STUDY ON THE RECONSTRUCTION OF THE FIRST IMPERIAL AUDIENCE HALL, NARA PALACE 2Wooden Parts（2010）

 

Research Report Vol. 81

Nara National Research Institute for Cultural Properties

 

English Contents and Summary

 

Contents

Chapter I Process of review and drafting the report   1

1 Progress of review of reconstruction plans for wooden parts    1

A 1982: Initial reconstruction plan    1

B 1992-93: Start of detailed review    1

C 1994: Drawing up of the original plan    3

D 1995-96: Structural analysis and problem identification    6

E 1997: Basic revision and presentation of the revised plan    7

F 1998-2000: Finalization of the revised plan    9

G 2001-09: Detailed review based on the revised plan    9

2 Documents for altering the existing state of a Historic Site    3

A Committee for examining reconstruction (Mar 2009)    11

B Explanation of alterations to the existing state    15

3 Drafting the report    18

 

Chapter II Case studies    20

1 Basic surveys of surviving structures    22

A Main Hall, Horyuji    22

B Eastern Pagoda, Yakushiji    28

C Main Hall, Toshodaiji    32

2 Dimensional plans    35

A Elevation plans    35

B Cross-sectional dimensions    37

C Dimensions of members    39

D The 10 m module    42

3 Structural plans    50

A Relationship of pillar placements and roof form    50

B Structure and significance of multistory buildings    54

C Two types of Nara period structural form     57

D Developments in structure from Nara to Heian    60

4 Bracket complexes    63

A Yakushiji three-stepped and Horyuji bracket complexes    63

B Typology of ancient three-stepped bracket complexes    65

5 Eave curvature    67

A From differences in pillar height    67

B From members comprising the eave    69

6 Designs and techniques of building details    73

A Surfaces for roofing    73

B Barge boards    77

C Ceilings    79

D Doors     82

E Balustrades    86

F Plaques    89

 

Chapter III Process of making the reconstruction design    96

1 Lower story horizontal plan    96

A Extent of excavated features    96

B Daigokuden at Kuni and Nara Palaces   98

C Interior finish    100

2 Basic structure    102

A Single or multistory?    102

B Shape of the roof     105

3 Original reconstruction plan    108

A Process of evaluating the structure and design    108

B Structural analysis and problem identification    114

4 Revised reconstruction plan    124

A Basic structural form of revised plan No. 1    124

B Structural analysis, partial revision, finalization   135

C Plan of design details and techniques    143

 

Chapter IV Reconstruction plan    159

 

　Summary

　In 710, the nation's capital was constructed in the area of the present Nara City. Nara Palace was located at the center of the northern area of the capital, extending approximately 1.3 kilometers east-west and one kilometer north-south. The palace included not only the imperial residence, but also areas for conducting rituals and the affairs of state. During the first phase of operation of Nara Palace, the First Imperial Audience Hall (Daigokuden) was built in its center. The Daigokuden was the main building of the larger Daigokuden complex. The capital was relocated to Kuni in 740, however, and the Daigokuden was moved to Kuni Palace at that time. In later years the Daigokuden was reused as the main hall of the Yamashiro Kokubunji temple, although it was dismantled in the intervening years. When the capital was relocated back to Nara in 745, anew palace had already occupied the site on which the Daigokuden was originally built, and the foundation platform of the Daigokuden had been largely removed. In 784, the capital was relocated again to Nagaoka, and the historic role of Nara Palace ended. Subsequently, in the early ninth century another palace was temporarily built at this site, but was maintained for only about fifteen years before being abandoned. As a result, the Nara Palace site reverted to agricultural use.

　In 1922 the Nara Palace site was designated a National Historic Site. From 1952, the Nara National Research Institute for Cultural Properties (NNRICP) has continuously undertaken excavation and research of the site. Further, the land has come under public ownership since 1963, in order to develop the site as a historic park. With regard to the first Daigokuden, the NNRICP has conducted excavations several times, and published an excavation report in 1982. In addition, as part of the total improvement project of the Nara Palace site, reconstruction of the first Daigokuden was proposed. From 1989, specific examination of plans for reconstructing the Daigokuden began, and in 1993, principles for making the reconstruction were adopted. Since then, after completion of studies and designs for reconstructing the Daigokuden, actual construction commenced in 2001. The work was completed in April 2010.

　This report explains the studies conducted by the NNRICP, and the grounds for its designs for reconstructing the Daigokuden. The report is divided into four volumes, as follows.

1 Platform/Foundation

2 Wooden parts

3 Coloring/Fittings

4 Roof/Tiles

Each volume includes the following content.

I Process of review and drafting the report

II Direct data/reference materials relating to the Daigokuden of Nara Palace

III Case studies

IV Process of making the reconstruction design

V Reconstruction plan

 

　The contents of the case studies give details of the basic research conducted in conjunction with considerations for the reconstruction. In addition, some findings of studies that were made after completion of the reconstruction design are also included. It should be noted the reconstruction plan shown in the report indicates only the ancient architectural form in theory, and that the building as it will actually be reconstructed has been changed slightly, taking into consideration factors of structural reinforcement and maintenance.

 

　This is the second of four volumes, covering the results of research for reconstruction of wooden parts in particular.

　

　Chapter I Process of review and drafting the report

　The progress of research since 1982 is outlined; documents are attached which relate to wooden parts, drawn up in conjunction with plans to alter the existing state of the Nara Palace Special Historic Site, an important watershed for the advancement of research for reconstruction; persons involved in the production of this report are listed.

 

　Chapter II Case studies

　This chapter notes the basic materials and methods for research aimed at reconstruction, and discusses the individual case studies of research that were based on those methods.

With regard to the wooden parts of the Daigokuden, considerations for reconstruction of a portion of them are possible because the horizontal plan of the foundation platform of the first Daigokuden at the Nara Palace can be reconstructed, based on its features excavated at Nara and at Kuni Palace where it was transferred in Tenpyo 12 (740). From the locations of pillar base stones and the dimensions of their seats for receiving pillars, the placement and diameters of the pillars are clear, and from the length of the eave overhang the structure of the bracket complexes and eaves can be largely inferred. On the other hand, as almost no information can be obtained for the structure of wooden parts from documentary sources, materials directly relevant for reconstruction are limited mainly to excavated features. In conducting research for the reconstruction, the following methods were established for inferring by analogy the portions, apart from those mentioned above, which were unclear.

 

　1. Principles regarding the structure, design, and techniques of ancient architecture were extracted thorough reexaminations of structures surviving from the same period, and reconstruction of the Daigokuden proceeded through the application of these principles to its extant features. In this process, the basic properties as natural material of the wood used for construction served as preconditions for envisioning the structure, design, and technique for each part of the structure.

　2. Regarding the structure, design, and techniques of ancient architecture, there are intimate relations between a structure’s parts and the whole, and direct connections based on form with regard to the flow of force. The character of ancient architecture, which could be called its structural rationality, was analyzed through diagnoses and experiments based on modern structural mechanics, and the suitability of architectural form checked.

　3 Compared to the first Daigokuden of Nara Palace, much is known through pictorial and literary materials about its successor, the Imperial Audience Hall of Heian Palace. While neither the horizontal plan nor the scale are identical, as a certain degree of 　continuity in the rituals conducted at the Daigokuden can be discerned, it is not unreasonable to assume a basic continuity in form. Accordingly, for the roof, the interior finish of the bays, the balustrade surmounting the foundation platform and so forth, the basic form is based on reference to the Daigokuden of Heian Palace.

　　

　Of the above, the first and second in particular reconstruct methods for designing ancient architecture, and these methods were used in taking surviving architectural examples, and applying them with adjustments for the form and scale of the remains of the First Imperial Audience Hall of Nara Palace.

The themes and basic outlines of research conducted through case studies are as follows.

 

　1 Basic surveys of surviving structures

　Surveys were newly conducted of the Main Hall of Horyuji, the Eastern Pagoda of Yakushiji, and the Main Hall of Toshodaiji, to use them as reference for the reconstruction design of the Daigokuden. The Main Hall of Horyuji is the lone surviving two-story example of its type, and research was conducted on its structural system. There was question about the fact that the pillars of the lower and second stories do not line up in the same manner, whereas the trusses of the second-story roof stand directly above the lower story pillars, but it became clear that this was a structural system unique to two-story architecture for separating the load of the trusses from that of the eaves in transferring load to the lower story. In this structure, because the entire load of the second story is transferred directly by its pillars to the lower story, there is no need in the core for large load-bearing beams.

　At the Eastern Pagoda of Yakushiji, an investigation was made of the differences in dimensions of the bracket complexes and eaves for each story, the characteristics of the uses of koguchimasu (bearing blocks set with crosscut sides facing outward), and the differential uses made of separate tree species. While it was learned that the dimensions of the bracket complexes and eave overhangs were adjusted in keeping with the gradual reduction in horizontal area it was also found that concern had been made for adjusting the elevation area by minutely changing the eave height. Regarding the koguchimasu, they are known for their use as a technique aimed at strengthening the structure by setting the grain of the bearing blocks perpendicular to that of the bracket arms, and it was found that with regard to the tree species, considerations for structural rationality were made, with the hard-wooded zelkova selected for locations such as the principal bearing blocks and hip rafters, where the load concentrates.

 

　2 Dimensional plans

　The plans of the elevation, cross-sectional dimensions, dimensions of the members, and the relationship between the length of members and the building scale are discussed.

　In the elevation plans, it was seen that in addition to the cumulative height of the architectural members, vertical proportions of the structure also appear to have been planned, using the foundation platform, eaves, and ridge as reference points for the height.

　Regarding the plans of cross-sectional dimensions, correlations were pointed out between the width of the bays and the pillar height, the height of the eaves and the overhang, the transverse beams of the core and the height of the ceiling, indicating a degree of relationship between the horizontal and cross-sectional plans of buildings.

　For plans of the dimensions of members, certain proportional relationships became clear between the width of the bays and pillar thickness, and between the main and secondary bearing blocks and the bracket arms; moreover, it was shown that in ancient architecture standardized materials of identical cross-sectional dimensions were used for long members such as bracket tie beams.

　Regarding the relationship between the length of members and the building scale, it was pointed out that in ancient architecture there was a maximum length for architectural members set at a 10 m limit, and this 10 m length is seen to have defined a module for the building scale.

 

　3 Structural plans

　The relationship between pillar placements and roof form, the structure and significance of multistory buildings, the two types of structural form in the Nara period, and developments in structure from the Nara to the Heian periods, are each discussed.

　For pillar placements and roof form, distinctions between hipped roof and hip-and-gable roof construction in terms of the pillar placements are discussed, with trends indicated for the bays in hipped roof buildings to narrow gradually in width going from the center of the longitudinal axis toward the corners, whereas in hip-and-gable construction the spans are equal in length across the central part of the axis, and interpretation was made of the reason from a structural perspective.

　For the structure of multistory buildings, taking among others the Main Hall of Yamadadera and the Middle Gate of Horyuji as examples, it was shown that horizontal layouts intended for multistory structures are found mainly in the Hakuho period.

　Regarding the two types of structural form in the Nara period, these were indicated as full-scale structures with eave purlins projected outward from the walls, namely buildings with three-stepped bracket complexes which insert multilayer bracket systems between the principals and the roof, and simple bracket buildings lacking projecting purlins, which extend the pillars directly up to the roof and insert brackets as shock absorbing materials, with the structural systems related to them being fundamentally different.

　As for developments in structure from the Nara to the Heian periods, the progressive separation of structural versus decorative materials is discussed, visible in both structural portions and the fine details, as symbolized by the appearance of hidden roof frameworks above the ceiling (nogoya) and latticed ceilings with recessed panels (kogumi gotenjo).

 

　4 Bracket complexes

　That the Horyuji style bracket complex is a three-stepped arrangement, and the possibility of categorizing three-stepped bracket complexes, including the Horyuji style, into four types from a structural perspective, are discussed.

　Three-stepped bracket complexes not only have the characteristic, in terms of external appearance, of supporting the eaves, but are also linked to the principal interior structure. Tail rafters or reinforcing rafters are drawn in as far as the position of the core pillars, making aright triangle along with the strut spanning the gap between their ends, and serving not only to support the eave but also strengthen the structure of the aisle surrounding the core. From this perspective, the Horyuji style which uses a cloud-shaped bracket arm can also be called one type of three-stepped bracket complex.

　The four types of three-stepped bracket complexes are classified based on this right triangle, and its relation to the perimeter and core pillars. They consist of cases where the right triangle sits directly atop the two pillars which are of the same height, cases where the pillars are of equal height but the triangle sits atop double layers of brackets, cases where the core pillars are taller, and cases where core pillars are lacking.

 

　5 Techniques for forming the curvature of the eaves

　The curvature of the eaves is determined by the difference in height of the corner and interior pillars, and the members comprising the eaves. In Nara period examples, the eaves were given a gradual curve from the center of the building to the corners using a difference in pillar height, then the curvature of the overhang was increased through the use of base rafters, hip rafters, eave supports and flying rafter supports, giving a sharp upturn to the ends of the eaves.

 

　6 Designs and techniques of the building details

　Data on examples were gathered and discussed for: the surface for roofing, barge boards, ceilings, doors, balustrades, and plaques inscribed with the building name.

　Regarding the surface for roofing, from features remaining in surviving structures it was seen that for buildings with three-stepped bracket complexes, an open space was provided above the rafters and a separate roofing surface made. In such cases, the roofing surface consisted of a double layer of vertically running boards, which also served as a temporary cover at the time of construction.

　Barge boards are regarded as covers for the space created at the gable ends between the roofing surface and the rafters. Accordingly, their dimensions are determined by the principal structure.

　Latticed ceilings strengthen the building laterally, with diagonal members called shirin functioning as braces for supporting the ceiling; both are seen as structural materials.

　Regarding doors, observations are made for inward and outward opening types, and methods of securing the doors are discussed. In Japan, doors were generally outward opening, but the existence of Chinese style inward opening doors, and three types of methods for securing, related to the door structure are indicated.

　For balustrades, it is seen that in the Nara period newel posts were not used, horizontal members lacked curvature, struts surmounted with bearing blocks had indentations on the top surfaces, and the topmost horizontal members were round or octagonal in cross-section.

　Regarding plaques bearing building names, it is shown that in addition to being items for securing the displayed surface as decoration, they also had the structural function of attaching to the building, and their scale and shapes bore relation to its structural form.

 

　Chapter III Process of making the reconstruction design

　While the design for reconstructing the Daigokuden was worked out over time, with the results of observations being accumulated and partial revisions made over and again, the changes added between the basic plan and the stage of the actual working plan can be called fundamental. Accordingly, the proposals for the reconstruction are explained by dividing them into the two types of original and revised reconstruction plans. The report takes up as much as possible the different proposals, and multiple alternatives for consideration, which came out of the reconstruction process.

　First, as content common to both types, the horizontal plan of the lower story and the basic structural form are discussed.

　For the horizontal plan of the lower story, the pillar arrangement and horizontal scale are known from excavated features of the Daigokuden at Nara and Kuni Palaces. The shape has a core of 9 x 2 bays, surrounded on all four sides by an aisle of single bay width. Regarding the interior finish of the bays, from the pillar base stones of the Kuni Palace Daigokuden it is seen that the rear and two lateral faces had walls and doors, but the front face of the building is thought to have been open, based on materials such as the form of the Heian Palace Daigokuden seen in the picture scroll Nenju gyoji emaki, and the shapes of pillar base stones of the main hall at the Daizaifu administrative headquarters for Kyushu.

　As for the basic structural form, whether it was single or multistory, and the shape of the roof, are discussed. No basis exists for deciding whether the Daigokuden was single or multistory from either its remaining features at Nara and Kuni Palaces, or from historic sources. Accordingly, as the main halls of temples of the same period were mostly multistory buildings, or if single-story structures were provided with pent-roofed enclosures giving the appearance of two stories, and because the Heian Palace Daigokuden was possibly multistory, the First Imperial Audience Hall, the highest level of palace architecture of the Nara period in its day, is thought to have been a multistory structure. Regarding the shape of the roof, from those seen for the Heian Palace Daigokuden in the picture scroll Nenju gyoji emaki and in multistory main halls of temples of the same period, and from the placement of the Daigokuden’s pillars, a hip-and-gable roof is considered appropriate.

　The original reconstruction plan was drawn up as a two-story structure with a horizontal area just slightly larger than the first-story core. To fit a ceiling for the lower story to the wide transverse bays of the core, large curved tie beams (“rainbow beams”) were used transversely for supporting the ceiling, and moreover greater ceiling height was obtained by lengthening the core pillars beyond those of the perimeter. As a result, the plan was modeled on the style of the Toshodaiji Main Hall. During the course of deliberations, as the horizontal plan of the second story was enlarged, struts were needed to support the middle portions of the bottommost transverse beams in the second-story roof trusses, and it was decided to have the rainbow beams receive these struts. The second-story horizontal plan became 3 shaku wider on every side than the lower story core, being envisioned as 9 bays and 125 shaku wide along the front facade, with the pillar spans gradually narrowing from the center toward the corners, and 3 bays and 42 shaku in the transverse direction. The bracket complexes were in the Yakushiji style three-stepped arrangement, and a balustrade was set on the second story. The roof trusses had a double rainbow beam arrangement, with both ends of the bottommost beams supported by struts, and a beam connecting the bracket complexes receiving the struts.

　Based on this original reconstruction plan, as structural analysis and experiments were conducted starting from 1995, fundamental problems concerning the weight became clear, stemming in particular from the placement of struts atop the rainbow beams and thereby producing extreme distortions in the members forming the lower story core, and it became necessary to make basic revisions to the reconstruction plan. Surviving structures from the ancient period were reexamined at that point from the perspectives of both structural techniques and structural mechanics, in search of principles of ancient architecture that should be applied for the Daigokuden. As a result, it was recognized that the structural system of the Main Hall of Horyuji, the lone surviving multistory example of this type of temple building, possesses a rationality making a two-story structure possible. In this system, the number of bays of the second story is reduced in each direction by one from the number for the lower story, second-story pillars are stood in the intervals between lower story pillars, secondary pillars are stood in the intervals between second-story main pillars, and the load of the trusses and that of the eaves are divided respectively between the second story’s secondary and main pillars and transferred to the lower story. Based on this structural system of the Main Flail of Horyuji, a revised reconstruction plan was developed. This became the final plan for the reconstruction.

 

　Chapter IV Reconstruction plan

　The revised reconstruction plan was drawn up through reference to the basic structural system of the Main Hall of Horyuji, with design of the details modeled after the temporally close Eastern Pagoda of Yakushiji.

　The structure as a two-story building is as follows. The scale of the second story is for the most part matched with the core of the lower story. The number of second-story bays for both the longitudinal and transverse directions are reduced by one, making it an 8 x 3 bay arrangement. Accordingly, second-story pillars stand at the intervals between pillars of the lower story. Secondary pillars are placed on the long sides of the second floor. The roof trusses are placed in line with the secondary pillars, and are supported by struts standing atop the second-story transverse beams. The load from these struts passes through the lengthwise ceiling joists of the lower story, from which diagonal members (shirin) serve as braces transferring the load to the lower story core. Accordingly the load of the trusses and that of the eaves are divided between the struts, standing along the lines of the secondary pillars, and the second-story main pillars, and transferred to the lower story core.

　For the vertical proportions, the height of the building and the width of the foundation platform are matched, and the eaves of the lower and second floors are positioned to divide the total height into thirds.

　For the framework of the building, in the lower story all pillars are of the same diameter and height, with no large rainbow beam in the core. The lower story has a latticed ceiling supported by ceiling joists and diagonal shirin. Second-story pillars stand atop pillar bases placed on top of lower story base rafters.

　The eaves have a double rafter structure of base and flying rafters, and the bracket complexes are modeled after those of the Eastern Pagoda of Yakushiji.

　The roof trusses are made with two-tiered transverse beams connected by struts.

　For the interior finish within the bays, the second story has mud plaster walls at the corners, and wooden lath windows elsewhere.

　The upper surfaces of the foundation platform and lower story roof are rimmed with balustrades, and in the center of the second story a tall plaque is hung, carved with the characters for Daigokuden.

 

2010年5月31日　発行

奈良文化財研究所学報　第81冊

平城宮第一次大極殿の復原に関する研究

2　木部