DESCRIPTION FRAME WORK STRUCTURE TECHNICAL FIELD The present invention relates to a frame work structure for replacing existing equipment on the frame work, which is assembled from pillar and beam members, with new equipment of different type. BACKGROUND ART Generally installed in a boiler plant is an air preheater to preheat air, which is to be introduced into a boiler body, with heat of exhaust gas discharged therefrom. As such air preheaters, a fixed-element type air preheater (so-called Rothemuhle type air preheater) and a rotatable-element type air preheater (so-called Ljungstr6m type air preheater) have been mainly employed. Fig. 1 is a plane view showing an example of a frame work with a fixed-element type air preheater as existing equipment in which beam members 3, 4 and 5 are arranged to bridge upright pillar members 1 and 2, thereby forming the frame work 6, and the fixed-element type air preheater 7 is arranged as existing equipment on the frame work 6 such that load supporting points P1 thereof are positioned on the beam members 4. In Fig. 1, reference numeral 4a denotes level-adjusting liners on the beam members 4 on which the fixed-element type air preheater 7 is mounted. In the frame work 6 as mentioned above, the load of the fixed-element type air preheater 7 as the existing equipment acting on the load supporting points P1 is transmitted from the beam members 4 via the beam members 3 and 5 to pedestals of the pillar members 1 and 2 to act as basic load on a base (not shown). When the fixed-element type air preheater 7 as the existing equipment is to be replaced with a rotatable element type air preheater as new equipment, the load supporting points thereof are displaced due to structural differences of the air preheaters. Then, it is ordinarily considered that the existing beam members 4 and 5 are to be removed and different beam members are to be added such that the basic load transmitted to the pedestals of the pillar members 1 and 2 becomes equal to that in the original construction before reconstruction. Conventionally, such operational work has not been conducted and there are no known patent documents thereon. The removal of the existing beam members 4 and 5 and the adding of the different beam members as described above, however, not only have difficulty in making the basic load transmitted to the pedestals of the pillar 2 members 1 and 2 substantially equal to that in the original construction before reconstruction, but also have a disadvantage that the added beam members must be at a higher level than the existing beam members 3 since a beam arranging operation is conducted within a limited narrow space in the frame work 6. Moreover, in order to counteract long- and short-term horizontal forces generated at the load supporting points of the rotatable-element type air preheater as the new equipment, horizontal trusses and the like must be arranged to extend outward, which makes it impossible to use existing maintenance floors (not shown). The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. SUMMARY OF THE INVENTION The present invention provides a framework structure for replacing existing equipment on a framework, which is assembled from pillar and beam members, with new equipment 3 of a different type, wherein a ring beam is mounted against load supporting points of the existing equipment on the existing beam members on the frame work via existing liners and level-adjusting liners and a new equipment is arranged on the ring beam, whereby the load of the new equipment is transmitted to the load supporting points of the existing equipment on the beam member. 3a According to the above-mentioned means, the following effects and advantages may be obtained by some, but not necessarily all, embodiments of the present invention. The load of the new equipment is transmitted via the ring beam to the load supporting points of the existing equipment on the frame work so that basic load transmitted to pedestals of the pillar members becomes substantially equal to that in the original construction before reconstruction, unlike a case where the existing beam members are removed and different beam members are added. In the above-mentioned frame work structure, the ring beam may be of box-shaped section, which makes it unnecessary to arrange horizontal trusses and the like to extend outwards in order to counteract long- and short-term horizontal forces generated at load supporting points of the rotatable-element type air preheater as the new equipment and makes it possible to use existing maintenance floors. Furthermore, in the above-mentioned frame work structure, the existing equipment may be a fixed-element type air preheater and the new equipment may be a rotatable element type air preheater. 4 BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a plane view showing an example of a frame work with a fixed-element type air preheater as existing equipment; Fig. 2 is a plane view showing an embodiment of the invention; Fig. 3 is a sectional view partially showing a ring beam of the embodiment according to the invention, which is taken along line III-III in Fig. 2; and Fig. 4 is a view looking in the direction of arrows IV in Fig. 3. BEST MODE FOR CARRYING OUT THE INVENTION Now, an embodiment of the invention will be described with reference to the drawings. Figs. 2 to 4 show an embodiment of the invention in which the reference numerals same as those in Fig. 1 denote the same parts and which is similar in basic structure to that of the conventional one shown in Fig. 1. The illustrated embodiment is characteristic in that, as shown in Figs. 2 to 4, it has a ring beam 9 which transmits load of a rotatable-element type air preheater 8 as new equipment to the load supporting points P1 of the fixed-element type air preheater 7 as existing element on the frame work 6. 5 In the illustrated embodiment, the load of the rotatable-element type air preheater 8 as the new equipment acts on load supporting points P2 on the ring beam 9. The load acting on the load supporting points P2 is transmitted to existing beam members 4 on the frame work 6 such that, as shown in Figs. 3 and 4, the ring beam 9 is mounted via existing liners 4a and level adjusting liners 10 against the load supporting points P1 of the fixed-element type air preheater 7 on the existing beam members 4. The ring beam 9 is, as shown in Figs. 3 and 4, of box-shaped section and has therein reinforcing ribs 11 at positions corresponding to the load supporting points Pl. Its interior may be lined with backing plates 12, as need demands. Next, the mode of operation of the above illustrated embodiment will be described. The load of the rotatable-element type air preheater 8 as the new equipment acts on the load supporting points P2 on the ring beam 9 and is transmitted via the ring beam 9 to the load supporting points Pl of the fixed-element type air preheater 7 as the existing equipment on the frame work 6. As a result, the basic load transmitted to the pedestals of the pillar members 1 and 2 becomes substantially equal to that in the original construction 6 before reconstruction, unlike a case where the existing beam members 4 and 5 are removed and different beam members are added. Further, in the illustrated embodiment, the ring beam 9 is of box-shaped section, which makes it unnecessary to arrange horizontal trusses and the like to extend outwards in order to counteract long- and short-term horizontal forces generated at the load supporting points P2 of the rotatable-element type air preheater 8 as the new equipment and makes it possible to use existing maintenance floors (not shown). Thus, the existing beam members 4 and 5 are retained and the basic load transmitted to the pedestals of the pillar members 1 and 2 can be made substantially equal to that in the original construction before reconstruction; and the existing maintenance floors can be made use of as it is. It is to be understood that the frame work structure according to the invention is not limited to the above embodiment and that various changes and modifications may be made without departing from the scope of the invention. As is described above, according to a frame work structure of an embodiment of the invention, existing beams members may be 7 retained and basic load transmitted to pedestals of pillar members may be made substantially equal to that in original construction before reconstruction; and existing maintenance floors may be made use of as it is. Thus, the invention may be suitable to replace existing equipment on a frame work with new equipment of different type. 8