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TOPIC OVERVIEW
TOPIC OVERVIEW
This module provides an overview of the seismic design of timber structures in earthquake-prone regions, starting from the basic concepts of earthquake engineering and concluding with the application of capacity-design principles to timber structures.
TABLE OF CONTENTS
TABLE OF CONTENTS
INTRODUCTION
INTRODUCTION
Preface
Preface
This module introduces the main topics related to the seismic design of timber buildings.Â
Its primary objective is to provide an overview of the fundamental principles governing the seismic design of timber structures, summarizing the entire design process from the basic concepts of seismology to the structural detailing strategies required to ensure adequate seismic energy dissipation.
The content first presents the main topics of seismology, structural dynamics, and earthquake engineering that define the seismic design of buildings. These principles are then specifically adapted to the design of multi-storey timber buildings, highlighting the distinctive mechanical behaviour of timber structural systems and the crucial role played by connections in controlling the seismic response. Diffrently from traditional construction materials (e.g. reinforced concrete, steel), in fact, timber members are designed to behave elastically during strong earthquakes, while the plastic behaviour and energy dissipation concentrated in mechanical connections. As a result, the design of timber joints is one of the most critical aspects of seismic design.
The page further introduces a summary of Seismic Force-Resisting Systems (SFRSs) for timber buildings, Particular attention is devoted to behaviour factors, the definition and design of dissipative zones, capacity design principles, and the detailing requirements necessary to achieve ductile behaviour and reliable energy dissipation during seismic events.Â
Prerequisites
Prerequisites
To fully benefit from this module, readers are expected to possess a basic understanding of timber engineering and structural mechanics. In particular, the following background knowledge is recommended:
Structural design of timber buildings under gravitational loads, including the analysis and design of beams, columns, floors, and walls subjected to permanent and variable actions.
Design and verification of dowel-type timber connections, including the fundamental failure mechanisms and load-carrying.
Timber structural systems, with an understanding of the behaviour and load transfer mechanisms of common timber construction systems, such as light-timber frame, cross-laminated timber (CLT), and post-and-beam structures.
Distribution of lateral loads to structural elements, including the principles governing the transfer of horizontal actions through floor diaphragms and the lateral load-resisting system to walls, frames, or bracing elements.
Fundamentals of structural dynamics, including the concepts of mass, stiffness, damping, natural vibration periods.
CONTENT TOPICS
CONTENT TOPICS
Principles of seismology and seismic-hazard assessment
Principles of seismology and seismic-hazard assessment
Topics:
What is an earthquake?Â
The nature of earthquakes (common mechanisms of earthquake sources, focus, epicentre, seismic waves, Plate tectonic theory, earthquakes worldwide and earthquakes in Europe).
Measurmentes and characteristic of Earthquakes (magnitude, intensity, Duration, PGA, Response Spectrum).
Seismic Risk vs Seismic Hazard.
Summary:
Earthquake is a unpredictable natural phenomenon, results of a sudden release of energy.Â
The nature of earthquakes can be explained by means of the plate tectonic theory.
Magnitude and Intensity are the most widely accepted indicators for the size of an earthquake.Â
The severity of an earthquake may be analyzed by some parameters (PGA, Duration, Response Spectrum,…).
Earthquakes do not kill people, unsafe structures do!!!!
Principles of Structural Dynamics
Principles of Structural Dynamics
Topics:
Dynamic response of a Single Degree of fredoom (SDOF) system under a seismic acceleration (Equation of motion, natutal paeriod. Duhamel's integral, Internal force and pseudacceleration).
Acceleration Response spectrum.
Dynamic response of a Multi Degree of fredoom (MDOF) system under a seismic acceleration (Equations of motion, mode shapes, natural periods, combination of effects (SRSS)).
Summary:
A structure can be considered as a filter that modifies the input signal (ground mootion) into an output signal (displacement, acceleration, velocity).Â
The response of the structure depends on the characteristcs of the ground motion and on its dynamic behaviour (natural period and damping). A ground motion can be very hard for some structures but not so for some others!
The response spectrum is a very useful tool to evaluate the maximum relative displacement or maximum absolute acceleration of structure subjcected to a ground motion.Â
A structure can be represented by a N-DOF system, whose response can be calculated as the linear combination of N independant S-DOF systems.
Fundamentals of Seismic Engineering for new Buildings
Fundamentals of Seismic Engineering for new Buildings
Topics:
Anelstic response of a SDOF system (ductility and Newmark hypothesis).
Design response spectrum and behaviour q-factor.
Capacity based design.
Regularity of buildings (in-plane regularity, in-elevation regularity).
Types of Seismic Analyses (Lateral Force method, Modal response spectrum analysis, Push-over analysis, Time History analysis).
Advanced seismic approaches (additional damping and isolated structures).
Summary:
According to a traditional seismic design, the structure damage allows to reduce the seismic forces, since the building can disspate the input energy by means of the hysteretic energy.
The design seismic force can be obtained dividing the elastic force by the q-factor, taking into account the ductility of the structure and structural redundency.Â
In order to achieve a high level of ductility the capacity design approach must be applied. Ductile failure modes should be enagaged while brittle ones must be prevented.
Advanced seismic approaches can be used to avoid the damage of structure reducing the input energy (isolation system) or increasing the viscous damping dissipated energy.
Seismic Engineering applied to Timber Structures: brittlness of wood and ductility of timber connections
Seismic Engineering applied to Timber Structures: brittlness of wood and ductility of timber connections
Topics:
Lightness and brittlness of timber structural elements.
Timber connections as dissipative zones.
Mechanical behaviour of connections under cyclic loads.
Summary:
Timber members have to be regarded as behaving elastically. Due to brittlness of timber energy dissipation cannot occurd in timber members.
The seismic energy can be dissipated in ductile laterally-loaded dowel-type connections.
The energy dissipation of mechanical connections is related to their mechanical failure which can involve the timber embedment strength and the yielding of fastener.
Pinching effect is observed in cyclic tests of connections.
Small diameter fasteners are prefferred to engage the fommation of plastic hinges.
Seismic Force Resisting Systems (SRFSs) in timber buildings: design concepts, behaviour factors, dissipative zones and structural details
Seismic Force Resisting Systems (SRFSs) in timber buildings: design concepts, behaviour factors, dissipative zones and structural details
Topics:
Ductility classes according to Eurocode 8.
Structural types and behaviour q-factors.
Ductility of connections.
Safety verifications.
Dissipative zones in timber buildings.
Summary:
According to European Standards (i.e. Eurocode 8), Earthquake-resistant timber buildings shall be designed in accordance with one of the following concepts: low-dissipative structural behaviour or dissipative structural behaviour.
Five categories of structural types are defined in Eurocode 8.
Values of the baheviour factors are defined for medium- (DCM) and high-ductility (DCH) classes
Connections shall be able to deform plastically for at least three fully reversed cycles, without more than a 20% reduction of their resistance, at a static ductility ratio of 4 for DCM and 6 for DCH.
Simplfied rules are introduced by Eurocode 8 as an alternative to experimental tests to satisfy the requirements in terms of ductility.
In light-frame timber builkdings ebergy dissipation is primarly reached in sheathing to framing connection. In CLT buildings dissipative zones are located in hold-downs, angle brackets and vertical joints in segmented shearwalls.
The capacity-based design approach for timber buildings
The capacity-based design approach for timber buildings
Topics:
Levels of capacity design: fastener, connection, structural system.
The weak-link concept.
Protecting the brittle elements (overstrength concept).
Summary:
Failue modes of laterally-loaded dowel-type connections should engage the formation of plastic hinges.
Brittle failure modes of steel plates and anchor bolts have to be protected. Partial nailing in hold-downs can promote yielding of fasteners.
Soft storey mechanism should be prevented by uniform ratios of capacity-demand along the height of the building.
📄 ASSIGNMENT - Seismic design of a 3-storey CLT buildingÂ
📄 ASSIGNMENT - Seismic design of a 3-storey CLT buildingÂ
Objective
Objective
The objective of this assignment is to apply the principles of seismic design to a 3-storey Cross-Laminated Timber (CLT) building in accordance with the provisions of Eurocode 8 (EN 1998-1). The readers are expected to develop a complete conceptual and preliminary structural design, identify the seismic force-resisting systems, evaluate the seismic response of the building, and verify the main structural components and connections.
A medium ductility class (DCM) is requested.
The assignment aims to strengthen the understanding of the seismic behaviour of timber buildings, the role of timber connections in energy dissipation, and the application of capacity design principles.
Building description
Building description
Geometry:
Number of storeys: 3
Storey height: 3.0 m
Total building height: 9.0 m
Plan dimensions:
X-direction: 8.0 m
Y-direction: 10.0 m
Structural system:
Roof and Floor elements are made with 5-ply CLT panels. The Thickness of the panels will be selected depending on the floor span and gravitational loads.
Wall elements are made with 5-ply CLT panels with thickness equal to 120 mm at the first floor and 100 mm at the second and third floor.
Propriatory hold-down are located at the bottom edge of each CLT shearwall to prevent uplift while angle brackets are uniformly dsitributed to resist shear laods.
The wall layout will be chosen in both direction (X and Y) by the reader consistently with gravitational load path in order to ensure regularity in-plane and in-elevation.
Vertical Loads:
The permanent load at roof level can be assumed equal to 1 kN/m2.
The permanenent load at floor levels is equal to 2 kN/m2.
Snow load is assumed to be equal to 0.8 kN/m2.
Live load at floor level is equal to 2 kN/mq (Residential occupancy, Category A).
Seismic load:
Ground type C
Design ground acceleration: ag = 0.15g
Importance class: II
Damping ratio: 5%
behaviour factor q: 2.5 (according to Italian Building Code)
Design tasks
Design tasks
Structural layout with specific attention to the location of shearwalls belonging to the SFRS.
Calculation of the total mass of the building.
Calculation of the natural period.
Determination of the base shear load due to seismic action. The equivalent lateral force procedure may be adopted.
Distribution of seismic loads to shearwall.
Design and verification of connections of one shearwall.
Application of capacity based design principle to a hold-down.
REFERENCES
REFERENCES
Villaverde R. , Fundamental Concepts of earthquake engineering, CRC Press 2009
Di Sarno L. – Elnashai A.S., Fundamentals of earthquake engineering ,WILEY 2008
Chopra A.K. – Dynamics of structures, PRENTICE HALL2007
Penelis G.G. , Kappos A.J. – Earthquake -resistant concrete structure, TAYLOR AND FRANCIS, 1997
Christopoulos C., Filiatrault A. – Principles of passive supplemental damping and seismic isolation, IUSS Press 2006
EN 1998-1 (Eurocode 8-1): Design of structures for earthquake resistance- General rules, seismic actions and rules for building
Blass H. J. - Sanhaas C. - Timber Engineering, Principle of design - KIT, Chapter G.3 Earthquake-compliant structural details - https://www.ksp.kit.edu/books/1340/files/ee4f3533-48c6-4390-b80e-6ffe7341f8ed.pdf
Gerhard Fink, Robert Jockwer, José Manuel Cabrero - Holistic Design of Taller Timber Buildings, Springer Tracts in Civil Engineering - Open Access, https://link.springer.com/book/10.1007/978-3-032-02098-7Â
Mirra M. et al. Lateral-Load-Resisting Systems (LLRSs) in Taller Timber Buildings, Holistic Design of Taller Timber Buildings, Springer Tracts in Civil Engineering, Open Access, https://link.springer.com/chapter/10.1007/978-3-032-02098-7_19Â
Naumavoski et al., Seismic Protection Technologies, Holistic Design of Taller Timber Buildings, Springer Tracts in Civil Engineering, Open Access, https://link.springer.com/chapter/10.1007/978-3-032-02098-7_20Â
Izzi M., Casagrande D. et al. - Seismic behaviour of Cross-Laminated Timber structures: A state-of-the-art review, Engineering Structures Volume 170, 1 September 2018, Pages 42-52, https://doi.org/10.1016/j.engstruct.2018.05.060Â
FPInnovations, Modelling Guide for Timber Structures, Chapter 10: Seismic Response Analysis, https://web.fpinnovations.ca/modelling/Â
Sandoli A., D'Ambra C., Ceraldi C., Calderoni B., Prota A. Sustainable cross-laminated timber structures in a seismic area: Overview and future trends, Applied Sciences (Switzerland), https://doi.org/10.3390/app11052078Â
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