Comparative of Study the Design Spectra Defined by Various Seismic Codes

Comparative of Study the Design Spectra Defined by Various Seismic Codes

K Hadar Elmi1, Erdal Coskun2

1- Institute of Graduate, Structural Engineering Program stanbul Kültür Üniversitesi 2- Department of Civil Engineering stanbul Kültür Üniversitesi.

Abstract:- Seismic codes provide to achieve desired performance that structure withstands the earthquake loads with minimum requirements. Many standard building codes developed and imposed to minimize damage and to ensure life safety and economic loss. In seismic regions, seismic design is very conscious. However, the lack of seismic design codes, engineers depends on the international guidelines. Therefore it is imperative to compare the international design codes once the building codes developed. To mitigate the hazard of earthquakes is the primary purpose used for seismic codes. The main objective of this study is to compare the various seismic codes defined by design spectra, and some fundamental points are defined generally included the elastic design spectra, seismic hazard specification, and soil classification. In this study, a particular site has chosen to achieve the comparison codes and different soil classes considered for the design. The result obtained by the design codes compared and highlighted by the similarity and difference among them, such as base shear, interstory drift, and displacement and the dynamic linear analysis used to implement the design to achieve the main goals. The analysis is performed in SAP 2000. V20.

Keywords: Comparative Seismic Codes, Elastic Design Spectra, Soil Classification, Seismic Hazard Specification and Base Shear

INTRODUCTION

Turkey is considered the most active seismic regions in the world, surrounded by different plates. In the past two decades, two strong earthquakes happened in Turkey (Kocaeli and Düzce), remarkably over 51,000 buildings were damaged or collapsed, and more than 18,000 have died [1] [2]. However, after that strong earthquake are revised the code and produced another seismic code which more confidential the previous building code. The material used for the design structures were poor and also not probably designed for structures as well as the magnitude of the earthquake is the major problems that cause this heavily casualties both for structures and human. In Van earthquake (2011), more than 604 lost their lives, and 1301 people got injured [3]. The past studied for the earthquake recognized that lateral loads are essential than the designing structures for a gravity load only. The standard seismic codes used for a force-based design for elastic analysis, while a designed an earthquake resistance design, however, the structure experienced more displacement during the strong earthquakes and expected to behave for nonlinear and energy dissipating is essential to account for that the displacement-based design developed later. The main seismic codes established to reduce the catastrophic

of earthquakes and to preserve structures can resist the earthquake loads. In the prediction of future earthquakes, it is imperative to define the seismic loads to recognize that structures withstand the earthquake loads. For a short period, earthquake induces forces that are significant structure able to withstand without damage or collapse. Structures should be able to resist the small magnitude with minor damage to reduce maintenance costs. Building codes provide to achieve desired performance that structure resists the earthquake loads with the least requirements. Many standard building codes developed and imposed to minimize damage and to ensure life safety and economic loss. In seismic regions, seismic design is very conscious. However, the lack of seismic design codes, engineers depends on the international guidelines. Therefore it is imperative to compare the international design codes once the building codes developed [4]. To evaluate the similarity and dissimilarity among seismic codes, many researchers have compared several seismic codes Tariq Nahass (2017) compared Saudi building code and UBC 1997 code to assess the resemblances and differences between these codes and methods of analysis done by modal response spectra and equivalent lateral force methods [5]. The response spectrum originated by Boit and advanced by Housner, and the theory of response spectrum earthquake engineers uses to estimate the peak response of a single degree of freedom system [6] [7] [8].

SEISMIC HAZARD SPECIFICATION

The hazard is defined in terms of a single parameter, and Eurocode uses reference peak ground acceleration on type A ground agR. The countrys national Annex found this parameter and derived for seismic zonation. The National Authorities chosen by reference peak ground acceleration for each seismic code corresponds to the reference return period. Eurocode recommended the return period for non- collapse performance level 475 year corresponding to 10% probability of exceedance in 50 [9]. United States Geological Survey (USGS) prepared by seismic maps are used to construct design spectra corresponding to the maximum considered earthquake (MCER) and earthquake design base. The probabilistic MCER earthquake proposed has a probability of exceedance of 2% in 50 years (2475 reoccurrence period) and the earthquake design base represented by 10% years in 50 years for return period 457 years. The new Turkish seismic code defined four different ground motion levels, DD-1: 2% probability of exceeding

in 50 years, and recurrence is 2475, DD-2: 10%, probability of exceeding in 50 years, and recurrence is 475, DD-3: 50% probability of exceeding in 50 years, and recurrence is 72 and finally DD-4: 50% probability of exceeding in 50 years, and recurrence is 43 [10].

SOIL CLASSIFICATION

The response of structures, local soil conditions becomes a vital role. After conducting a research study for earthquakes, Loma Prieta 1989, Northridge 1994, and Kobe 1994 and other earthquakes, seismic building codes recognized the importance of local site conditions. The rock and soil at a site have particular features that can ominously amplify the incoming earthquake motions traveling from the earthquake source [11]. The structures with a similar construction type, it was detected that

structures with deep soils are more damaged than the structures with founded by on rock. Building codes define different categories into account the site effects. The most useful parameter for site classification is Vs.30 and describes the average shear wave velocity of the upper 30 m of the soil profile. According to the EC8 code, VS.30 parameter uses to classify the soil along with NSTP and plasticity index PI and undrained shear strength CU. Eurocode expresses five ground types (A to E) along with two soil factors. Turkish code considers similar to the Eurocode and uses Vs.30 to classify the soil classes [12]. The new Turkish code defines a five soil classes varying A to E. The American code ASCE7-16 considers both undrained shear strength and Vs.30 to classify the soil classes. The soil classes ASCE7-16 varying A to E. Table 1 shows the soil classes according to these seismic codes.

Table 1 Soil Classification

ELASTIC DESIGN SPECTRA

The response spectrum is a widely accepted method for earthquake resistance design, and most seismic codes use. Earthquakes induce ground trembling that commonly epitomized by the form either acceleration or displacement response spectra. Magnitude, soil conditions, duration, and epicentral distance are earthquake parameters and influenced by the form and the amplitude of response spectra [13]. The elastic shape defined by Eurocode provides expressions in both vertical and horizontal components of seismic actions. The control points TB, TC, and TD and of the soil factor defined by the elastic spectrum shape depends on the ground type. The soil factors defined in Eurocode depends on the damping factor and ground type. The design spectrum derived from the elastic response spectrum based on no collapse adopting by

modification factor q, which Eurocode called by behavior factor. The map prepared by USGS to estimate the seismic hazard covered the whole United States. Probabilistic and deterministic hazard spectra both considered. Once the seismic map provides the spectra values SS and S1 for short and long periods respectively, this information will use to determine the response spectrum curve, and soil amplification factors (Fa and Fv) used to modify the response spectrum curve. The design spectrum SMS and SM1 represented by short and long periods and are computed by 2/3*MCER. The points TO and TS define the acceleration control region, and the points TS and 1.0 sec describe the velocity control region, and the corner TL defines where the displacement control region started, however, figure 1 describes the elastic design spectrum according to the ASCE7-16. The new Turkish seismic

codes define spectral ordinates, and the seismic hazard map provides the spectral acceleration for short and long period SS and S1 for T=0.2 s and T=1 s respectively, to construct the elastic design spectra. The local soil coefficient used to

modify the response spectrum to get spectral design acceleration SDS and SD1. The corner periods TA and TB found from the related ratios of SDS and SD1.

Table 2 The elastic ordinates defined by the seismic codes compared TSC-2018 ASCE7-16 EC8-2004

0 T TA

0 T TA

T

0 T TB

Sae(T) = (0.4 + 0.6

T

T

T

) SDS

A

Sae(T) = (0.4 + 0.6 ) SDS

T

T

A

Se(T) = (1 + ( 2.5 1))

T TB Sae(T) = SDS

TB T TL

T TB Sae(T) = SDS

TB T TL

B T TC Se(T) = s 2.5

TC T TD

SD1

S (T) =

S (T) =

TC

s 2.5 )

Sae(T) =

SD1 T

ae T

TL T

e ( T

TD T 4s

TL T

SD1TL

S (T) =

S (T) =

TCTD

s 2.5

( ) SD1TL

ae T2

e T2

Sae T = T2

BUILDING MODEL

A particular location has chosen to achieve a better comparison among seismic codes, and Avclilar in Istanbul has chosen. The regular building has four bays, both longitudinal and transverse direction (20 m), and the geometry of the building presented by Fig 1. The total height of the building is 18 m and the building designed by a high seismic zone with medium soil. The ground motion designed a moderate or level DD-2 as described by Turkish

code, which describes a 10% probability of exceeding in 50 years, corresponding to a return period of 475 years for very stiff soil. The compressive strength assumed M25 and reinforcement steel S420, the structural elements such as beams (30 x 50) mm, columns (50 x 50) mm, and thickness of slab 150 mm used for design, respectively. Table 3 describes all the details about the preliminary data used for the design.

Table 3 Preliminary Data

Figure 2: Building layout

RESPONSE SPECTRUM

Figure 3: 3D Model

acceleration. Most seismic codes use acceleration elastic

The response spectrum is a widely accepted method for earthquakes resistance design. Boit developed, and Housner improved it later. This method plots the maximum response of a single degree of freedom corresponding to the natural period in terms of acceleration, velocity, and

response spectra. For a given earthquake, the design spectra considered the peak values of the performance of each variation mode is a computed modal combination method. The peak values achieved by each mode combined either SRSS or CQC. This study, the CQC, used to

combine the maximum modal contributions, and the modal damping assumed should be 5% for all modes. The modal response spectrum in modes should not be less than 90% the mass of the structure (EC8-2004), while American code considers 85% the modal analysis in base shear force [17]. The design implemented by using SAP 2000.v.20.

BASE SHEAR

Base shear is a fundamental parameter that controls the performance of structures by given ductility class. Base shear is the maximum expected lateral force that earthquake induces the base of the structure as know that earthquakes generate large loads that more than expected loads. The elastic force designed by earthquakes is more

economical and not indicated the actual behavior of the structures as know that even small magnitudes causes a small deformation, however, most codes us ductility levels to reduce the elastic design force and deformation caused by an earthquake can sustain a period before the collapse. This study was compared three seismic codes to evaluate the difference and similrity among them. However, the main factors that base shear depends on are the peak ground acceleration and reduction factor. This study shows that the Turkish code is more base shear than other codes, although the difference between the highest and lowest base shear shows 30%. This difference comes from the different peak ground acceleration considered the design and reduction factors consider for the design.

Base Shear X- Direction

Base Shear X- Direction

EC8

2251

EC8

2251

ASCE7-16

2397

TSC-2018

ASCE7-16

ASCE7-16

2397

TSC-2018

ASCE7-16

TSC-2018

EC8

0

TSC-2018

EC8

0

Base Shear Kn

Base Shear Kn

2937

2937

1000

1000

2000

2000

3000

3000

Figure 4: Base shear comparison seismic codes

INTERSTORY DRIFT RATIO

The prevention and awareness for non-structural and structural damages for structures, the past decades become prominence for the building located by seismic areas, exclusively subsequent reflection on the catastrophic left by seismic actions. Story drift defined by the percentage among the two floors divided by the height of the floor. It is vital and essential to describe the story drift. Building codes provide guidelines to reduce the potential damage to

the structures caused by serve earthquakes. Nevertheless, the valuation and design of structures should be established the earthquake demand for inelastic deformations. Under low to medium for intensity seismic actions, the building codes limited the damage. The seismic design emphasizes to control the damage of non-structural damage by limiting the inter-story drift. The results found building codes demarcated are safe when compared to the allowable drift that seismic codes limit.

Interstory drift

7

6

5

4

3

2

1

0

EC8

ASCE7-16 TSC-2018

Interstory drift

7

6

5

4

3

2

1

0

EC8

ASCE7-16 TSC-2018

0

0.01

0.02

Interstory drift ratio

0.03

0.04

0

0.01

0.02

Interstory drift ratio

0.03

0.04

Story

Story

Figure 5: Interstory drift ratio

CONCLUSION

In this study, the seismic codes USA, TSC-2018, and EC8 have been compared; the primary purpose of the comparison is to investigate the similarity and difference among them. Many standard building codes developed and enforced to minimize damage and to ensure life safety and economic loss. Seismic codes designed and set a minimum requirement to mitigate the hazards of earthquakes. In seismic regions, seismic design is very sensitive. Nonetheless, the absence of seismic design codes, engineers depends on international guidelines. Therefore it is imperative to compare the international design codes once the building codes developed. In this study, a particular site has chosen to achieve the comparison codes and different soil classes considered for the design. The result obtained by the design codes compared and highlighted by the similarity and difference among them, such as base shear, inter-story drift, and displacement and the dynamic linear analysis used to implement the design to achieve the main goals. The analysis is performed in SAP 2000. V20.

• The design spectra are the main difference among these seismic codes. However, codes describe for ground motion design on different levels. The Turkish seismic code describes four different ground motions for a return period of earthquake DD-1: 2% probability of exceeding in 50 years, and return is 2475, DD-2: 10%, probability of exceeding in 50 years, and recurrence are 475, DD-3: 50% probability of exceeding in 50 years, and recurrence is 72 and finally DD-4: 50% probability of exceeding in 50 years. EC8 defines the seismic hazard at 10% probability exceedance in 50 years for return period 475 years as the reference of the seismic action. ASCE7-16 defines the return period 2475 years, the probability of 2% seismic inputs exceeded in 50 years, matching the MCER.

• The results obtained show that the Turkish code is more base shear than other codes, although the difference between the highest and lowest base shear shows 30%. This difference comes from the variation peak ground acceleration considered the design and reduction factors used for the design.

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