Lateral Force,
Base Shear Evaluation – Lateral Distribution of Base Shear, Behaviour of Unreinforced
and Reinforced Masonry Walls
A. Seismic Design Requirements:- Seismic design is a crucial aspect of structural
engineering, especially in regions prone to earthquakes. The primary objective of seismic
design is to ensure that structures can withstand the forces generated by seismic events,
minimizing damage to property and reducing the risk of injury or loss of life. Here are
some key requirements and principles of seismic design:
i) Building Codes and Regulations: Seismic design is governed by building codes and
regulations specific to the region where the structure is located. These codes provide
minimum standards and requirements for seismic resistance based on the seismicity of
the area.
ii) Site Specificity: Seismic design takes into account the geological and geotechnical
characteristics of the site, including soil types, ground motion parameters, and local
seismic hazard assessments.
iii) Structural Analysis and Modeling: Engineers use advanced analytical techniques
and computer simulations to model the behavior of structures under seismic loading. This
involves analyzing the response of the structure to different seismic forces and ensuring
that it remains stable and safe during an earthquake.
iv) Strength and Ductility: Seismic-resistant structures are designed to have sufficient
strength to resist seismic forces without collapsing and adequate ductility to deform and
dissipate energy during an earthquake without failing catastrophically.
v) Redundancy and Continuity: Seismic design incorporates redundancy and continuity
in structural elements to ensure alternative load paths and prevent sudden structural
failure. This can involve designing redundant lateral load-resisting systems and providing
continuity in load transfer mechanisms.
vi) Lateral Load-Resisting Systems: Buildings and structures are equipped with lateral
load-resisting systems such as shear walls, moment frames, braced frames, or
combinations thereof to resist horizontal forces generated by earthquakes.
vii) Foundation Design: Foundation systems are designed to adequately support the
structure and mitigate the effects of ground shaking. This may involve deep foundations,
pile foundations, or other specialized techniques to improve soil stability and reduce
settlement.
viii) Seismic Retrofitting: For existing structures, seismic retrofitting techniques may be
employed to enhance their seismic resistance. This can include strengthening existing
structural elements, adding supplemental lateral bracing, or improving foundation
systems.
ix) Performance-Based Design: In some cases, performance-based design approaches
are used, where the performance objectives of the structure under seismic loading are
explicitly defined, and the design is optimized to achieve these objectives.
x) Quality Control and Construction Practices: Quality control measures are essential
during construction to ensure that the design specifications are followed accurately.
Proper construction practices, material quality, and inspection procedures are critical for
ensuring the effectiveness of seismic design.
By adhering to these requirements and principles, engineers can design structures that
are resilient and capable of withstanding the forces generated by earthquakes, thereby
reducing the risk to life and property during seismic events.
Seismic Design Methods:-
1. Linear method
i) Static method – Equivalent static method
ii) Dynamic – Response Spectrum method
2. Non-linear
i) Static method – Pushover analysis method
ii) Dynamic – Time-History analysis
B. IS Code provision for Vertical Irregularity: – The vertical irregularity may be defined
as per Table 6 of IS 1893:2016. The vertical geometric irregularity shall be considered to
exist, when the horizontal dimension of the lateral force resisting system in any storey is
more than 125% of the storey below.
IS Code provision for Mass Irregularity:- The mass irregularity may be defined as per
Table 6 of IS 1893:2016. Mass irregularity shall be considered to exist, when seismic
weight of any floor is more than 150% of that of floors below.
IS Code provision for Torsional Irregularity:- The Torsional irregularity may be defined
as per Table 5 of IS 1893:2016. In torsionally irregular building, when the ratio of
maximum horizontal displacement at one end and the maximum horizontal displacement
at other end is more than 1.5.
C. Behaviour of Unreinforced and Reinforced Masonry Walls
The behaviour of Unreinforced and Reinforced Masonry Walls are given in clause no. 7.9
of IS 1893:2016. In RC building with moment resisting frames and unreinforced masonry
(URM) infill walls, variational of storey stiffness and storey strength shall be examined
along the height of the building considering in-plane stiffness and strength of URM infill
walls.
We need to understand 4 aspects of the masonry infill
1. Stiffness – In plane stiffness is very high, out of plane it is very less
2. Strength - In plane strength is very high, out of plane it is very week
3. Behaviour - Due to high strength and stiffness, infill wall influences the dynamic
properties of structures such as natural period and mode shapes. Natural
frequency will decrease and it will attract more design base shear.
When the wall fails or cracks, immediately the wall action becomes frame action.
Eventually the bending moment decreases and axial forces increases.
4. Stress resultant – It is obtained from the result of any model.
If the stiffness of any storey is less than the storey above is called soft storey
If the strength of any storey is less than the storey above is called week storey
Reinforced masonry wall - A reinforced masonry wall is a structural element used in
building construction that combines masonry units (such as bricks, concrete blocks, or
stone) with additional reinforcement to enhance its strength, stability, and resistance to
various loads and stresses. The key points about reinforced masonry walls:
i) Composition: Reinforced masonry walls are typically composed of masonry units
(bricks, blocks, or stone) bonded together with mortar. In addition to the masonry units
and mortar, these walls incorporate reinforcement, such as steel bars or mesh, to improve
their structural performance.
ii) Reinforcement: The reinforcement in reinforced masonry walls provides tensile
strength and ductility, which are typically lacking in traditional masonry construction. Steel
bars or mesh are embedded within the mortar joints or placed within cavities in the
masonry units to enhance the wall's resistance to tensile and shear forces.
iii) Design and Construction: Design considerations for reinforced masonry walls
include determining the appropriate type and spacing of reinforcement, as well as
ensuring proper detailing for load transfer and structural integrity. Construction techniques
involve careful placement of reinforcement during wall assembly and adherence to
engineering specifications for mortar strength and bond.
iv) Applications: Reinforced masonry walls are commonly used in various building types,
including residential, commercial, and industrial structures. They are particularly suitable
for applications requiring high load-bearing capacity, such as retaining walls, shear walls,
and load-bearing exterior walls.
v) Advantages: Reinforced masonry walls offer several advantages, including durability,
fire resistance, and thermal mass properties. By combining the inherent strength of
masonry construction with the added reinforcement, these walls can withstand significant
loads and resist deformation under adverse conditions.
In summary, reinforced masonry walls provide a robust and versatile solution for structural
support in building construction. By incorporating reinforcement with traditional masonry
materials, these walls offer enhanced performance and reliability in withstanding various
loads and environmental factors.
