__Paper presented to Joint Technical Sessions Between__

__Jamaica____ Institution of
Engineers (JIE) and the __

__Caribbean____ Division of the
Institution of Structural Engineers (____UK____)__

__Recent Code Developments for Earthquake Resistance__

**By Alfrico D. Adams F.I.Struct.E,
FJIE**

**May
27, 2004**

__DESIGN__

1.0 __Introduction __

The more mature among us engineers would have grown with the SEAOC Code, the Recommended Lateral Force Requirement of the Structural Engineers Association of California. The design philosophy of the SEAOC Code was that the buildings designed to the code should:-

(a) Resist Minor Earthquakes without damage

(b) Resist Moderate Earthquakes without structural damage, but with some non-structural damage

(c) Resist Major Earthquakes without collapse, but with some structural and non-structural damage

This philosophy has served us for at least three decades, but more and more it has become evident that rational means must be introduced to distinguish the treatment of special buildings from conventional buildings and ensure different levels of performance for different buildings based on characteristics such as their use and occupancy, the economic costs of the contents of the buildings and the need for some essential buildings to come unscathed through extreme events.

2.0 __Scope of this Presentation__

This presentation lists some of the developments in earthquake resistant design procedure, aimed at refining the procedure and at selecting values and methods of design which are consistent with the

(a) earthquake risk to be experienced

(b) the building performance desired for the particular level of earthquake chosen

It also gives an introduction to the procedure now being proposed by the International Building Code (USA).

3.0
__Development of Our Current Methods__

Quite early in the development of earthquake resistant design technology it became evident that conventional buildings could not resist the forces from the predicted major earthquakes if their structural frames were to remain in the elastic range.

Any attempt to ensure totally elastic behaviour would result in prohibitive costs of construction. The next step was therefore to accept inelastic behaviour and by appropriate selection of materials and detailing practices, to encourage well-conditioned behaviour, beyond the elastic range.

Fig. 3.1 and 3.2 show the relationship between lateral shear and deformation which illustrates the range of damage control.

The early codes
(e.g., SEAOC 1959) expressed the lateral force for design as __V=KCW.__K varied from 0.67 to 1.33 and
depended on the type of structural system It reflected among other things, the
redundancies in the building systems and the ductility of the element of the
system. __C__ was related to the natural period of the structure and was
inversely proportional to it.

Later introductions were:-

Z - A zone factor which allowed for varying
seismicities, suitable for locations other than

S - A soil factor to allow for the thickness and or firmness of the supporting soil. Later this was incorporated in to the C factor.

I - An importance factor which introduced higher design forces for buildings which housed critical post-disaster facilities, or housed large numbers of persons.

R_{w} - A Response Modification Factor which
represented damping, ductility and observed performance in earthquakes. This replaced the K-factor.

These
developments culminated in the expression; V = __ZIC__.W. in
the 1980’s

R_{w}_{}

and this form with minor variations appeared in the SEAOC, UBC and CUBiC Codes of that decade.

4.0
__Other Research and Development Efforts__

Concurrent with
all these developments research was pursued by the Applied Technology Council,

4.1 __Applied Technology Council__

The ATC3-06 which was parallel with the other codes, but somewhat more advanced used the expression:

V = C_{S}W

Where C_{S} = __1.2
AvS__ where Av represented mapped acceleration

RT^{2/3
}

4.2 __National Earthquake Hazard
Reduction Program__

This American research program introduced the concept of Building Performance Levels.

In working to develop guidelines for Seismic rehabilitation, it was recognised that some owners might desire to have better building performance than that implied by the traditional SEAOC methods, i.e. that buildings would:

· Survive moderate earthquakes with only minor repairable damage to the structure.

· Survive major earthquakes by ensuring life safety, but sustaining damage which may be beyond repair.

Furthermore, the recognition that non-structural damage often represented an even greater economic loss than structural damage, urged them to introduce performance levels which included non-structural considerations.

The following are definitions of the various Building Performance Levels; Structural Performance Levels, Non-structural Performance Levels and the Basic Safety Objective.

4.2.1 __Building
Performance Levels__

1. Operational Level

2. Immediate Occupancy Level

3. Life Safety Level

4. Collapse Prevention

Building Performance comprises structural & non-structural performance levels.

__Determination of S _{DS}__

From 1615.1.3

The 5% damped
design spectral response acceleration at short periods S_{DS} is
determined from:

S_{DS } = 2/3 S_{MS}

S_{MS }= Maximum Considered
Earthquake Spectral response acceleration for short period as determined in
Section 1615.1.2

Note: similarly S_{D1 }= 2/3 S_{M1}, where

S_{M1 }= Max. Considered
earthquake spectral response acceleration for 1 second period.

From
1615.1.2 S_{MS } =
F_{a} S_{S}

Where F_{a}_{ }= Site Coefficient from site classes definition
table 1615.1.1 and table 1615.1.2-1.

S_{S }= Mapped spectral acceleration
for short period

Typical values Site Class Definition

* Stiff Soil Profile* SPTN 15≤N≤50 – Site Class
Definition

Then for S_{S}_{ }=
0.5 or Interpolating for S_{S} = 0.4

F_{a}_{ }=
1.4

S_{S }= The mapped spectral
acceleration for short periods as determined in Section 1615.1

4.2.2 __Basic
Safety Objective__

This is met when a building can satisfy two criteria:-

1. The Life Safety Building Performance Level, i.e. Structural and Non-Structural Life Safety Performance Levels, for Basic Safety Earthquake 1.

2. The Collapse Prevention Performance Level, which only pertains to Structural Performance, assuming the stronger shaking from the less frequent Basic Safety Earthquake 2.

Figure 4.2.1 shows the hierarchy of Building Performance Levels.

Figure 4.2.2
illustrates the Rehabilitation Objectives and the relative costs of the
objectives contemplated by NEHRP in their attempt to develop systematic
guidelines for seismic rehabilitation of buildings in the

4.2.3
__Rationalising the Terms used in NEHRP Guidelines for Seismic
Rehabilitation__

__NEHRP – National Earthquake Hazard Reduction
Programme__

One phase of NEHRP was aimed at Seismic Rehabilitation of Existing Buildings. For this, a range of Rehabilitation objectives are listed:

- Operational Performance Level – Non-Structural facilities useable after event (e.g. elevator, escalator etc)

- Immediate Occupancy Level – say for E/q 50% Probability of exceedance in 50 years.

- Life Safety Level of Performance – 10% Probability of exceedance in 50 years

The Basic Safety
Earthquake 1 – BSE1 is the 10% probability of exceedance
in 50 years earthquake. This is
equivalent to the __Design earthquake spectral response acceleration__
parameters for short and long period earthquake.

BSE2 is the maximum credible earthquake (mapped MCE response).

BSE1 can either be 10% probability of exceedance, 50 year ground shaking or 2/3 of BSE2, whichever is smaller.

BSE2 can be taken as 150% of BSE1.

4.3 __Seismic Design by IBC 2000__

4.3.1 __Design
Accelerations__

Select Site Class A-F

When insufficient data is available Site Class can be considered to be D i.e. assuming unconfined shear strength ≤ 1000 pst.

S_{MS }= Maximum considered short
period (0.2 sec.) spectral acceleration.

S_{M1 }= Maximum considered long
period (1 sec.) spectral acceleration.

The mapped values are:

S_{S} = Mapped spectral response
acceleration for short period (0.2 sec).

S_{1} = Mapped spectral response
acceleration for long period (1.0 sec).

The above sets of
values are linked by a coefficients F_{a} and F_{v} derived
from the site class and mapped acceleration.

F_{a} applies to the short
period values.

Hence the maximum considered spectral accelerations are:

SMS = Fa SS

S_{M1} = F_{v} S_{1}

**TABLE 4.3.1.1 - TYPICAL SITE CLASS DEFINITIONS**

SITE
CLASS S |
SOIL PROFILE NAME |
SPT (N) |
SHEAR WAVE VEL. or SHEAR STRENGTH |

A B C D E F |
Hard Rock Rock Very Dense Soil Soft Rock Stiff Soil Profile Soil See IBC |
Not Applicable Ditto N>50 15≤N≤50 N<15 See IBC |
Refer to IBC |

4.3.2. __Design
Spectral Response Acceleration Parameters__

4.3.2.1 __S _{DS}
and S_{D1}__

These are based on 5% damping.

S_{DS } = 5% damped design spectral
acceleration at short periods

S_{D1} = 5% damped design spectral
acceleration at 1 second period

S_{DS} = 2/3 S_{MS} – where S_{MS
} = maximum considered E/q spectral response –
short period.

S_{D1} = 2/3 S_{M1} – where S_{M1} =
maximum considered E/q spectral response acceleration for 1 second
period.

4.3.2.2 __General
Procedure Response Spectrum - S _{a}_{}__

The design
spectral acceleration is S_{a}_{}

1. For period less than
T_{o}

the formula S_{a}
is 0.6 __S _{DS}__ T + 0.45S

T_{o
}

T_{o} is taken =
0.2 S_{D1}/S_{DS}

2. For periods between T_{o}
and T_{s}

S_{a}
=
S_{DS}, where T_{s}
= S_{D1}/S_{DS}

3. For period greater
than T_{s }

S_{a} = S_{D1}/T

Where T = Fundamental period (in seconds) of the structure.

Fig. 4.3.2.2 illustrates these relationships.

4.3.3
__Seismic Design Category__

Buildings are
classified into Seismic Use Groups I to III varying with the importance of the structure
(see table 4.3.3.1), Table 4.3.3.2 gives Seismic Design Category from A-D
depending on the Use Group and the design acceleration S_{DS} or S_{D1.}

4.3.4. __Design
Requirements__

__Seismic Category A__

For Seismic Category A, a nominal Minimum Lateral Force of

F_{x} = 0.01w_{x} is assigned.

Certain irregularities are ignored for one and two storey buildings for Category A and also for B&C.

4.3.5 __Seismic
Category B&C__

The equivalent
lateral force procedure V = C_{S}W is permitted in Seismic Category
B&C for reinforced concrete buildings 2-storey and over.

For 3-storey light frame buildings or for one and two storey building in

Use Group 1
Simplified Method (V = __1.2S _{DS}__ W) is permitted.

R

4.3.6 __Plan
and Vertical Structural Irregularities__

These are classified and the building assigned to various more stringent design categories e.g. D, E &F depending on the nature and extent of the irregularity.

Included for some of these categories is a requirement for dynamic model analysis. In addition, certain types of construction are not permitted in these more stringent categories.

4.3.7 __Seismic
Design Procedure – In Accordance with IBC 2000__

Say building exceeds two storeys and is of Reinforced Concrete construction.

Refer to Section 1617-IBC 2000

EARTHQUAKE LOADS – Minimum Design Lateral Force and Related Effects

The loads computed here are for use in the Load Combinations set out in Section 1605.

e.g. Strength
Design: 1.2D + 1.0E + (f_{1}L or
f_{2}S) or 0.9D + (1.0E or 1.3W)

L = Live Load

S = Snow Load

f_{1}^{ }= 0.5 exception for certain cases

f_{z} = 0.2 for most roofs

It should be noted: It seems obvious that the earthquake loads computed are ultimate loads, unlike the values previously computed for SEAOC and CUBiC.

__To Compute Seismic Load Effect ‘E’__

E = ρ Q_{E} + 0.2S_{DS}D

This is where seismic and gravity loads are additive, where:

E = combined effect of horizontal and vertical earthquake loads.

Ρ = a reliability factor based on system redundancy (Note: For Seismic category A-C, ρ = 1.0)

Q_{E} = The effect of horizontal
seismic forces.

S_{DS}
=
The design spectral response acceleration at short periods obtained from
Section 1615.1.3 or Section 1615.2.2.5 of IBC

D = The effect of dead load

Where the effects of gravity and seismic load counteract each other plus changes to minus.

__Analysis Procedures__

(For light framed structures up to 3 stories and for other types up to 2- stories in Seismic Use Group 1). The SIMPLIFIED ANALYSIS PROCEDURE 1616.6.1 is allowed as indicated in 1617.5 of IBC

i.e. V = __1.2
S _{DS}__

R

S_{DS}
=
The design elastic response acceleration for short period as determined
in accordance with: 4.3.2.1 herein

R – Response Modification Coefficient

W – Effective Seismic Weight

V – Seismic Base Shear

(Note: the above
appears to include Group 1 for __all__ Seismic Categories.)

__For Other
Structures in Seismic Category B or C__

The analysis procedures are to be in accordance with Section 1617.4 of IBC where:

V = C_{S}W

C_{S}
= The
seismic response coefficient determined in accordance with 1617.4.1.1 i.e. S_{DS} = 2/3 D_{MS}

R = Response Modification Factor

I = The Occupancy Importance factor determined in accordance with 1616.2

C_{S }need not exceed __S _{D1}__; S

(R/I) T

Nor be less than
C_{S} = 0.444 S_{D}

(Note: There are other rules for Categories E & F)

__Period Determination__

Fundamental
Period Ta = C_{T}h_{n}^{3/4}

CT = Building Period Coefficient

= 0.035 for moment resisting frames of steel resisting 100% of the required seismic force.

(Note: Metric coefficient is 0.085)

= 0.030 for moment resisting frames of reinforced concrete resisting 100% of the required seismic force.

(Note: Metric coefficient is 0.073)

= 0.030 for eccentrically braced steel frames

= 0.020 for all other building systems.

(Note: Metric coefficient is 0.049)

h_{n} = The height (ft or m) above the base to the
highest level.

Alternatively for up to 12 stories and minimum storey heights = 10ft (3m)

T = 0.1N, where N = number of stories.

4.4 __Example__

(a) Consider RC Frame Building 5-storeys.

Choose Life Safety Performance level for this Example.

Consider Mapped Accelerations

Say, Short Period Acceleration SS = 0.75

Say, Long Period Acceleration S1 = 0.3

(b) Select Site Class from SPT N value between 15 & 50

Say Site Class C

Then F_{a} = 1.1

F_{v}
=
1.5

(c) Max considered earthquake accelerations

S_{MS} =
F_{ax} S_{S}
= 1.1 x 0.75 =
0.825

S_{M1} =
F_{v} S_{1}
= 1.5 x 0.3 = 0.45

(d) Design Spectral Response Acceleration 5% Damped

S_{DS} =
2/3 S_{MS} = 2/3 x 0.825
= 0.55

S_{D1} =
2/3 S_{M1} = 2/3 x 0.45
= 0.3

(e) Choose Seismic Design Category

Assume Seismic Use Group 1

- For Short Period

If S_{DS}
between 0.33g and 0.5g therefore Seismic Category C

- For 1 Second Period

S_{D1}
exceeds 0.2g therefore Seismic Category D

Hence Category D controls as the more severe of the two.

The Seismic Category determines:

(1) What irregularities, if any, must considered

(2) What analysis procedure must be used

__For Seismic
Category D__

The requirements would be:-

1. Plan structural irregularities and vertical structural irregularities would all need to be allowed for.

2. The Equivalent Lateral Force procedure must be used for design

V = C_{S}W where
C_{S} = S_{DS}/(R/I)

Upper and
lower limitations of C_{S} are set.