Paper presented to Joint Technical Sessions Between
Jamaica Institution of Engineers (JIE) and the
Caribbean Division of the Institution of Structural Engineers (UK)

 PLANNING BUILDINGS TO RESIST EARTHQUAKES

By Wayne Adams MIStructE, MJIE & Alfrico Adams FJIE FIStructE
Thursday, Dec.16, 2004

 

1.0 INTRODUCTION
  This paper will attempt to address structural form but will, where possible, reference prevailing code provisions as examples of the application of configuration assessment in the design process.
  The code referenced here is ASCE 7-02, which is very similar to the IBC 2000
  Other aspects of the structural design such as site effects, seismicity, structural system and material selection are outside the scope of this paper.
  A well planned structural configuration or form, in conformance with code requirements, is expected to achieve objectives such as
   

-
-
-

Predictable behavior
Simplicity of design & construction
Minimized structural cost for the required function
  The objectives of the architectural and other design team members however, may not primarily be these goals as aesthetics and specialized function, to name a few, may have equal or greater priority. Moreover, they may be in direct conflict with the goals of structural design.
  Early cooperation with other design team members, who have a stake in the structural form, can be very effective in realizing the desired objectives and achieving the cost/benefit trade offs that are acceptable to all. The structural engineer, being aware of the implications of building configuration on the stated objectives, is obliged to inform the team of any adverse impacts preferably at the earliest opportunity in the planning process.
  The current seismic provisions of current codes referenced in this paper, namely American Society of Civil Engineers 7-02 and International Building Code 2000 reflect these basic intents of structural design.
   

2.0

DESIGN CRITERIA
  The criteria for planning structural forms should take into account

a)
b)
c)

Function
Cost
Reliability

 

Function
    Function, in most cases, is prescribed by the architect & owner but will demand of the engineer a safe and serviceable structure. The code performance criteria previously
     

No collapse under a major earthquake; some damage under moderate earthquake and little or no damage from minor earthquakes.

This criteria will vary depending on the expected occupancy and use of the structure.

Drift is the criteria of performance most relevant to felt effects on a building’s occupants and protection of contents. Post disaster response capabilities and high occupancy classification are two typical reasons why a structure would attract stringent drift requirements for design.

Strength would dictate the load carrying capacity of the structure during the event.
Refer figures 1 and 2

 
 

Cost

   

Choices of structural form determine regular or irregular classification of structures, as defined in current codes.

Structural form will be shown to have direct influence on these code design forces. In most cases, the design force increases, from code prescribed penalties on irregular structures, lead to more expensive structural provisions by the engineer.

   

Reliability

    Serviceability and Strength are related to the probabilistic approach of design where the limit states provide the following end user requirements.
   
Strength – Load carrying capacity and collapse prevention

Serviceability

– repairability, damage to contents, post disaster response capability limits on deformation

These are satisfied to provide a code defined level of seismic resistance

Reliability requires that the structure behave predictably to satisfy these two limit states.
 

    Structural configuration has also been illustrated to influence aspects of reliability.
A conforming structural configuration is expected to have predictable behavior in the design event by virtue of distributed post yield behavior.
Alternatively, a non-conforming structure, because of concentrated yielding, is likely to have less predictable behaviour. It is expected to require significantly greater design effort on the part of the engineer to have the structure satisfy the limit states.

Seismic load compared to the other forms of structural loads e.g. dead, live, wind, snow etc. is well known for its high variability. A probabilistic definition is found in ASCE7-02 & IBC2000

From the perspective of reliability of the design load, the maximum considered earthquake has 2% probability of occurrence or exceedence in 50 years. This translates to a 2500 year return period. On a deterministic basis, the maximum ground motion possible from a tectonic feature defines the maximum considered earthquake.
The subject codes require that their Equivalent Lateral Force procedure utilize an event having 2/3 of the spectral response acceleration produced by the maximum considered earthquake.
 
3.0

STRUCTURAL CONFIGURATION

A few concepts in structural design are reviewed in defining structural configurations for resisting this relatively unpredictable load type. They list as follows:

-
-
-

Continuous load paths
Redundancy
Stable energy dissipating post yield behavior
   
   

Continuous Load Path

     

All members and connections are expected to have sufficient strength & flexibility (deformation capability) to transmit loads through successive supporting structural elements in a path to the ground without collapse or catastrophic failure.

     
    Redundancy

Systems that provide an alternative load path, in case of failure of the primary one, is considered as having a redundancy. A common example is dual systems consisting of lateral load resisting frames and shear walls.

 

Stable Energy Dissipating Post Yield Behavior

This post yield behavior involves sustained vertical load carrying capability under deformation from lateral loads. As the structure cannot resist all the loads from the design level earthquake, damage takes place. This results in yielding and energy dissipation.

 

Structural Forms

The general configuration of the structure in plan and elevation constitutes the most basic but arguably the most influential factors with respect to reliable structural response. In many cases these factors are as follows:
-
-
-
-
-
-
Symmetry – influences concentration of forces & stresses from asymmetric behavior
Length – Long buildings can have non-uniform response at different points along their length.
Aspect ratio – A low aspect ratio leading to increased prevents higher mode and other dynamic effects
Uniformity - Uniform structures will most likely have uniformity of stress form earthquakes.
Stiffness – Low stiffness will lead to a structure of low frequency long period that may be dynamically sensitive and also lead to excessive drift.

Refer Figures 3 & 4 for illustrations of these generalized categories.

 
4.0

CODE CLASSIFICATIONS OF BUILDINGS IBC & ASCE7-02

4.1 Seismic Design Category
Codes provide classification procedures for earthquake resistant design, which initially requires assignment to a Seismic Design Category.
These categories range from A to F where A attracts the least stringent force and detailing requirements and F attracts the most stringent.
 
The design category is dependent on:

a)
b)
c)

Occupancy/Use considerations
Site specific hazard depending on considerations of seismicity, local subsurface condition
Building height

 
4.2 Basic Seismic Force Resisting System

The codes define Basic Seismic Force Resisting Systems, which are identified by

-
-

Structural system of vertical and lateral load resistance eg. moment resizing frame, shearwalls.
Material eg. concrete, steel, masonry, wood, precast etc.

The further subdivision of the Seismic Force Resisting Systems into regular and irregular structures are made with their respective design implications.

 

4.3

Regular & Irregular Structures

The codes presume regularity and then tests the assumption against defined criteria that defines irregularity.

Irregular structures are less numerous and attract more rigorous analysis, larger forces and stricter detailing requirements and are subdivided in the code as:
Diaphragm Flexibility
Plan Irregularity
Vertical Irregularity
Refer to Figure 5
Refer to Figures 6 & 7
Refer to Figure 8
 

Structural behaviour of regular structures is characterized by distributed inelastic behaviour, which is predictable and allows the structure to maintain its vertical load carrying capacity throughout an earthquake event showing load deformation characteristic. Ref. Figure 9

 

Irregular structures on the other hand, have concentration of inelastic behavior in regions of the structure which are not only unpredictable but very rapidly exceed strength and deformation limits. This results in rapid reductions of the vertical load carrying capacity during earthquake events. Refer Figure 9

 
 

Code provisions to mitigate against the effects of irregularity consist of at least one of the following: -

-
-
-
-

Analysis Procedures
Penalties on design force
Prohibition of the structural irregularity
Stringent detailing requirements

 

4.4 Analysis Procedures for Categories D, E & F (per ASCE 7-02)
Structural Characteristic Least Stringent Analysis Procedure
     
1. Seismic Use Group (SUG 1)
Building of light frame construction
£ 3 storeys
(SUG-1 is standard occupancy)
Equivalent Lateral Force Analysis (ELF)
     
2 Other SUG-1 buildings
£ 2 storeys
ELF
 
3.

Regular Structures with
T < 3.5 Ts and all structures of
Light frame construction

ELF
 
4.

Irregular structures with
T < 3.5 Ts and having only
Plan irregularities:
Reentrant corner, diaphragm, out of plane or
Vertical irregularities:
In plane discontinuity and weak storey

ELF
   
5.

All other structures

Modal Response Spectrum Analysis
 

For conditions in Jamaica, 3.5 Ts is a building period of approximately 1.4 seconds. This would be above the fundamental period for most regular building under 10 storeys and not possessing a long period.

As a general qualitative statement it could be said that code provisions for analysis suggest a dynamic analysis for certain irregularities.

The ELF can be used for majority of building structures with notable exceptions.

-

Long period building structures

·

·

·

Many storeys
Slender
Very long span
 

-

Irregular structures of type

·
·
·
·

Torsion
Mass
Geometric
Stiffness
 

 Table 3 shows the analysis procedure requirements for IBC2000

 
4.5 CODE PENALTIES ON DESIGN FORCES FOR IRREGULAR CONFIGURATIONS

The descriptive of ‘penalty’ is a label adopted in this paper to represent Code prescriptions for undesirable building configurations.

 
4.5.1

Category B &C ‘Weak’ Storey Limit

Discontinuities in vertical system lateral strength capacity shall not occur in structures over 2 stories or 30ft

Weak storeys are not permitted

Where Columns are supporting discontinuous walls or frames
  E = Wo QE + 0.2SDS D
For regular structures E = rQE + 0.2SDS D
Where 1<r<1.5 defines redundancy factor
In this case of irregularity 2 > Wo > 3
     
Where E
W
QE
SDS
Earthquake effects
overstrength factor
effect of horizontal seismic forces
Design earthquake spectral response acceleration
     

An exception is if the structure can resist 75% of deflection amplified force

 

Cd x (V=CsW)

NB. Cd varies from 1 ½ to 6 ½
This would indicate a potential base shear increase of 50% to 550%
For higher category’s ie. D, E &F this irregularity is prohibited.
 

GENERAL PROVISIONS

 

Torsion
The amplification to forces determined by structural displacements results in a force amplification of up to 3 governed by the equation.

(     dm      )  < 3
( 1.2 davg )²

Diaphragm
Overstrength factors W are to be reduced by 1/2 for flexible diaphragms
Design forces for diaphragm connection to vertical elements are increased by 25%
Flexible Diaphragm require design for an out of plane load at
Fp = 0.8 SDS I Wp

Vertical Irregularity
MR i   < 60% MR i+1 or VR i < 60% VR i+1
Where MR & VR represent the moment and shear resistance of storeys.
The code requires that the engineer adjust strength to compensate for this deficiency.

   
4.5.2

Category E & F

The following irregularities are prohibited
Extreme Torsion Irregularity
Vertical Irregularity

-
-

Extreme Soft Storey
Weak Storey
Force comparisons are summarized for irregular structures in Tables 1 & 2
 
4.6

Redundancy

The codes now require that the effect of failure of one component, connection of the lateral force resisting system would have on overall structural stability, be assessed by the engineer.

 
5.0 COMMON IRREGULARITY CONDITIONS IN JAMAICA

Because of the inherent stiffness of concrete blocks, when they are used as interior non loading bearing partitions they may attract lateral load from an earthquake and create an unintended irregular configuration. We are not aware of studies that have been done on local infill masonry panels in Jamaica to determine to what extent they would withstand a design level earthquake but it is expected that building resistance to at least the initial high energy pulses, would be influenced by infill walls

The most common irregularities created are:

-
-
-

Plan stiffness irregularity
Vertical stiffness irregularity
Vertical strength irregularity

 

A specific example of the vertical strength irregularity, seen in the 1992 earthquake, was the Strong Beam – Weak Column Effect. Refer Figure 10
Exterior infill masonry was terminated at partial height of columns for creation of window openings.
Adjacent masonry and beam form the strong beam and the portion of column not adjacent to the masonry form the weak column.
Premature failure of the column in shear occurred large forces were attracted to the elements due to the stiffness of the beam and short column.

 
6.0 CLOSING REMARKS

Building Configuration has been seen to influence the structural design and construction cost through application of code requirements.

Code prohibitions for particular configurations of extreme Torsional irregularity, extreme soft storey and weak storey often requires the most creativity from the responsible engineer to offer viable alternations to the owner and the rest of the design team.