For the feasibility study of using New RC materials to this type of structure, a power plant building, which elevations are shown in Fig. 9.18, was designed.
The building is 100 m high, consists of four box reinforced concrete columns of 10 m square, supporting steel top girder grill. At the center of this top girders is the boiler hanging. Figure 9.19 shows the plan of the foundation and four box columns, and Fig. 9.20 shows the plan of the top girder grill. It has cantilevers on one side of the square plan, and Fig. 9.21 illustrates the section of the building including this cantilever.
Unlike feasibility studies in the preceding two sections, this study aimed at the more practical feasibility. Hence the material selected for this study was 60 MPa concrete and SD 685 steel. In other words, they were selected from the Zone I material range. For the top girders, structural steel of grade SM570 was used.
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Figure 9.22 summarizes the section of box columns. Its outside measure- ment is 10 m square, and the wall thickness varies from 700 mm at the base to 500 mm at the top. These box columns have many openings in the wall, and the reinforcement shown in Fig. 9.22 was determined by the proportioning of a column with the largest openings.
Figure 9.23 illustrates the schedule of top girders. Girders are made of built- up I sections with the depth of 3.5 m. GO girders suspending the boiler and G l ( G l A ) girders holding GO girders have midspan depth of 6 m. The portions of top girder grill that rest on the box columns are called crown elements, which have to be rigidly connected to the top of box columns. For this purpose the wall thickness at the top of box columns is increased from 500 mm to 1200 mm, and anchor bolts of SD685 with D51 size, 200 mm on centers, are embedded in the wall. Crown element itself is 3.5 m deep, consists of flanges 950 mm wide and 125 mm thick and web plates 80 mm thick with stiffners and rib plates.
For the anchorage of high strength steel anchor bolts, use of anchor plates or splicing to wall bars are temporarily considered, however its detail is yet to be developed. Also it is desirable to increase the stiffness of crown elements to avoid stress concentration at the corners and to evenly distribute the reaction forces. This is another points to be explored in future.
Foundation of the structure is to be supported by cast-in-place concrete piles. The plan shown in Fig. 9.19 illustrates arrangement of 1.5 m diameter piles, which was determined by assuming an imaginary site with relatively deep bed rock. Footings measure 20.5 m square or 17.5 m square, both 7.5 m deep, and foundation beams have 12 m x 7.5 m section. It is considered necessary in future to investigate means to reduce amount of material for the foundation, to investigate the evaluation method of pile group effect including proper pile
arrangement, alternate use of continuous underground walls in place of cast- in-place concrete piles.
Design seismic forces were determined independently to four box columns from the preliminary response analysis. Design shears and design moments were independent as they do not necessarily act on the box columns simul- taneously. Design stress distribution for top girders and box columns were analyzed using linear elements for members and plate elements for joints. In addition to gravity loading and seismic loading, stress distribution for gravity loading considering the erection progress was also analyzed.
Rigidity of connection between top girder crown elements and anchor bolts to box columns was a concern from the early stage of the feasibility study. An additional analysis using a model with spring elements between crown elements and box columns showed that the flexibility of connection did not affect very much on the stress distribution in the top girders, crown elements and box columns.
Top girders hanging the boiler will be subjected to considerable effect of vertical earthquake motion. Dynamic response analysis was conducted for Hachinohe UD waveform corresponding to level 2 intensity. It was shown that the top girder end moment increases due to vertical response by 10 to 20 per- cent from the values due to horizontal response. Top girders were designed to remain elastic even under the combined effect of vertical motion.
Another consideration was the effect of temperature change of box columns and top girders. It was shown that the temperature effect was negligible as it increases the member forces not more than 2 percent from the design values.
Earthquake response analysis was conducted against four waveforms using base-fixed model and sway-rocking model considering deformation of piles.
Input waves were assumed to act in x- and y-directions, as well as in the 45 degrees direction. Design criteria to evaluate the response analysis results were set to be similar to those in Chapter 6. This was determined after the following consideration. On one hand this structure is composed of only four columns and is basically a single story structure, hence the degree of statical indeterminateness is low, which may lead to more conservative design criteria.
On the other hand the structure is used as power plant, supporting boiler and other equipments, and no heavy human or furniture occupancy is expected.
Considering these two contradicting factors to influence on the decision of design criteria, it was concluded to adopt similar criteria as for general highrise residential or office buildings, or those in Chapter 6.
Response drift under level 1 earthquake motions in x- and y-directions fell well below the design criterion, being 0.15 to 0.27 percent. Box columns and top girders did not show any yielding under level 1 input. It is anticipated from the box column deformation that columns would not even crack at this stage.
Response drift under level 2 earthquake motions, being 0.40 to 0.72 percent, also satisfied the criterion. Box column reinforcement did not yield, but yield hinges were formed at the ends of top girders. The rotation angle of column bottom of 0.38 percent corresponds, according to the experiments mentioned later, to fiexural cracking with steel strain about half-way to yielding.
Response under level 1 and 2 earthquake motions in the diagonal direction was essentially similar to that in the x- and y-directions. Strain in the re-bars of box columns was higher, but it was still in the elastic range. Top girders produced yield hinges, but the stress was lower than the previous case.
Analysis of sway-rocking model showed slightly larger response drift, but still conforming to design criteria. Elastic behavior of box columns and forma- tion of yield hinges at top girder ends were also similar to those of base-fixed model.
Finite element static analysis of box columns was conducted to investigate effect of openings to the overall stress distribution and also the local stress concentration around openings. The overall stress distribution significantly changes due to openings, and it could not be corrected by providing additional reinforcement around openings. Hence it is necessary to evaluate overall stress distribution considering the size, shape, and distribution of openings. Openings are to be provided with additional periphery reinforcement, and it is generally understood that the periphery reinforcement improves the structural behavior after cracking, but it does not prevent cracking itself. From the FEM analysis, concentrated arrangement around the periphery is found to be more effective for stiffness as well as strength than the diffusive arrangement. Also it was found that cracking at the opening corners did happen at the early stage of loading, but it did not lead to the re-bar yielding, and overall stiffness and strength were not affected very much.
Experimental works were also conducted of two box column specimens in 1/7 scale, that is 1.4 m square and 4.2 m long, and they were subjected to bidirectional reversal of loading, one in 0 degrees-90 degrees directions, and another in 45 degrees-135 degrees directions. They behaved elastically up to deformation drift of 0.12 percent, and bar yielding was initiated at drift of
0.5 to 0.75 percent. The specimen in 0 degrees-90 degrees directions failed by a sudden crushing in the compression flange at the side of opening at the box column base. The failure occurred after the maximum load of 942 kN was reached, at the deformation drift of 1.39 percent, without being accom- panied with the strength reduction. The specimen in 45 degrees-135 degrees directions showed concrete crushing at the box corners, then shear compression failure progressed gradually, maximum load of 939 kN being observed at drift of 1.05 percent. Both of them showed 5-type load-deflection curves under load reversal, with relatively small hysteresis loop area, or in other words small en- ergy absorbing capacity. Observed damage and failure at various deformation stages were directly useful in evaluating the structure's behavior under levels 1 and 2 earthquake input.
Finally, method of construction should be mentioned. Two most important construction stages are the construction of box columns and the erection of top girders including crown elements. Several construction methods for these two stages were selected and compared. As to box column construction, both slip forms and jump forms were found to be applicable, with slight advantage of slip forms in the reduction of construction period. The erection of top girders are to be as follows. Top girders together with cantilever portions and divided crown elements are up-lifted first, then slided laterally at the column top, jacked down to the position, connected together and to the anchor bolts, and then central portion of top girder grill is lifted up. Such construction process is judged to be the most superior in terms of quality control, cost, construction period, and construction safety.
Thus it was concluded that a thermal power plant boiler building utilizing reinforced concrete box columns, 10 m square and 100 m high, was a feasible structure with the use of material combination of Zone I of New RC project.