تقنية أجهزة تبديد الطاقة الزلزالية Dampers

Bridge Energy-dissipating devices

· في الماضي اشترط ان يكون المنشأ مرناً بشكل كامل اثناء حدوث الزلازل

· ثم استحدث مفهوم المطاوعة Ductility الذي يسمح للمنشأ ان يتعرض للتمفصل اللدن (المطاوعة). هذا الأسلوب في التصميم أكثر اقتصادية بشرط التأكيد على المحافظة على مستوى جيد من الأمان

· مقاربة أخرى لتعزز الأداء الزلزالي هي زيادة قدرة النظام على توهين الذبذبات increase system damping بتضمين أجهزة تبديد الطاقة في بعض المواقع ضمن الجسر. مما ينتج عنه زيادة في الأمان والمردود على المدى الطويل

· من هنا فان الكثير من الدول تعمل على التخفيف من حدة الكوارث الزلزالية بالمحافظة على الجسور القائمة بإضافة مبددات الطاقة التي تعمل على تحسين أداء هذه الجسور عند الاحداث الزلزالية

The seismic protection system used for this bridge comprises a combination of non-linear fluid viscous dampers and pot bearings. The bearings were designed and installed according to the previous design.

Reference AASHTO LRFD 2012 Bridge Design Specifications 6th Ed (US).pdf

1.3—DESIGN PHILOSOPHY

1.3.3—Ductility

The structural system of a bridge shall be proportioned and detailed to ensure the development of significant and visible inelastic deformations at the strength and extreme event limit states before failure.

Energy-dissipating devices may be substituted for conventional ductile earthquake resisting systems and the associated methodology addressed in these Specifications or in the AASHTO Guide Specifications for Seismic Design of Bridges.

For the strength limit state:

η_D ≥ 1.05 for nonductile components and connections

= 1.00 for conventional designs and details complying with these Specifications

≥ 0.95 for components and connections for which additional ductility-enhancing measures have been specified beyond those required by these Specifications

For all other limit states:

η_D = 1.00

C1.3.3

The response of structural components or connections beyond the elastic limit can be characterized by either brittle or ductile behavior. Brittle behavior is undesirable because it implies the sudden loss of load-carrying capacity immediately when the elastic limit is exceeded. Ductile behavior is characterized by significant inelastic deformations before any loss of load-carrying capacity occurs. Ductile behavior provides warning of structural failure by large inelastic deformations. Under repeated seismic loading, large reversed cycles of inelastic deformation dissipate energy and have a beneficial effect on structural survival.

If, by means of confinement or other measures, a structural component or connection made of brittle materials can sustain inelastic deformations without significant loss of load-carrying capacity, this component can be considered ductile. Such ductile performance shall be verified by testing.

In order to achieve adequate inelastic behavior the system should have a sufficient number of ductile members and either:

· Joints and connections that are also ductile and can provide energy dissipation without loss of capacity; or

· Joints and connections that have sufficient excess strength so as to assure that the inelastic response occurs at the locations designed to provide ductile, energy absorbing response.

· Statically ductile, but dynamically nonductile response characteristics should be avoided. Examples of this behavior are shear and bond failures in concrete members and loss of composite action in flexural components.

· Past experience indicates that typical components designed in accordance with these provisions generally exhibit adequate ductility. Connection and joints require special attention to detailing and the provision of load paths.

· The Owner may specify a minimum ductility factor as an assurance that ductile failure modes will be obtained. The factor may be defined as:

The ductility capacity of structural components or

connections may either be established by full- or large- scale testing or with analytical models based on documented material behavior. The ductility capacity for a structural system may be determined by integrating local deformations over the entire structural system.

The special requirements for energy dissipating devices are imposed becaus of the rigorous demands

placed on these components.

1.3.4—Redundancy

Multiple-load-path and continuous structures should be used unless there are compelling reasons not to use them.

For the strength limit state:

η_R ≥ 1.05 for nonredundant members

= 1.00 for conventional levels of redundancy, foundation elements where  already accounts for redundancy as specified in Article 10.5

≥ 0.95 for exceptional levels of redundancy beyond girder continuity and a torsionally-closed cross-

section

الكود الكندي

4.7.4.2.4 Column shear and transverse reinforcement

The factored shear force, Vf , on each principal axis of each column and concrete pile bent shall be as specified in Clause 4.4.10.4.3.

The amount of transverse reinforcement shall not be less than that determined in accordance with Clause 8.9.3.

The following requirements shall apply to the plastic hinge regions at the top and bottom of the column and pile bents:

(a) In the plastic hinge regions, when the minimum factored axial compression force exceeds 0.10fc’Ag , Vc shall be as specified in Clause 8.9.3. Vc shall be taken as zero when the minimum factored axial compression force is zero. For values of minimum factored axial compression force between zero and 0.1fc’ Ag , linear interpolation may be used to determine the value of Vc.

(b) The plastic hinge region shall be assumed to extend from the soffit of girders or cap beams at the top of columns to the top of foundations at the bottom of columns. This distance shall be taken as the greatest of

(i) the maximum cross-sectional dimension of the column;

(ii) one-sixth of the clear height of the column; or

(iii) 450 mm.

The plastic hinge region at the top of the concrete pile bent shall be taken as that specified for columns.

At the bottom of the pile bent, the plastic hinge region shall be considered to extend from three times the

maximum cross-section dimension below the calculated point of maximum moment, taking into account

soil-pile interaction, to a distance of not less than the maximum cross-section dimension, but not less

than 500 mm above the ground line

Pier 4 and 5, the tallest along the viaduct, are provided with fixed pot bearings so that in the absence of any seismic protection device, all the seismic loads are concentrated at these locations and their capacity exceeded by the seismic demand.

The addition of viscous dampers at all the expansion piers provided two effects: first of all, the piers were called to withstand a longitudinal seismic force (force distribution) and second, their action dissipates the earthquake-induced energy introduced by the ground motion. As a result, the longitudinal computed action resulted lower than the available capacity, thus providing a demand/capacity ratio lower than one.

In the transverse direction, the bridge was deemed to have sufficient capacity to withstand the new level of design acceleration due to the good distribution of the seismic load as well as intrinsic pier strength.

In order to estimate the actions on the structure, a step-by- step non linear analysis was performed using three artificially generated accelerograms compatible with design spectra (AASHTO, Soil Type II).

The maximum results were used for design purposes. A parametric analysis was performed to evaluate damper characteristics. Their force capacity was estimated to be 500 kN each, considering the installation of one unit at each expansion pier, so that a total of 12 units were provided for this project.

The maximum seismic design displacement was evaluated as ±68mm. Thus, in view of the fact that dampers should be designed to also allow for thermal deck expansions, devices providing a ±200mm stroke were considered.

Figure 10 shows a drawing depicting the dampers as installed at each expansion pier.

The structure comprises two continuous concrete decks (33m+14@50m+33m) of the pre-stressed concrete box type. The piers are characterized by height ranging from 7 to 12 m.

The original bearing layout comprised pot bearings of the fixed type, at the central and tallest piers.

In order to protect the substructure from a seismic event having a peak ground acceleration of 0.154g, the designer again suggested adding a damping system along the longitudinal axis of the bridge.

Figures 12 and 13 shows a photo of the bridge is an a damper installed.

Pier 8, located in the middle of the bridge, is equipped with fixed pot bearings so that in the absence of any seismic protection device, all the seismic loads are concentrated at these locations and their capacity exceeded by the seismic demand.

The addition of viscous dampers at some of the expansion piers provided two effects: first of all, the piers are called to withstand the longitudinal seismic force (force distribution) and second, their action dissipates the earthquake-induced energy introduced by the ground motion. As a result, the longitudinal computed action results lower than the available capacity and thus provides for a demand/capacity ratio

lower than one.

In order to estimate the actions on the structure, a step-by- step non linear analysis was performed using ten artificially generated accelerograms compatible with the design spectra (AASHTO, Soil Type II, PGA 0.154g). Average of results was used for design purposes. The maximum seismic design displacement was evaluated as ±40mm. Thus, considering the fact that dampers should be designed to also allow for

thermal deck expansions, devices providing for a ±120mm stroke were considered. Their force capacity was estimated as 850 kN each and one unit was installed at the expansion Piers P5-P7 and P9-P11, so that a total of 12 units was provided for the above project

A two-span, skewed, cast-in-place prestressed concrete bridge with an outrigger bent is examined. The bridge is located in a highly seismic area of Southern California. It is observed that dampers alleviate the torsional movement and reduce the transverse and longitudinal movements of the superstructure.

INTRODUCTION

It is common practice today that structural engineers do not design their structures to remain fully elastic during a seismic event as in the past.

في الماضي اشترط ان يكون المنشأ مرناً بشكل كامل اثناء حدوث الزلازل

// energy dissipation by preforming plastic hinging

Instead, they allow structures to experience plastic hinging in certain areas that are carefully detailed for this particular reason.

Thereby, energy dissipation is achieved through hysteretic damping.

This concept of ductile مطاوع لدن design leads in general to more economical designs provided that a certain level of safety is still maintained.

ثم استحدث مفهوم المطاوعة Ductility الذي يسمح للمنشأ ان يتعرض للتمفصل اللدن (المطاوعة). هذا الاسلوب في التصميم أكثر اقتصادية بشرط التأكيد على المحافظة على مستوى جيد من الامان

// energy dissipation by energy dissipation devices

Another approach to enhance seismic performance is to increase system damping by introducing energy dissipation devices in certain areas within the structure. The objective here is to have structures meet code strength requirements without the devices and reduce displacement demands through increased damping by utilizing energy dissipation devices and thus, improve seismic performance. This in turn results in safer and more cost effective structures in the long run.

مقاربة اخرى لتعزز الاداء الزلزالي هي زيادة قدرة النظام على توهين الذبذبات increase system damping بتضمين اجهزة تبديد الطاقة في بعض المواقع ضمن الجسر. مما ينتج عنه زيادة في الامان والمردود على المدى الطويل

Several types of energy dissipation devices (passive, active, or semi-active) have been proposed or applied to various structures throughout the world during the past three decades. In this study, the use and effects of fluid viscous (hydraulic) dampers are investigated in a bridge application by performing nonlinear time history analysis.

Various applications of passive viscous dampers in bridges have been reported in the past. The first use of viscous damper stoppers in bridge application was performed on the Tokyo Metropolitan Expressway, Tokyo, Japan in 1962 (Kawashima 1992). It was a five span, 116.9 m long, 16.7 m wide, four-cell box girder structure. The dampers were installed between the superstructure and substructure at both abutments and at the top of the piers at bents three and four. Yamadera and Uyemae (1979) also reported the use of viscous damper stoppers in at least ten bridges in Japan after the Tokyo Metropolitan Expressway including the Kaihoku Bridge, a five span 285 m long one-cell box girder bridge that suffered no damage during the strong earthquake of Miyagi-ken-Oki in June of 1978.

As another bridge application of dampers in the United States, Brown (1995) referred to a retrofit project on Pennsylvania S.R. 29 over the Schuylkill River in Montgomery County. The bridge, which is a five span, four girder structure with a central fixed pier at the center, is upgraded to sustain earthquake forces by placing dampers that work as lock-up devices on two of the four girders at the three expansion joints.

The damping force that the devices produce is the result of the different pressure across the piston head and is given by the equation F=C*V exp(n), where,

F is the damping force,

C is a constant, called the viscous damping coefficient

V is the piston rod velocity,

n is a predefined coefficient in the range of

0.4 to 2.0. When n=1, the damper is linear.