Proceedings of the Ninth Pacific Conference on Earthquake Engineering Building an Earthquake-Resilient Society

14-16 April, 2011, Auckland, New Zealand

Paper Number 188

Seismic strengthening of the Synchronous Condenser Transformer Building at Haywards

K. Grinlinton, D. Wood, F. Ayan and P. Clayton Beca, Wellington, New Zealand.

ABSTRACT: As part of its HVDC Pole 3 project, Transpower commissioned Beca to provide the detailed assessment and strengthening design for the existing Haywards Synchronous Condenser Transformer building. Earthquake effects up to a 2500 year return period event were considered. Existing schematic studies had concluded that a downslope sliding displacement of the whole building was possible, resulting in a preliminary strengthening scheme that included extensive anchoring into the slope. Geotechnical investigation and subsequent time history analysis suggested a much more limited extent of sliding. It was also necessary to derive response spectra at the mounting levels of the existing synchronous condensers and the proposed new transformers to limit damage to plant and connections in a severe earthquake. A range of damping levels was considered to estimate the effect of both building and plant damping on seismic acceleration response.

A strengthening solution evolved which balanced the needs of the transformer manufacturer (seismic accelerations limited to achievable levels) with a preference to avoid significant work in confined spaces and/or ground anchoring. The paper describes the time history analysis undertaken and the advantages derived in relation to the building strengthening decision making and the resulting scheme.

1 INTRODUCTION

1.1 Context

Transpower New Zealand Ltd (Transpower) commissioned Beca Carter Hollings & Ferner Ltd (Beca) to prepare detailed design recommendations (DDR) for seismic strengthening of the existing Condenser Transformer Building (CTB) at Haywards Sub-Station. Output response spectra were also required at the mounting levels of the transformers and condensers to allow for appropriate seismic design of this equipment.

Transpower’s Seismic Policy (2009) sets the policy for the seismic performance of Transpower’s assets by reference to AS/NZS 1170 (2002) and IEEE 693:2005. It specifies the required performance at two levels:

Serviceability-Level Earthquake: Equivalent to AS/NZS 1170 Serviceability Limit State 2 (SLS2), this level has an annual probability of exceedance of 1/500. Essential equipment and facilities are required to continue to operate without interruption, with unimpaired operational ability, and with retention of full electrical safety factors during and following SLS2 shaking.

Design-Level Earthquake: This level has an annual probability of exceedance of 1/2500. Essential equipment and facilities are required to have damage limited to that which can be repaired as soon as reasonably practicable following earthquake shaking at this level, having consideration for the criticality of the site. If replacement is available, in the form of spares or an alternative supply, the annual probability of exceedance may be reduced to 1/1000.

A base assumption of this study is that the repair of transformers in the aftermath of a severe

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earthquake would be unlikely to be possible within a short or satisfactory period of time, and therefore that new transformers must be capable of withstanding the 1/2500 year accelerations experienced at the mounting level. Bushes and internal connections must not be damaged to the point of failure in such an event.

The building has therefore been assessed at the Design-Level (1/2500 year) earthquake only.

1.2 Building and site description

The CTB is located in Switchyard A at the Haywards Substation, off SH58, Lower Hutt. The existing 2-storey building was designed by the then Ministry of Works in the early 1960s. The building is sited on the edge of a cut/fill platform on a slope consisting of shallow soils over weak rock. It consists of a transformer level at the top of the slope, and a lower condenser level with a cable reticulation basement underneath. It is a 67 m long structure divided longitudinally into five structurally separate bays by contraction joints One typical bay is shown in Figure 2. In the transverse direction large buttress walls at the back and top level of the building provide most of the lateral support.

The 12 existing single-phase transformer units supported by the buttress walls are being replaced with four transformers, and the modelling takes account of this new configuration. The four synchronous condenser units are to remain in their current positions.

An earlier study, known as the Solutions Study Report (SSR) concluded that extensive stabilisation works, including ground anchors, ground beams and piles, were required to prevent instability during the design-basis earthquake shaking. This was based on an elastic design approach. An initial review by Beca suggested that dynamic analysis may show these stabilisation works to be unnecessary.

Figure 1 –Transverse cross-section of building

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Figure 2 –3D view of typical bay of base building

Further geotechnical investigations, comprising two additional machine boreholes north and south of the building and a series of cored holes through the basement to investigate the rock level below the building, were completed.

Based on the results of these investigations, a ground model was prepared and a limit equilibrium/simplified Newmark sliding-block approach was used to provide an initial assessment of overall stability and a check on the subsequent dynamic analysis.

2 MODELLING AND ANALYSIS

2.1 General approach

The structural analysis was completed using two models.

A two-dimensional (2D) transverse model with non-linear soil spring supports was built using SAP2000, and analysed using time-history earthquake records. The analysis allowed for the potentially critical down-slope movement of the building. Geotechnical investigations showed that the foundation key engages the bed-rock which was modelled by soil springs.

A response spectrum at the lowest (basement) level of the building was derived from the 2D time-history analysis of the soil-building structure interaction.

A three-dimensional (3D) modal response spectrum model was built for assessment of the elemental building actions. This analysis utilised the response spectrum obtained from the 2D transverse model, which was input to the 3D model, to determine the seismic strengthening design requirements for the building retrofit. For analysis purposes, one typical bay of the structure was modelled.

Finally, output response spectra were derived from the 2D model at the mounting levels of the transformer and condenser units after including the new structural properties from the retrofitted building design. The output response spectra are to be used by others for the seismic design of the new transformers.

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For the derivation of the global seismic response, the application of the equipment masses at their mounting levels (rather than at their centres of mass) was considered to be a satisfactory approximation because the equipment masses are a low percentage of the overall structural mass (2.5 % for transformers, and 4.3 % for condensers).

2.2 Input parameters

Soil stiffnesses and strengths were derived from a ground model assembled from soil investigations. These were converted into bi-linear varying stiffness soil springs for use in the dynamic analysis to simulate elasto-plastic soil behaviour.

Non-linear horizontal and vertical springs were defined using the soil stiffness information provided by the geotechnical engineer. The type of link used had a multi-linear plastic hysteresis loop, and the links were selected as one or two-joint depending on their orientation. In many cases, multiple links were needed at each node. Springs representing the key stiffnesses were modelled at the end-nodes along the base of the structure.

Non-linear time-history analyses could then be undertaken using the SAP2000 software. The integration time-step for displacements and forces in all of the soil springs was 0.01 seconds, with properties of the springs being re-determined at each time-step.

Springs representing shear friction and passive pressure were modelled separately, so that some joints have four springs attached to them. Time-history analysis runs were also made for half and double the spring stiffnesses, without modification of the force plateau values. The effect on resultant spectra was not sufficiently large to warrant further consideration. Refer to section 5 herein.

Figure 3 – Labelling of spring supports

Figures 4 and 5 below are typical of the SAP2000 input required for defining the horizontal and vertical non-linear springs. The input parameters shown are those calculated from the values provided by the geotechnical report and the base area corresponding to each node, for support locations defined in Figure 3.

Figures 6 and 7 below are analysis output showing the force displacement graph for representative springs. The vertical spring response shown in Figure 7 (Link 21) indicates that uplift has occurred. This uplift is a maximum of 3 mm. In the SAP2000 model, the horizontal springs are still acting when this uplift occurs, which is not the situation in reality, but is the best modelling representation that could be achieved. Due to the minimal uplift (3 mm maximum) and small period of time over which this occurs, together with the passive resistance due to keying into rock, it is considered that this

A

B

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approximation does not have a significant impact on the final results.

Figure 4 - Typical SAP2000 input for a horizontal

shear spring located at node A

Figure 6 - Typical SAP2000 input for a horizontal

shear spring located at node A showing hysteresis

response

Figure 5 - Typical SAP2000 input for a vertical soil spring

located at node C

Figure 7 - Typical SAP2000 input for a vertical soil spring

located at node C showing elastic response

2.3 Time-History Analysis

The 2D model was analysed using non-linear time-history analysis techniques. This was necessary because of the non-linear nature of the soil springs, and to enable output response spectra to be plotted for the transformer and condenser support levels. Analyses were completed three times in the

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horizontal direction - once for each chosen earthquake record in one of the two orthogonal directions given, in combination with vertical inputs. Three response spectra (one for each record) were obtained and a recommended design spectrum for use with the 3D model was derived from them.

Five earthquake records were provided by GNS along with the scaling factors required to match them to the seismic design level of shaking for this site. Three of these records were used, namely: Mexico Cale (1985), USA El Centro (1940), and Mexico Union (1985). GNS’s calculations of the variation of the response spectra of each scaled record with the design spectrum, using the root mean square method over the relevant natural period ranges, indicated that they are a good match in each case.

As no vertical scale factor was provided, a conservative approach was taken for the vertical inputs by adopting the larger of the K1 scale factors provided by GNS for each earthquake record.

The time-history analyses were run with initial conditions set to the results of the self-weight case so that the vertical soil springs were pre-loaded in compression before the time-history analyses were run.

Proportional damping was used with 5 % damping specified at natural periods of 0.4 and 0.1 seconds (first and third-mode periods obtained from the modal analysis). The changes in the structure were tracked every 1/100th of a second for 40 seconds to ensure that the maximum response of the structure to the earthquake was captured.

3 STRENGTHENING SCHEME

The input response spectrum derived from the base response of the 2D time-history model was scaled up slightly to reflect the longer period of the slightly softer 3D model, because the period of the first mode response for the structure lies in a zone of the response spectrum where accelerations increase with period increase. Fixing the 2D model at all base nodes and running a modal analysis produced natural frequencies and mode shapes very similar to those from the 3D model - confirming that the two models are closely comparable. The period of the first natural mode of the fixed-base 2D model was 0.052 seconds, compared with 0.057 seconds computed from the 3D model. This resulted in a seismic base acceleration of 1.9 g for the 3D model- slightly increased from the 2D time-history model.

3.1 Transverse

The CTB is relatively open in the transverse direction. At a typical bay, the lateral actions at the transformer level are primarily resisted by the six existing buttress walls directly beneath the transformer floor in each structural bay (corresponding with each switchroom). The existing endwall at one end of each switchroom is not sufficiently reinforced to offer useful resistance, and serves only as fire protection and partitioning.

The six buttress walls have an overturning stability issue under the design-level earthquake. This results in excessive rocking actions of the structure, and appears to cause undesirable amplification of motions in the transformers.

The adopted strengthening scheme adds two new full-width transverse concrete walls, one at each side of the existing contraction joints; 8 walls (4 pairs) in total. The existing fire or partition wall can be incorporated into the new wall. The provision of a shear wall at each end of each switchroom improves the torsional response of each bay and helps to obviate the tying together of bays across existing contraction joints. The wall is to be extended into the buttress zone behind the switchrooms to control rocking. Our dynamic analyses show that the rocking action is reduced by approximately 50 % by the addition of these walls.

New columns and strengthening of the existing columns are required to carry the new wall actions to the ground.

In the event that the horizontal building accelerations at the level of the transformer slab exceed the limitations of the equipment or their supports, a further strengthening option is available. This involves the addition of vertical ground anchors installed along the northern side of the building beneath the

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external wall under the transformers at grid A in Figure 8. These anchors would further minimise the rocking action of the building and reduce corresponding accelerations.

3.2 Longitudinal

Strengthening in the longitudinal direction is minimal when compared to that previously proposed in the SSR. Pounding is not considered to be a damaging event given the very low predicted building displacements in the longitudinal direction, so linking across the existing contraction joints has been dispensed with.

Figure 8 – Typical cross-section showing strengthening

Figure 9 – Plan layout of strengthening at switch room level

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4 RESULTS AND COMPARISONS

From the 2D time-history analyses, the maximum horizontal movement of the building was computed to be 90 mm, with the building essentially returning to its original position after the earthquake. The majority of this displacement occurs at the ground interface. The lack of permanent displacement was expected after it was confirmed that the majority of the building was sitting in rock. We believe this level of temporary displacement will be acceptable - based on our understanding of the nature of the interconnections between the CTB and adjacent structures.

It is therefore considered that anchoring of the building to the slope is unnecessary to achieve acceptable performance, but care will need to be taken to ensure that any in-ground services coming into or leaving the building are able to accommodate this movement at the interface between the building and the ground.

A key feature of time-history analysis results is the ability to plot output response spectra. Vertical and horizontal response spectra at both the condenser and transformer base levels have been determined for the existing structure prior to any strengthening works, and also for the structure after strengthening works have been accounted for.

Figure 10 shows the horizontal response at transformer level from three earthquakes and Figure 11 shows the recommended response spectrum derived from these. Figures 12 and 13 correspond for the as-strengthened building.

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T (s)

Spec

tral

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eler

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)

Cale

El Centro

Union

Figure 10 - Horizontal response spectrum at transformer-base level for un-strengthened case, 5% damping

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El Centro

Union

Figure 12 - Horizontal response spectrum at transformer-base level for strengthened case, 5 % damping

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tral A

ccel

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(g)

Figure 11 - Smoothed horizontal response spectrum at transformer-base level for un-strengthened case, 5% damping

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Figure 13 - Smoothed horizontal response spectrum at transformer-base level for strengthened case, 5 % damping

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Figures 14 - 17 provide a comparison between the output response spectra at the transformer and condenser slab levels for the un-strengthened base building, for the strengthened building, and the underlying GNS input rock spectrum.

The amplification due to structure response and sub-structure interaction can be seen, and is most significant for the horizontal response of the new transformers.

At the condenser level, there is noticeably less amplification over the bed-rock acceleration, because most of the effect is derived from rocking of the walls above this level.

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tral A

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(g)

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Un-strengthened Base Building

Strengthend Building

GNS Input Rock Spectrum

Figure 14 - Summary of horizontal response spectra at transformer-base level, 5 % damping

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Strengthened Building

GNS Input Rock Spectrum

Figure 16 - Summary of horizontal response spectra at condenser-base level, 5 % damping

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Strengthened Building

GNS Input Rock Spectrum

Figure 15 - Summary of vertical response spectra at transformer base level, 5 % damping

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Un-strengthened Base Building

Strengthened Building

GNS Input Rock Spectrum

Figure 17 - Summary of vertical response spectra at condenser base level, 5 % damping

5 SENSITIVITY TO INPUT PARAMETERS

5.1 Comparison with building anchored at rear

Figure 18 shows the reduction in peak acceleration that can be achieved if rear anchors were used for additional strengthening. This reduction would not be as great at the condenser level.

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Strengthened Design

Strengthened Design with Rear Anchor

Figure 18 – Comparison with rear-anchored building - horizontal response spectrum at transformer base (5 % initial damping input & output, Mexico Cale record)

5.2 Soil effect – pinned vs sprung

Figure 19 shows the significant difference in the transformer level spectrum when soil springs are modelled as opposed to assuming a fixed-base structure. The sprung response spectrum is taken from the un-strengthened structure.

5.3 Spring stiffness variation

Figure 20 shows the change to the transformer response spectra when the spring stiffness values are increased and decreased. Decreasing the spring stiffness increases the peak by around 1 g from the original values, and also pushes the peak out to 0.45 seconds. This sensitivity to spring stiffness values was considered acceptable when compared with the other potential sensitivities.

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Spring Model

Pinned Model

Figure 19 - Comparison of pinned and sprung model - horizontal response spectrum at transformer base (5 % initial damping & output, Mexico Cale record)

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Double Horizontal Spring Stiffness

Original Spring Stiffness

Half Horizontal Spring Stiffness

Figure 20 - Horizontal response spectrum at transformer base with varying horizontal spring stiffness (strengthened design, 5 % initial damping & output, Mexico Cale record)

5.4 Comparison of damping levels

Figure 21 indicates the difference in peak response when a different initial damping value is chosen. This represents the damping inherent in the structure itself, and does not include damping from the non-linear soil springs which is calculated intrinsically. In this case, 5 % was assumed for all analyses.

Figure 22 above shows the response spectra for different levels of equipment damping. If the transformers have 10 % inherent damping, the peak acceleration is reduced by around 2 g, which is a significant decrease.

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5% Initial Damping

10% Initial Damping

Figure 21 - Horizontal response spectrum at transformer base with varying initial damping (strengthened design, 5 % damping output, Mexico Cale record)

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2% Damping5% Damping10% Damping

Figure 22 - Horizontal response spectrum at transformer base for various output damping (5 % initial damping, Mexico Cale record)

6 CONCLUSIONS

The elastic design approach used in the earlier Solutions Study Report indicated that strengthening work, including extensive ground anchoring, was required to prevent instability and excessive damage occurring during a design-level earthquake. The subsequent dynamic analysis design approach has resulted in a considerably reduced earthquake demand on the Condenser Transformer Building.

Analysis of the base building indicates that strengthening of the structure is still required to prevent both instability and major building and equipment damage.

For the structural type and low ductility demand implied by the designed strengthening, we would not expect significant building damage at the design-level earthquake.

Floor-response spectra determined for the existing base building indicate high amplification of horizontal accelerations at the transformer-slab level which would be expected to jeopardise the transformer design.

However, with the addition of new transverse walls to control rocking motions, the response spectra at the transformer slab level are reduced. Subject to the transformer supplier confirming that the spectral accelerations provided by this study will not damage the transformers or their internal or local interconnections and parts, it is expected that transformers will be operable immediately after a design-level (1/2500) earthquake.

REFERENCES:

NZSEE 2006, Assessment and Improvement of the Structural Performance of Buildings in Earthquake, New Zealand Society for Earthquake Engineering, 2006.

Transpower New Zealand Ltd, Seismic Policy, TP.GG 61.02, Issue 2, April 2009.

Structural Design Actions, AS/NZS 1170, Australian and New Zealand Standards.

IEEE Std 693: 2005: IEEE Recommended Practice for Seismic Design of Substations.

HVDC Inter Island Link, Pole 1 Replacement Haywards Substation Solution Study Report –Detailed Seismic Assessment of Condenser Transformer Building (AECOM, ref 60048466, 29 Sep 09, Rev 3). Also referred to as “Solutions Study Report (SSR)”.

Recommended Accelerograms and Vertical Spectra for Haywards – GH McVerry, (GNS Science Consultancy Report 2007/362, November 2007).

HVDC Pole 3, Strengthening of Synchronous Condenser Transformer Building – Factual Geotechnical Report (Beca, ref 2871795, 26 Apr 10, Rev A).