Background of the Invention 1. Field of the Invention

The present invention is broadly concerned with stiffness decoupling assemblies to be used in the con ^¬ struction of earthquake-resistant structures such as multi-story buildings, bridges, and smaller structures such as single family houses having basements. The assemblies of the invention effectively decouple the lateral stiffness of the structure in question from the load-bearing strength of the supporting column system for the structure. In this way, the dynamic behavior of a structure under seismic excitation is effectively con ^¬ trolled, while nevertheless retaining the necessary load- bearing strength, damping strength and natural period for the structure. Advantageously, the stiffness decouplers of the invention include a plurality of elongated, con- crete-filled pipes rigidly secured to a structure, to ^¬ gether with a surrounding, primary load-bearing column extending between an underlying foundation and the struc ^¬ ture, and receiving the pipes; low-friction bearings are provided between the columns and structure, in order to permit relative lateral movement therebetween. In the case of single family houses or similar small structures, the basement walls can be used as the primary load-bearing member for the associated concrete-filled pipes.

2. Description of the Prior Art

Architects and structural engineers have long grappled with the problem of designing buildings, bridges or other structures in areas prone to seismic events. The recent earthquake in San Francisco is but one of many examples of the potentially catastrophic consequences of improper building design in such locales.

Many proposals have been made in the past aimed at increasing the safety of earthquake-resistance of various structures. In general, most modern day proposals have attempted to combine the qualities of strength (that is, the ability to withstand large forces while remaining elastic), deformability and energy-absorbing capacity. For example, it is known to employ large elastomeric bearings to support ductile reinforced concrete frame structures, in order to isolate the structure from its underlying foundation. However, such bearings can be expensive, and moreover some are subject to environmental degradation. It has also been suggested in the past to make use of mild steel energy-absorbing devices which are rigid under service-type loading, but yield and absorb energy under large earthquake-type loading. Such schemes rely on the hysteretic energy-absorbing capacity of steel bars used as base-isolating devices.

Despite intensive on-going research in this area, however, workers in the art have failed to hereto¬ fore develop a truly effective base isolation system which is economical, easy to install, long-lived and capable of absorbing potentially destructive seismic forces while preventing collapse of the supported structure.

Summary of the Invention

The present invention overcomes the problems outlined above and provides a novel stiffness decoupling

assembly designed to give enhanced earthquake protection to structures such as buildings and bridges. The decoupl¬ ing assembly of the invention serves to decouple the lateral stiffness from the load-carrying strength of the column system supporting the structure, to thereby reduce the transmissibility of ground acceleration to the iso¬ lated structures.

In preferred forms, the invention contemplates use of a plurality of elongated, relatively flexible, hollow pipes rigidly connected adjacent the upper ends thereof to a protected structure, with the pipes extending downwardly towards the underlying foundation for the structure. At least certain of the pipes (and preferably all) are filled with material for damping induced movement of the pipes; preferably, the pipes are filled with concrete for this purpose. In addition, the overall decoupling assembly includes means operatively coupling at least certain of the pipes to the foundation for resisting overturning of the structure. Advantageously, such pipes are coupled to the foundation in a manner permitting limited upper shifting movement thereof against an in¬ creasing biasing force.

A primary load-bearing member also forms a part of the complete decoupling assembly and is located in spaced relationship to the plurality of pipes. Typically, a hollow, unitary square or circular in cross-section reinforced concrete column is employed for this purpose, with the plural pipes extending downwardly through the column. This load-bearing member rests upon the founda- tion and extends upwardly toward the structure to present an upper end. Bearings are interposed between the load- bearing member and the structure for engaging both of the latter and permitting relative lateral movement therebe¬ tween. In the case of smaller structures having base-

ments, the basement walls can be used as the primary load- bearing member in lieu of separate structural columns.

In actual practice, a given structure will be provided with decoupling assemblies, provided at the location of all conventional load-bearing columns, but provided with the bearing structure and internal pipes described previously.

Brief Description of the Drawings Figure 1 is a fragmentary view, with parts broken away for clarity, of the structural frame of a multi -story building, with the decoupling assemblies of the present invention in place between the frame base and an underlying foundation; Fig. 2 is a fragmentary view, with parts broken away and certain parts shown in phantom, illustrating important components of a decoupling assembly;

Fig. 3a is a sectional view taken along line 3a- 3a of Fig. 2 and with parts broken away illustrating the construction of a decoupling assembly making use of a hollow, square in cross-section reinforced concrete column with respective low- friction bearings at the column corners;

Fig. 3b is a view similar to that of Fig. 3a, but showing a decoupling assembly wherein use is made of a hollow, circular in cross-section column and spaced low- friction bearings;

Fig. 4 is an exploded view illustrating the components of a preferred bearing for use in the inven- tion;

Fig. 5 is an enlarged vertical sectional view illustrating one preferred spring-biased coupling means for securing the lowermost ends of pipes to the underlying foundation of a structure;

Fig. 6 is a view similar to that of Fig. 5, but showing another type of spring-biased pipe coupling means;

Fig. 7 is a sectional view illustrating compo ^¬ nents of a decoupling assembly in accordance with the invention, wherein use is made of a bundled plurality of pipes within a hollow column;

Fig. 8 is an enlarged, fragmentary view illus ^¬ trating the orientation of pipes in the Fig. 7 embodiment;

Fig. 9 is a fragmentary view illustrating the securement of one of the secondary reinforcing cables to the building frame base;

Fig. 10 is a sectional view taken along line 10- 10 of Fig. 5 and further illustrating the pipe coupling arrangement; Fig. 11 is a fragmentary, essentially schematic side view illustrating use of the decoupling assemblies of the present invention in the context of a multi -tiered basement forming a part of a large building, wherein the decoupling assemblies are interconnected between the supported structure and the first basement tier to provide a false ground for the assemblies;

Fig. 12 is an essentially schematic plan view illustrating the use of the present invention in the context of a small building having a basement, wherein the basement walls serve as the primary load-bearing member;

Fig. 13 is a fragmentary, essentially schematic side view depicting the interconnection of the decoupling assemblies in the Fig. 12 embodiment;

Fig. 14 is a cross-sectional view of one of the preferred concrete-filled pipes forming a part of the decoupling assemblies used in the Fig. 12 embodiment;

Fig. 15 is a plan view of a corner bearing plate used in the Fig. 12 embodiment;

Fig. 16 is an end view of the bearing plate depicted in Fig. 14;

Fig. 17 is a plan view of a rectilinear bearing plate used between corners in the Fig. 12 embodiment; and Fig. 18 is an end view of the bearing plate of Fig. 17.

Description of the Preferred Embodiments

Turning now to the drawings and particularly Fig. 1, the skeletal frame 20 of a multi -story building is illustrated, in conjunction with a plurality of stiffness decoupling assemblies 22, the latter being interposed between the base 24 of frame 20 and an underlying rein¬ forced concrete foundation 26. Broadly speaking, each decoupling assembly 22 includes a plurality of elongated, relatively flexible hollow pipes 28, a hollow, unitary, upright, load-bearing column 30 in surrounding relation¬ ship to the pipes 28, and bearing means broadly referred to by the numeral 32 operatively interposed between the underside of frame 20 and the upper ends of the respective columns 30. In more detail, the skeletal frame 20 is en¬ tirely conventional and includes, in addition to the base 24, the usual upright columns 34 and separate story floors 36, 38. The frame 20 is typically formed of any desired construction material such as reinforced concrete and presents, at strategic locations, conventional support areas 40 forming a part of the base 24. Each of the support areas 40 presents, in the illustrated example, a pair of transverse horizontal beams 40a, 40b.

Likewise, the foundation 26 is of the usual variety (except for the modifications herein described associated with the pipes 28) , and has footings 42; the foundation 26 is also formed of reinforced concrete.

Attention is next directed to Figs. 2 and 3a which illustrate in greater detail one of the decoupling assemblies 22. It will be observed in this respect that

the plural pipes 28 are arranged in spaced relationship to each other and present an array with peripheral pipes 28a and a central pipe 28b. These pipes are of conventional, thin-walled metallic construction, and would typically range in diameter from about 3/4 inch to 3 inches. Each of the pipes 28a, 28b is filled with an appropriate damping material, here concrete 44. The uppermost ends of the pipes 28 extend into and are embedded within the reinforced concrete of support area 40. As viewed in Fig. 1, it will be seen that the pipes extend upwardly through base 24 and into the associated column 34. In order to enhance the rigid connection of the pipes 28 to the frame 20, use may be made of laterally extending flanges or collars (not shown) on the pipes. More broadly, however, the pipes 28 may be secured to a structure such as frame 20 by any convenient and appropriate means, so long as a rigid connection is effected between the structure and the individual pipes. In the embodiment illustrated, the pipes 28 extend into and are embedded within the underly- ing footing 42 of foundation 26. Here again, other appropriate means of operatively connecting the pipes 28 to foundation 26 may be employed, and two preferred options are described in detail hereinafter.

Each overall assembly 22 further includes an upright, hollow, unitary, primary load-bearing column 46. In the embodiment of Figs. 1 - 3a, the column 46 is square in cross-section and includes vertical stiffeners 48 under each bearing. As illustrated in Fig. 2, a metallic reinforcement 50 passes through each stiffener 48 and into the underlying foundation 26, in order to enhance the rigidity and lateral stability of the columns 46. In this respect, it will be seen that each of the columns 46 rests atop a footing 42, and extends upwardly towards the skele¬ tal frame 20; at the upper end of each column 46, bearing supports are provided.

The overall bearing means 32 is made up of a number of identical bearing assemblies 54, with each of the latter being positioned beneath a respective beam 40a, 40b. Each of the bearing assemblies 54 (see Fig. 3a) includes a base 56 of truncated triangular configuration, the latter supporting an upstanding bearing pad 58 formed of material having a relatively low coefficient of fric¬ tion (e.g., bearing alloys formed of bronze, steel, lead or powdered sintered metals, with or without lubrication) . The base 56 (see Fig. 4) is provided with an aperture 60 adjacent each corner thereof in order to permit connection of the base to support plate 52. In addition, a pair of oppositely tapered, mated, slotted shims 62, 64 are stacked beneath each corner of base 56, with the slots thereof in registry with the associated aperture 60. A total of three somewhat J-shaped threaded connectors 66 are embedded within the column 46 for each bearing assem ^¬ bly 54 and extend upwardly to pass through the shim slots and apertures 60. Nuts 68 are then employed to secure the bearing assemblies in place on the column. It will be appreciated that provision of the mated shims 62, 64 allows proper adjustment of the height and location of each bearing assembly 54, so as to prevent uneven loading on the bearings and/or to establish a desirable normal loading on the bearings.

The respective bearing pads 58 are adapted to engage the underside of a support area 40 and to permit relative lateral movement between the load-bearing columns 46 and the frame 20. In order to facilitate such opera- tion, a metallic slide plate 70 is secured to the under ^¬ side of each support area 40 at the region where the pads 58 contact the support area. Each slide plate 70 is secured in place by a number of headed studs 72 embedded within the concrete or otherwise fixed to the associated support area formed of conventional building materials.

Fig. 3b illustrates a similar decoupling assem ^¬ bly 22 wherein use is made of a hollow, circular column 72. The latter is also provided with vertical stiffeners 74 at 90° intervals, and the latter have the interconnect- ing reinforcement 50 embedded therein, as in the case of square column 46. The decoupling assembly 22 of this embodiment also includes the plural, concrete-filled pipes 28, as well as a total of four bearing assemblies 54 above the stiffeners 74. Four separate slide plates 78 are affixed to the underside of the associated support area 40, and coact with a respective bearing pad 58 of each bearing assembly 54.

In preferred forms of the invention, at least the peripheral pipes 28a of each array within a respective column 46 or 72 are coupled to the foundation 20 in a manner to permit limited upward shifting movement of the peripheral pipes against an increasing biasing force. Attention is first directed to Figs. 5 and 10 which illustrate one such coupling arrangement. In particular, a pipe coupling device 80 is provided which includes a pipe-receiving base rigidly secured to a footing 42 and presenting a lowermost support plate 82 and an upper annular retaining ring 84 disposed in spaced relationship above the support plate. In the particular embodiment illustrated, the support plate 82 and ring 84 are embedded within the concrete of the footing 42. Moreover, the retaining ring is further secured against uprooting by means of nut and bolt assemblies 86 likewise embedded in the footing 42. The lowermost end of a pipe 28a received within device 80 is provided with an abutment plate 88 which is affixed with welding or other convenient means. The abutment plate 88 is configured and arranged for captively retaining the lowermost end of the pipe 28a between support plate 82 and retaining ring 84. A coil spring 90 is located between abutment plate 88 and ring 84

and disposed about the lowermost portion of pipe 28a received within device 80. As can be readily understood from the drawings, upward movement of the pipe 28a is against the bias of coil spring 90. Fig. 6 illustrates another similar pipe coupling device 92. In this instance, the device 92 includes a base 94 rigidly secured to the footing 42 and presenting an upwardly extending pin 96, the latter having an abut ^¬ ment plate 98 securely fixed to the upper end thereof. The base 94 is secured against uprooting by means of embedded nut and bolt assemblies 100.

In this embodiment, the lowermost end of pipe 28a is hollow and is provided with an engagement plate 102 and a retaining ring 104. As viewed in Fig. 6, the plate 102 is spaced upwardly from the lowermost butt end of the pipe 28a and is within the confines thereof. On the other hand, retaining ring 104 is located below the plate 102 but likewise within the confines of the pipe 28a. The ring 104 is of annular configuration and is adapted to slidably received pin 96 as shown; in such orientation, the abutment plate 98 and ring 104 cooperatively serve to retain the upper end of the pin 96 between the engagement plate 102 and retaining ring 104. A coil spring 106 is located between the retaining ring 104 and engagement plate 102 and is disposed about the upper received end of pin 96. Here again, it will be readily observed that upward movement of pipe 28 is against the bias of spring 106.

Although the pipes 28 illustrated in the embodi - ments of Figs. 1 - 3a and 3b are located in spaced rela¬ tionship to each other, the invention is not so limited. For example, a plurality of pipes 108 may be employed (see Figs. 7-8) wherein the individual concrete- filled pipes are placed in contact with one another to form a bundled array. Such pipes would also be substantially filled with

concrete 110 or similar damping material. While the particular type of pipe array is not critical, it is important to locate the pipes in sufficiently spaced relationship from the defining walls of the surrounding column or other support member to prevent significant contact between the pipes and the support member during a seismic occurrence. As best seen in Figs. 3a and 7, the minimum distance between the load-bearing member (i.e., column 46) and the pipe located closest thereto is greater than the maximum cross-sectional dimension (i.e., the diameter) of the closest pipe. Finally, while in pre¬ ferred forms use is made of unitary hollow support col ^¬ umns, the invention may also be practiced by using spaced apart upright plates or similar expedients which are dis- posed about a pipe array.

In order to provide the most effective earth¬ quake protection, the decoupling assemblies of the present invention may be used in conjunction with other devices designed to enhance the earthquake resistance of a given structure. For example, and referring again to Fig. 1, it will be seen that crossed flexible cables 112, 114 extend between and are embedded within the base 24 of frame 20 and foundation 26. At the time of installation, these cables are relatively loose and are maintained in a suspended condition (for example above a doorway 116) by means of a retaining spring 118. As those skilled in the art will appreciate, the cables 112, 114 serve to prevent undue lateral movement of the frame 20 relative to founda ^¬ tion 26 under exceptionally violent earthquake conditions. The connection of the cables 112, 114 can be effected by any convenient means, such as through the use of an embedded cross-pin 120 within base 24, with the end of a cable looped around the cross-pin and secured by connec¬ tors 122.

As an additional measure, reinforced concrete load-bearing walls 124 are advantageously provided between foundation 26 and base 24. In particular, the upper load- bearing surfaces of the walls 124 are disposed slightly below the engagement between the bearing means 32 and base 24. Thus, in the event of a complete failure, the build¬ ing structure may settle upon the load-bearing walls to prevent complete collapse of the entire structure.

During normal use of the decoupling assemblies 22, the primary structural load is borne by the upright columns 46 or 72, through the medium of the individual bearing assemblies 54. The concrete-filled pipes carry only a minor load in compression. As described, the primary columns 46 or 72 have no shear and moment connec- tors with the supported structure.

In the event of a seismic disturbance, the concrete-filled pipes associated with the support columns serve to substantially reduce the transmissibility of ground acceleration to the isolated structure, and as a consequence also reduce interstory drift. The concrete fill within the individual pipes serves as a local stiff- ener and damper during movement. In particular, the concrete fill tends to fragment and dampen movement much in the manner of a shock absorber . The pipes also serve as tension rods during such occurrences, to assist in prevention of separation and overturning of the protected structure. In this connection, use of spring-biased pipe connectors of the type described in Figs. 5 and 6 is particularly advantageous in that as overturning moments increase, the tension applied to retard overturning also increases. Finally, the concrete- filled pipes control the natural period of the protected structure and provide a restoring force to return the structure back to its neutral position after a seismic event.

The bearing assemblies support the structure and transmit eccentric loads to the support columns, in order to keep the structure in equilibrium while maintaining its stability under the motion of forced vibrations. There- fore, the bearings are designed to function much like roller supports with very little resistance to relatively lateral motions between the structure and the support columns .

The crossed cables 112, 114 serve as a non- linear spring to prevent excessive lateral displacement, i.e., the lateral resistance of the structure will be increased as needed. The load-bearing walls 124 are designed to be separate from the protected structure as long as the deformation is small. These walls become effective when the level of earthquake is excessive, i.e., when the vertical displacement induced by lateral deforma¬ tion of the structure is large enough to make contact between the protected structure and the load-bearing walls, the walls provide additional bearing strength to support the deformed structure and to provide friction forces to absorb the kinetic energy and reduce the ampli¬ tude of oscillation.

The invention therefore provides a number of advantages not attainable with prior designs. For exam- pie, the decoupling assemblies of the invention are passive devices and can absorb both compressive and tensile loads. The assemblies are long lasting and experience little deterioration in strength and function over time; this is to be contrasted with damaging aging processes which occur in resilient rubber bearing pads heretofore used. Most importantly, however, the decoupl¬ ing assemblies of the invention remain stable even under excessive, earthquake-induced lateral displacements to keep the protected structure in equilibrium both during and after strong earthquakes. At the same time, the

assemblies provide high tensile strength to resist over ^¬ turning moments.

Fig. 11 illustrates use of the present invention in the context of a large building 125 having a multi - tiered basement 126, i.e., a basement having subfloors 128, 130 and lowermost foundation 132. The subfloors 128, 130 would normally be provided with reinforcing metallic girders 134 as shown, with subfloor 128 having a rein¬ forced concrete section 136. As illustrated, the building 125 is supported by basement 126 and foundation 132. To this end, the building 125 presents a lowermost floor slab 138 having a reinforced concrete base cap 140. An exem¬ plary large hollow column 142 (shown in section in Fig. 11) extends between cap 140 and subfloor 128 and serves as a primary load-bearing member. It will also be observed that similar columns 144 and 146 are provided between subfloors 128, 130, and subfloor 130 and foundation 132, thereby providing complete structural support for the building 125. Of course, it will be readily apparent that the building 125 would have a number of similar hollow columns as depicted in Fig. 11 to provide adequate struc¬ tural support.

The decoupling assembly 148 is in the form of a plurality of upright, cement- filled metallic pipes 150 essentially identical with those described previously.

The lower ends of the pipes 150 are embedded with section 136, whereas the upper ends thereof extend into and are embedded within base cap 140 and slab 138. The pipes 150 operate during a seismic event in the same manner as described above. In this case, however, it is impractical to extend the pipes 150 from the structure 125 to the lower foundation 132. Accordingly, the section 136 effectively defines a false ground or foundation for the lower ends of the pipes 150.

It will also be seen that the upper surface of column 142 is provided with a series of bearings 152, which are in contact with slide plates 154 affixed to the underside of base cap 140. These bearing assemblies thereby permit relative lateral movement between the column 142 and the building 125.

Attention is next directed to Figs. 12-18 which illustrate use of the invention in the context of small buildings such as single family dwellings having base- ments. As depicted in Figs. 12-13, a house 156 is posi ^¬ tioned above and supported by interconnected, reinforced concrete basement walls 158. The house 156 is itself entirely conventional, and presents a lower floor struc ^¬ ture 160 typically including support beams 162 resting atop the basement walls 158. The basement structure includes the walls 158 as well as a concrete foundation 164 supporting the walls 158. As particularly shown in Fig. 12, the interior of the house basement may be pro¬ vided with non-load-bearing interior walls 166. Stiffness decoupling is provided by means of a plurality of upright cement-filled pipes 168 which extend from foundation 164 to floor structure 160. As best illustrated in Fig. 13, each of the pipes 168 is embedded within foundation 164. In order to afford an adequate connection, the lower ends of the pipes 168 may be pro ^¬ vided with radially enlarged collar structure 170. The upper ends of the individual pipes 168 are connected to house 156 by any convenient means serving to transmit shear, moment and tension. In the illustrated embodiment, the pipes 168 are affixed to beams 162 by means of connec ^¬ tors 172. Fig. 14 illustrates an exemplary pipe 168 in cross-section, where it will be seen that the pipe is filled with concrete 174.

Again referring to Fig. 12, it will be observed that clusters of pipes 168 are provided at corner regions

of the basement at the interconnection of basement walls. In addition, individual pipes 168 are provided in spaced relationship along the lengths of the walls. As also shown, pipes 168 may be disposed within the interior walls 166. This expedient permits certain of the pipes to be effectively hidden without detracting from their operabil- ity.

The upper surfaces of the basement walls 158 are provided with a series of bearing plates 176, 178. As shown, the bearing plates 176 are essentially L- shaped in plan configuration and are shown in detail in Figs. 15 and 16. Specifically, each of the plates 176 presents an upstanding section 180 presenting a planar uppermost surface 182. Apertured side flanges 184, 186 are also provided which permit mounting of the plates 176 through the use of threaded connectors 188 embedded within the concrete structure of the underlying basement walls 158. As can readily be seen from a review of Fig. 12, the plates 176 are secured to the orthogonal corners defined by the basement walls 158.

The plates 178 are virtually identical with the plates 176, except that the former are rectilinear and not L-shaped. As best seen in Figs. 17 and 18, each of the bearing plates 178 presents an uppermost bearing surface 190 and apertured side flanges 192 permitting mounting via connectors 188. The plates 178 are positioned in spaced relationship along the length of the basement walls 158 between the corner bearing plates 176.

Respective metallic slide plates or pads 194 are affixed to the underside of floor structure 160 (Fig. 13) in locations for engaging the upper surfaces of the bearing plates 176, 178. In this fashion, relative movement is allowed between the load-bearing basement walls 158 and the supported house 156.

As a matter of structural design, when the aspect ratio (length to height) of a basement wall 158 is greater than one, intermediate, inwardly extending struc¬ tural partitioning walls may be provided or the basement walls may be formed with integral vertical rib stiffeners. This insures that the basement wall structure has suffi¬ cient resistance to bending in order to properly accommo¬ date pressures induced by seismic events. Alternately, a soft filler having a thickness greater than that of the adjacent basement walls can be used between the soil and the basement walls to reduce earth pressures.

As illustrated in Fig. 12, the pipes 168 are positioned in spaced relationship to the adjacent basement walls 158. Although not fully shown in Fig. 12 because of the scale of the drawing, it should be understood that the pipes are placed at a distance at least twice the diameter thereof from the closest basement wall surface, i.e., the distance from the center of a given pipe to the closest wall surface should be greater than twice the diameter of that pipe.

Properly constructed houses or other small buildings employing the stiffness decouplers of the invention operate in the manner previously described to provide protection against earthquakes. That is, the pipes 168 resist overturning of the protected structure while the dead load of the house is borne almost entirely by the basement walls 158. The basement walls 158 more¬ over have no shear or moment connection with the supported house 156. While the primary utility of the invention has been described in terms of providing earthquake resis¬ tance, the invention is also useful in other contexts. For example, decoupling lateral stiffness from the sup¬ porting strength of columns can alleviate temperature induced stresses in long span rigid frame bridges.