預力預鑄自行復位建築構架設計與耐震性能

Abstract

預力預鑄自復位構架是利用後拉預力工法提供梁與柱或柱與基礎結合時所需的預力，此種形式的抗彎矩構架已發展成為可替代傳統抗彎構架的新建築結構，在以往的研究中不乏針對預力梁柱接頭的研究。本研究建議梁柱及柱底接頭均採預力接合以避免接頭區產生殘餘變形，且樓板僅與預力構架單跨接合傳遞地震產生的樓層側力，以避免樓板對預力構架產生束制效應，此外針對預力界面開合造成柱對梁產生的束制效應，提出以柱整體變形之撓曲勁度計算柱束制對梁產生的軸向壓力增量，並提出預力預鑄自行復位構架的設計方法及設計流程。 當預力抗彎構架中柱的數量為兩支以上時，梁柱界面因開合效應產生的間隙會受到柱的束制，進而影響梁軸力的變化。本研究提出以預力建築構架中每一樓層開合效應對柱產生的變形，計算柱的撓曲勁度及梁軸向壓力的方法。由三層樓預力建築構架的電腦模型分析中顯示為了符合柱的整體變形，一樓處梁軸向壓力會較鋼腱拉力大，但在二樓與三樓處會較小。由三層樓兩跨建築物中擷取兩跨一樓的構架試體進行試驗，以驗證本方法的準確性；試驗中梁翼加勁板的細節在考慮柱頂無束制與柱頂為鉸接的情形下，發現梁軸力較鋼腱拉力分別增加2 %與60 %，此結果指出若二樓層處柱假設為鉸接時，柱會提供過大的束制行為造成高估預力構架中梁軸力，因此，假設柱在樓層中為鉸接的方式，並不適用於預測柱在預力建築構架整體側向變形時產生的撓曲勁度及軸力。 本研究建議一種樓板的接合細節，使得樓板在與預力構架接合時可提供滑動行為，並降低樓板限制預力構架伸展時產生的束制效應，並以縮尺兩跨一層樓的預力構架試體進行震動台地震測試。結構體包含一組預力構架(PT Frame)及兩組承重構架(Gravitational Frame)，其中兩承重構架是由鋼梁、鋼柱與鉸支承以鉸接方式接合，僅提供垂直向重力支撐，並分別平行放置預力構架的兩側，樓板與承重構架間以剪力釘固接，但僅與預力構架一跨的鋼梁以剪力釘接合，為使樓板能在預力構架上產生滑動，在樓板梁與預力構架間提供滑動支承，並分別以1994年北嶺地震(CNP196)與1999年集集地震(TCU074)對縮尺構架試體的耐震性能進行測試。在樓板無損害的情況下，預力構架的側向位移與承重構架的側向位移相似，且都擁有自行復位的能力。構架試體在加載1999年最大地表加速度1830 gal的集集地震時，構架的最大側位移角為7.2%，可觀察到梁下翼板靠近柱面承壓處產生挫屈，並造成柱與梁上初始預力分別下降50%與21%，但構架試體的殘餘變形量僅約0.01%。 為驗證依照本研究建議之設計方法設計之預力預鑄構架能符合設計規範要求，分別以5個地震共15組測站之地表加速度測試預力預鑄構架，同時，比較在相同的梁柱尺寸下，不同柱底及梁柱接頭的構架在地震反應下的結果，並針對各樓層的位移、殘餘變形、加速度及消釋能量進行比較，以判別各式構架的優劣情形。

The first generation for the post-tensioned (PT) self-centering (SC) system, which incorporates the PT technology to beam-to-column connections, exhibits good seismic performance with small residual deformations except for the first floor. Instead of using the fixed column base, the column PT to the base affects the seismic performance of frames, especially for residual deformations. A primary design procedure used for SC system was roughly described in this study. Cyclic tests on post-tensioned (PT) beam-to-column connections have demonstrated self-centering capabilities with gap opening, closing at the beam-to-column interface. Gaps, however, between beam-to-column interfaces in a real PT self-centering frame with more than one column are constrained by the columns, which causes beam compression force different from the applied PT force. This study presents an analytical method for evaluating column bending stiffness and beam compression force by modeling column deformation according to gap-openings at all stories. The predicted compression forces in the beams are validated by a cyclic analysis of a three-story PT frame, which is modeled with numerous axial springs in connections to capture the gap-opening behavior of the frame, and by cyclic tests of a full-scale, two-bay by first-story PT frame, which represents a substructure of the three-story PT frame. The proposed method shows that compared to the beam strand tensile force, the beam compression force is increased at the 1st story but is decreased at the 2nd and 3rd stories due to column deformation compatibility. The PT frame tests demonstrate that the proposed method reasonably predicts beam compression force and strand force. Test results also show that beam compression force is 2% and 60% larger than the beam strand force with respect to a minor restraint and a pin-supported boundary condition, respectively, at the tops of the columns. This indicates that assuming a pin-supported boundary condition at the upper story column can cause inaccurate estimation of column bending stiffness and beam compression force. This study proposes slab details, which allow for sliding of the slab and minimize restraints on the expansion of the PT frame. A composite slab is rigidly connected to the beams in only one bay of the PT frame. A sliding device is provided between the floor beams and the PT beams in other bays, where sliding of the slab is allowed. Several shake table tests were conducted on a reduced-scale, two-by-two bay one-story specimen model, which was composed of one PT frame and two gravitational frames. The scaled specimen model was excited by the 1994 Northridge and 1999 Chi-Chi earthquakes to examine its seismic performance. A PT frame and gravitational frames possessed the self-centering capability throughout the tests, responding in phase with minor differences in peak drifts due to the expansion of the PT frame. When the specimen was excited by the 1999 Chi-Chi earthquake with a peak ground acceleration of 1830 gal, the maximum interstory drift was 7.2%. Buckling of the beam bottom flange was observed near the compression toe, and the initial post-tensioning force decreased 50% and 22% in the columns and beams, respectively. However, the specimen remained operable and its residual drift was 0.01%. A three-dimensional analytical model with rotational springs in the PT connection and PT column base was introduced to capture shake table test results of the frame subassembly. The same modeling approach was adopted to one MRF and three SC frames to study the effects of column base on the seismic response of frames under the design based and maximum considered earthquakes. The monotonic, cyclic pushover, and time-history analyses were conducted for these frames. Analytical results showed that (1) the residual drift of the first floor could be significantly minimized by using the PT column base but the maximum interstory drift in the SC frame increased with decreasing fixity at the column base, (2) the largest maximum interstory drifts of the SC frames were larger than those of the MRF due to the low-to-medium structural period and high yield strength, and (3) the SC frame with the PT column base effectively decreased column restraining forces to the first floor compared to that with the fixed column base.