2 SUPPORT STRUCTURE DESIGN FOR NEW ZEALAND FOREST PRODUCTS LTD ' S NO . 5 RECOVERY BOILER

Internationally, the seismic resistance of large industrial boilers appears to be addressed by the most simple application of relatively low equivalent static lateral forces which are resisted elastically. This p a p e r describes measures taken to ensure a predictable and controlled seismic performance of such a boiler during a major earthquake. Inelastic time-history methods of analysis were used to confirm that the desired perforamce would be achieved. As a result the client was able to purchase a relatively standard boiler in the international marketplace and still achieve a level of seismic resistance consistent with the best NZ practices.


INTRODUCTION
New Zealand Forest Products No. 5 Recovery Boiler is a large steam generator of similar size and form to the largest of those installed in thermal power stations.The slag, unlike that of a fossil-fuelled boiler, forms a molten heap.Any failure of the boiler which causes water to be dumped onto this slag may lead to the regeneration of sufficient steam to create a catastrophic explosion.Controlled earthquake performance is therefore important to the owner provided this can be achieved without unreasonable cost.To allow for its large thermal expansion, the 1500 tonne boiler is hung from the roof beams of a 50 m high steel support structure.It is normal to guide such a boiler laterally (while allowing vertical sliding at the guides) at a small number of positions up the boiler height.The external frames of this structure are cross-braced to provide lateral strength.Industry practice has been to force the boiler and its supporting structure to move horizontally together during a large earthquake by providing laterally strong and rigid guides.In general, as boilers are much stiffer than the supporting structures, it could be expected that the boiler would act as a seismic lateral load-resisting element until either the lateral guides or the boiler failed.In a move that appears to be without precedent elsewhere, the lateral guides have been deliberately detailed to yield at predictable load levels and the support structure design has then been based on analyses which accurately take this into account.By choosing to adopt this approach the owner has been able to take advantage of the cost saving of installing a boiler generally the same as those manufactured for less seismic regions of the world and therefore available competitively, and at the same time achieve controlled earthquake Associate, Beca Carter Boilings & Ferner Ltd, Consulting Engineers, Wellington.was decided that the support structure should be proportioned so as to resist without damage (except to the yielding boiler guides) earthquakes within a return period of 1000 years.Because of the small risk of such an earthquake occurring during the life of the plant and because of the partial isolation of the boiler from the support structure when the guides yield, it was furthermore decided that a full capacity design of the support structure would be unnecessary.This approach had the double advantage of reducing the complexity of the design process while ensuring that the structure, in remaining elastic to a higher than normal level of earthquake loading, would remain virtually undamaged even in a major earthquake.At the same time the boiler guides would be proportioned to accept all the imposed deformations in a controlled manner and thus keep relative movements between boiler and support frame within acceptable limits.
If the boiler and support structure system would remain substantially elastic during a strong earthquake then the seismic lateral loading could be determined by distributing the design base shear up the height of the structure by either a manual method or by recourse to spectral modal analysis.However, the presence of the yielding guides meant that these methods would not be sufficiently valid and the more appropriate technique would be to subject an analytical model of the frame to records of several earthquakes which were representative of the 1000 year return period event using the "time-hi story" method.An envelope of the maximum member forces and displacements could then be found, by taking worst values from the results of each of the several earthquakes applied.
The specification of the 1000 year return period earthquake for the site of the boiler (Kinleith) was assembled by using skills and computer programs developed by BCHF over several years.From a catalogue of recorded strong motions four were chosen which were likely to have the greatest effect on the boiler support structure and which could also be considered consistent with the local site conditions.These records were scaled to be consistent with the parameters established in the risk study.Each frames was subjected to an elastic time-history analysis for the four different earthquake records.For each earthquake, plots of each guide force versus time were obtained and ductility demand assessed to enable appropriate detailing.The guides are intended to be replaced after a major earthquake.Similarly the relative deformations of boiler and support frame were monitored to enable the process engineers to check that important pipelines, controls and ducts could endure their magnitude and number of cycles imposed.For all structural members, envelopes of maximum forces were produced to enable their elastic design to be checked.-exterior support structure modelled by beam members and axial load carrying members

CONCLUSION
at this time were used as the basis for seismic load and clearance information included in the tender specification.Design-supply tenderers for the boiler were required to offer a support structure layout capable of being designed in accordance with the specified design philosphy.They had the option of also offering to supply the support overall timetable for the project it was necessary to produce sufficient information to indent the main steel members within six weeks of being commissioned.Therefore to meet the time restraints and the design philosophy concurrently it BULLETIN OF THE NEW ZEALAND NATIONAL SOCIETY FOR EARTHQUAKE ENGINEERING, Vol. 20, No. 1, March 1987 FIG. 1 -DIAGRAM TO ILLUSTRATE STRUCTURAL LAYOUT

FIG. 3 -
FIG. 3 -FORCES IN TOP AND BOTTOM BOILER GUIDE MEMBERS