18th International Conference on Structural Mechanics in Reactor Technology (SMiRT 18) Beijing, China, August 712, 2005 SMiRT18K072
UPGRADING THE SEISMIC PERFORMANCE OF THE INTERIOR
WATER PIPE SUPPORTING SYSTEM OF A COOLING TOWER
George C. Manos
Professor, Aristotle University
Lab. Of Strength of Materials and Structures
Fax: 00302310 995769
Email: gcmanos@civil.auth.gr
Thessaloniki, Greece
Vasilios J. Soulis
Postgraduate Student
Aristotle University
Thessaloniki, Greece
ABSTRACT
This paper presents results from a numerical study that was performed in order to simulate the seismic behavior of the interior support system of the piping and cooling features of a cooling tower in one of the old power stations located in an area at the NorthWestern part of Greece. This cooling tower has a diameter of 60m and a height of 100m. The interior piping support system consists mainly of a series of ninemeter high precast vertical columns made by prestressed concrete; these columns, together with reinforced concrete precast horizontal beams that are joined monolithically with the columns at their top, form the old interior supporting system. This system represented a very flexible structure, a fact that was verified from a preliminary numerical analysis of its seismic behavior. The maximum response to the design earthquake levels resulted in large horizontal displacements at the top of the columns as well as overstress to some of the columns. The most important part of the current numerical investigation was to examine various strengthening schemes of the old interior support system and to select one that will demonstrate acceptable seismic behavior.
Keywords: Cooling Tower, Seismic Upgrade, Interior Water Piping System.
1. INTRODUCTION
For Greece, new earthquake resistant design provisions were published in the Government's Gazette of October 1992 and they became mandatory in 1994 as Seismic Code of Greece. Figure 1 depicts the seismic
zonation as included on this 1992 seismic code. On May 13th, 1995, a Ms=6.6 earthquake occurred in the
prefecture of Kozani in the Northwest of Greece. The epicenter of this earthquake was quite close to the city of Kozani, the capital of the prefecture. To the North of this city and at distances varying from 10km to 30km, a number of coalburning thermoelectric Power Generating Stations are located. Some minor damage was reported for these facilities during this 1995 seismic event.
1.1 Seismic Zonation
After the 1995 event, a revision was introduced to the 1992 Provision which included a change in the seismic zonation of this particular region (EAK 2000). The city of Kozani together with the SouthWest region were placed in seismic zone II (the second lowest from a total of four seismic zones) whereas the Northern region, where the power stations are located, was left unchanged in seismic zone I (the zone with the lowest value of design peak ground acceleration, see figure 1).
the lowest seismic zone (design peak ground acceleration 0.12g) by introducing only three seismic zones. Consequently, a large part of regions belonging at present to zones I, II and III are assigned through the proposed change larger values of design peak ground acceleration than the values that are currently assigned. This increase is quite substantial; from 0.12g to 0.16g, from 0.16g to 0.24g and from 0.24g to 0.36g. It is evident from these changes that this 2003 proposal is in line with a continuous trend which has come out during the last 50 years in Greece regarding issues of seismic zonation. This trend is best described by the following:
 Every earthquake sequence that is accompanied by structural damage is followed by introducing higher
forcing levels to the corresponding locality through a local change in the seismic zonation. Latest examples of this trend are the region in Kozani prefecture (Kozani earthquake 1995) and the region of the Athens Metropolitan area (Athens earthquake of 1999).
 A systematic gradual increase throughout the last 50 years of the forcing levels springing from either the
seismic zonation change or the equivalent ground acceleration coefficients. The latter source of forcing levels’ increase has been indirectly moderated by the relevant response modification factors that are also part of the current seismic code, and have been left unchanged since their introduction 10 years ago.
IV(0.36) IV(0.36) III(0.24) III(0.24) II(0.16) II(0.16) I(0.12) I(0.12) Seismic zones Seismic zones
IV(0.36) IV(0.36) III(0.24) III(0.24) II(0.16) II(0.16) I(0.12) I(0.12) Seismic zones Seismic zones
Figure 1. The seismic zonaion of Greece according to EAK 2000 (reference 1)
modification factors) the seismic code regulators prefer to make seismic zone changes like the ones described above; these are not derived from any longterm studies or qualified geotectonic scenarios. A further consequence of such changes is that structures which were designed and built prior to these changes, according to the seismic zonation which was valid at the time, are downgraded as structures designed to withstand seismic loads below the levels specified after such changes are introduced.
For the problem of the cooling tower a comparison of the forcing levels, as represented by the effective ground acceleration, at various stages is as follows.
Forcing levels during the original design 1960 (0.06g Zone I, soil B) Forcing levels during the upgrade carried out in 2002 (0.087g , see section 5 ) Forcing levels after the proposed seismic zone change in 2003 (0.116g).
The 2003 seismic zone change results in a 33% increase in seismic forces compared to the seismic loads assumed for the 2002 upgrade of the water pipe support system.
III(0.36) III(0.36) II(0.24) II(0.24) I(0.16) I(0.16) Seismic zones Seismic zones
III(0.36) III(0.36) II(0.24) II(0.24) I(0.16) I(0.16) Seismic zones Seismic zones
Figure 2. Seismic zonation of Greece according to the June 2003 proposal
1.2 Investigation into the strength of materials and structural elements
A laboratory investigation was performed in order to obtain measured values of strength for materials and structural elements of the old pipesupport system as well as for those that were utilized in the upgrading scheme.
1.1.1 Mechanical properties of materials and elements of the old pipesupport system.
Table 1. Laboratory tests on steel specimens taken from the existing old piping support system
NominalDiameter (mm)
Ultimate Tensile Load (KN)
Ultimate Tensile Stress (MPa)
Remarks for the formation of the tested specimen
4.05 7.21 560 machined central part
4.05 7.21 560 machined central part
6.00 14.225 503 No part is machined
Tests were performed at the laboratory with samples from elements named “Moise” that were the filling inplane horizontal elements of the old pipe supporting system. These samples were removed from the old system and were tested in flexure. These elements have reinforcement only at their bottom side. The ultimate positive and negative bending moments, found from these tests, are equal to 1.206KNm and 0.313KNm, respectively. These “Moise” were removed from all the bays of the interior frames of the upgraded system.
1.1.2. Mechanical properties of materials and elements used in the upgrading scheme.
Fiber reinforced polymer profiles (FRP) were utilized in the upgrading scheme. The merit of employing such
structural elements springs from their high strength and their durability in the humid environment of the interior
of the cooling tower. Certain laboratory tests were performed with samples taken from these FRP structural
elements.
For the FRP
[
200mm x 80mm x 8mm profile 4 samples (No. 1,2,3,4) were taken with their axes parallel to thelongitudinal axis of such a structural element and two samples (No. 5,6) with their axes parallel to the tangential axis of this type of element. The cross section of all these samples was 23.2mm x 8mm. The following table 2 lists the tensile strength measured for all these samples in the laboratory.
Table 2. Results from laboratory tests on FRP specimens which were used on the upgrading
scheme.
Sample Number
Ultimate Tensile Load (KN)
Ultimate Tensile Stress (MPa)
Remarks for the formation of the tested specimen
No. 1. 73.575 386 Parallel to Long. axis
No. 2. 97.119 510 Parallel to Long. axis
No. 3. 74.556 392 Parallel to Long. axis
No. 4. 68.670 360 Parallel to Long. axis
No. 5. 2.453 12.9 Parallel to tang. Axis
Figure 3. Axial Compression Test
of an FRP cross section
An average tensile strength equal to 412MPa or 12.9MPa results from the above measurements along the longitudinal and tangential axis. The observed longitudinal tensile strength is well above the specified tensile strength, which is provided by the manufacturer to be equal to 240MPa.
A specimen from the same cross section (FRP [ 200mm x
80mm x 8mm) was formed with a height equal to 198.7mm and was subjected to axial compression along the longitudinal axis. The ultimate compression stress found in this way was equal to 175.4 MPa. The ultimate compression stress accepted in this study was taken equal to 40% the specified tensile strength (the corresponding value given by the manufacturer was equal to 240MPa); thus the initial compression strength was taken equal to 96MPa. For internal forces dominated by the seismic actions this limit strength value was increased by 33%. In order to check the validity of these assumptions two samples were formed from the rectangular profile [] 75mm x 75mm x 6mm.
The height of the first sample was equal to 897mm whereas the height of the second specimen was equal to 2370mm. The ultimate compression strength found from the first test was equal to 314MPa whereas for the
second specimen was found equal to 67.7MPa. The behavior of the 1st specimen was not influenced by buckling,
whereas, on the contrary, the 2nd specimen developed significant outofplane buckling behavior at ultimate load (figure 3). The reduction factor that would be assumed for this specimen, according to the procedure followed in this study, would have had a value equal to 1.62. Thus the assumed ultimate compression stress would be equal to 96/1.62 = 59.26MPa, which is 14% lower than the observed value (67.7MPa).
The flexural behavior of the FRP profiles was tested by subjecting a 1200mm long specimen of [] 75mm x 75mm x 6mm to three point flexure. The test was stopped when local failure developed at the two pointload location at the upper side of the specimen for a load equal to 26KN. The corresponding lower limit of an ultimate stress is equal to 235MPa.
2. LOADING CONSIDERATIONS
The new support system for the piping, which was designed to replace the old system, introduced a new distribution of the masses. The masses were concentrated at two levels; the first was at a level +7.7m from ground level (9.0m from the base of the columns) and the second at a level +5.30m from ground level (6.60m from the base of the columns). For the new structural system the upper level consists of structural elements supporting at this level the primary and secondary water piping system; the lower level consist of structural elements supporting at this level the water cooling plastic elements. More details of the structural system are given in the next section. The weight that is concentrated at the upper level is equal to 2298KN whereas at the
lower level is 5670KN. Thus the total weight (D) for the whole structure is 7968KN. The combination of loading
arrangements that were considered are as follows:
D + Ex + 0.3Ey (where Ex, Ey the seismic load in the xx and yy directions, respectively).
Due to the cyclic symmetry of the structural system the above load combination leads to an identical response as that of the combination D + Ey + 0.3Ex
For the radial beams of both the upper and lower levels the additional load combination 1.4 D + 1.7 L (where L
3. STRUCTURAL SYSTEM
The piping support system was a system with cyclic symmetry with a central axis passing through the central axis of the cooling tower (figure 4). A view of the interior of the cooling tower where the old piping support system can be seen is depicted in figure 5.
Both the old and the new support system can be envisaged as being formed by twelve identical substructures, joined together in a cyclic symmetrical way. Such a unit substructure, representing one twelfth (1/12) of the whole structure, is depicted in figure 6.
3.1. Structural elements of the old system
All the structural elements of the old system, apart from the “Moise”, became part of the new system. The “Moise” at the interior of the cooling tower were dismantled (figure 5). In contrast, the vertical prestressed concrete columns, present in the old system, were retained in the new structural system without any modification (see also figure 5). For the final numerical simulation of the structural system, these columns, with a crosssection 220mm by 250mm, were considered to be hinged at their ground support, as requested by the client, although an elastic fixity condition is thought to represent better the actual support conditions. The hinged support was thought to be more demanding for the additional structural elements. In alternative numerical simulations the absolute fixity of these columns to the ground support was also considered. Because these columns were prestressed their crosssection was taken as 100% active in deriving their stiffness properties. At the upper level (+7.7m) there are 12 cocentric rings, formed by horizontal precast beams that are linked monolithically to the vertical columns. The beams of the internal ring have a width of 220mm and a height of 160mm. For the beams of this internal ring 40% of the axial and bending stiffness were adopted in the numerical simulation. The beams of the remaining 11 rings have a width of 80mm and a height of 160mm. The connection of these cocentric ring type beams and the columns was considered to be monolithic (see again figure 5).
Figure 4. Outside view of the cooling tower Figure 5. View of the interior of the cooling
tower with the old piping support system
3.2. Additional structural elements to form the new structural system
8mm made by FRP. A single [ section is employed at the level +7.7m whereas a ] [ double section is employed at the level +5.5m. These beams are considered to be connected with absolute fixity conditions with the existing columns (this was done employing the appropriate connection detailing)
 Cross diagonal bracing was added at the central plane of each of the 12 substructures. Each such central
plane was formed by 6 bays (see figure 4); four of these bays can be considered as internal bays whereas two of them as external. The employed diagonal bracing connected the base of one column with the +5.3m level
of the adjacent column for each bay. As diagonal bracing elements for the internal bays square closed FRP
sections [] 160mm x 70mm x 5mm ]were employed; for the external bays square closed FRP sections []
75mm x 70mm x 5mm were employed instead.
 An internal horizontal central ring was added at level +5.3m. This ring connected all the central columns of
the existing structural system in a way similar to the existing central ring at the +7.7m level. This additional
central ring was formed by two square closed FRP sections [] 75mm x 75mm x 6mm.
The new components of the upgraded structural system are depicted in figure 6 as well as in figure 7 as they were installed in place during the upgrading works at the interior of the cooling tower.
Figure 6. A unit substructure representing 1/12 Figure 7. View of the upgraded system
of the whole system(the diagonals shown here are
part of the upgraded system)
4. MODAL ANALYSIS – DERIVATION OF THE SEISMIC LOAD
The upgraded structural system (“new system”), which was formed by the old and the new structural elements, was considered as a whole (with all the twelve substructures) in the modal analysis. The two first eigenmodes of this structure were the xx and yy translational modes both at 1.126 seconds, mobilizing 91.3% and 91.0% of the total mass, respectively (see figure 8). The structural system without any upgrading was simulated by assuming base fixity for the vertical columns at their foundation level as well as by neglecting the stiffness of the “Moise” (this is named “old system”). The corresponding modal frequencies of this “old system” was estimated to be in the range of 2.0 seconds. The modal frequencies, that were found for the upgraded system, led to a design spectral acceleration value equal to Rd=0.087g, based on the current seismic code of Greece and the seismic zonation conditions posed by the client. These were as follows:
Peak Ground Acceleration α=0.12g, Soil Category B, Importance factor γ=1.0, Response modification factor
T=1.126sec
THZ THZTHZ_{THZ} THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ_{THZ} THZ THZTHZTHZ THZTHZTHZ
THY THZ THY THZ THY THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZTHZ THZTHZ THZ THZTHZ THY THZ THY THZ
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THZ THZTHZ
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THZ_{THZ} THZ_{THZ} THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZTHZ THZ THZ THZ_{THZ} THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ_{THZ} THZ THZTHZTHZ THZTHZTHZ
THY THZ THY THZ THY THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZTHZ THZTHZ THZ THZTHZ THY THZ THY THZ
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THZ_{THZ} THZ_{THZ} THZ_{THZ} THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZ THZTHZ
Figure 8. Translational eigenmodes xx and yy
5. NUMERICAL ANALYSIS
As already mentioned in section 3, the weight of the various structural and nonstructural components (dead load) was applied together with the seismic forces that were derived as described in section 4 (earthquake load). The seismic forces were applied simultaneously with 100% of the earthquake load along the direction yy and 30% of the earthquake load along the direction xx, together with the dead load. In what follows a summary of the obtained results is presented.
5.1. Displacement Response and Stress Resultant Response
The peak displacement and stress resultant response, obtained for the upgraded system, is listed in tables 3 and 4.
In figures 9 and 10 the employed mesh and the deformed shape for the load combination D + Ey + 0.3 Ex is
shown. The axial stress distribution is depicted in figure 11 and 12. Figure 12 depicts in particular the concentration of axial stress at one of the diagonal FRP bracings. In figure 13, the concentration of moments at the old R.C beams of central ring at level +7.7 , together with the concentration of moments for the FRP beams at level +5.3 are shown (see also figure 15). Finally, in figure 14 the concentration of Shear Forces at the weak direction of the columns is depicted.
Table 3. Peak Displacement Response of the “new system” (Columns hinged at the base,
Radial Beams fixed with the columns at levels +7.7m and 5.3m)
(Design Spectral Acceleration equal to Rd = 0.087g ) Simulation of the Upgraded System*
Code number of the numerical analysis 5.3
Load combination D + Ey + 0.3 Ex
Peak horizontal displacement at level +7.7m (resultant) Central axis / external peripheral ring
38.22mm / 39.76mm
Peak horizontal displacement at level +5.3m (resultant) Central axis / external peripheral ring
30.78mm / 32.22mm
Table 4. Peak StressResultant Response of the “new system” (Columns hinged at the base,
Radial Beams fixed with the columns at levels +7.7m and 5.3m)
(Design Spectral Acceleration equal to Rd = 0.087g )
Code number of the numerical analysis 5.3
Load combination Ey + 0.3 Ex
Peak Axial Force at the old columns due to earthquake loading
level +5.3m
±19.153(KN) (Μ=3.643KNm weak )
(substructure y  y 30ο)
Maximum Bending Moment at the old column * (level 5.3m)
Weak direction (Μz)
10.990KNm (Ν=3.780KN)
(substructure y  y +30ο) Maximum Bending Moment at the old column (level 5.3m)
Strong direction (Μy)
4.908KNm (Ν=2.082KN)
Peak Axial Force at the columns (level –0.93m) ±33.305KN (Μ= 0 ) (sub. y  y +30ο)
Maximum Shear Force at the old columns – Weak Direction (Qy) Level –0.93m / level +5.3m / level +7.7m
16.336KN / 1.788KN / 21.820KN (substructure y  y +30ο) Maximum Shear Force at the old columns – Strong Direction
(Qz) level –0.93m / level +5.3m / level +7.7m
0.734 / 0.734 / 6.132 (substructure x  x +30ο)
Maximum Axial Force at the old peripheral beams (level +7.7m) ±17.380 KN (Μz , My = 0.052 KNm)
Maximum Bending Moment at the old peripheral beams (level
+7.7m) Weak Direction (Mz) (maximum torque = 0)
0.699 KNm (Ν=14.170 KN)
0.609 KNm (at the column’s face)
Maximum Bending Moment at the old peripheral beams (level
+7.7m) Strong Direction (My) (maximum torque = 0)
1.264 KNm (Ν=5.364 KN)
1.093 KNm (at the column’s face)
Maximum Shear Force at the old peripheral beams (level +7.7m) Qy=0.945 KN Qz=1.371 KN
Max. Axial Force at the old R.C beams of central ring (level +7.7) (KN). (Maximum torque = 1.031 KNm)
±5.333KN (Μz =12.377 KNm, My =0.356
KNm) ( substructure y  y +30ο)
Max. Bending Moment at the old R.C beams of central ring (level+7.7) Strong Direction (Μz.) Weak Direction (Μy)
Μz =12.900KNm, N= 4.745KN (sub y  y)
My = 1.611KNm, N= 5.070KN (sub x  x )
Maximum Shear Force at the old R.C beam of the central ring (level +7.7). Strong Direction (Qy) Weak Direction (Qz)
Qy = 14.030 Qz =1.688
Peak Axial Force for the FRP Diagonal Bracing
External Bays / Internal Bays
±17.040(KN) (substructure y y – 30ο) ±20.930(KN) (substructure y  y – 30ο)
Maximum Axial Force at the radial FRP beams (level +7.7m) ±23.890 KN (substructure y  y +30ο)
Maximum Axial Force at the radial FRP beams (level +5.3m) ±36.470KN (substructure y  y +30ο)
Max. Bending Moment at the radial FRP beams (level +7.7m)
Strong Direction (My) (Qy = 0.107 KN)
3.060 KNm (Ν= 16.748KN)
(substructure y y +30ο)
Max. Bending Moment at the radial FRP beams (level +7.7m)
Weak Direction (Μz.) (Qz = 1.777 KNt)
0.199 KNm ( N=0.339 KN) (substructure y y )
Max. Bending Moment at the radial FRP beams (level +5.3m)
Strong Direction (My) (Qy = 0.179 KN)
4.408 KNm (Ν=36.413 KN)
(substructure y y +30ο)
Max. Bending Moment at the radial FRP beams (level +5.3m)
Weak Direction (Μz.) (Qz = 2.391 KN )
0.343 KNm ( N=0.012 KN) (substructure y y ) Max. Axial Force at the FRP beams of the central ring
(level +5.3) ( Maximum torque = 0)
±3.573 KN (Μz = 16.620 KNm, My =
0.040 KNm) ( substructure y  y +30ο)
Max. Bending Moment at the FRP beams of the central ring
(level +5.3) Strong Direction (Μz.) Weak Direction (Μy)
Μz =16.840KNm, N=3.139 KN (yy 30ο)
My =0.191KNm, N= 0.594 KN(x x 
30ο)
Maximum Shear Force at the FRP beams of the central ring
(level +5.3) Strong Direction (Qy) Weak Direction (Qz)
Qy =18.330 KN , Qz =0.208 KN
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Figure 9. Finite Element model of the upgraded pipe supporting system.
Figure 10. Deformed Shape of the upgraded scheme.
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Figure 11 . Concentration of Axial Forces in the Upgraded Scheme.
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Figure 12 . Axial Forces for the FRP Diagonal Bracing External Bays / Internal
Bays.
1 6 7 03 . 6 1 4 3 17 . 4 1 1 9 31 . 1 9 5 4 4.9 2 7 1 5 8.6 9 4 7 7 2.4 6 2 3 8 6.2 3 0 2 3 86 .2 3 4 7 72 .4 6 7 1 58 .6 9 9 5 44 .9 2 1 1 93 1 .1 1 4 31 7 .4 1 6 70 3 .6 1 9 08 9 .8 CONT OURS OF M z ST RESS
Co m b i n ati o n 3
Figure 14 .Concentration of Shear Forces at the weak direction of the columns
Combination 3 ST RESS CONT OURS OF Fy
22000 19066.7 16133.3 13200 10266.7 7333.33 4400 1466.67 1466.67 4400 7333.33 10266.7 13200 16133.3 19066.7 22000
T HY T HZ T HZ T HZ T HZ T HZ T HZ T HZ T HZ_{T HZ} T HZ
T HZT HZT HZT HZ T HZ T HY T HZT HZT HZ T HZ T HZ T HZ T HZ T HZ T HZ T HZT HZ
T HZ T HZ T HZ
T HY T HZ T HZ T HZ T HZ T HZT HY T HZ
T HZ T HZ T HZT HZ
T HZT HZ T HZT HY T HZ
T HY T HZ T HY T HZ T HY T HZ
T HY T HZ T HY T HZT HY T HZ T HZT HZ
T HZT HZT HZT HZT HZ
T HZT HY T HZT HZT HZT HZ
T HY T HZ T HY T HZ T HZ
T HY T HZ
T HZT HZT HZT HY T HZT HZT HZT HZT HZT HZ T HZ T HZ T HZ
T HZ T HZ T HZT HZ
T HZ T HZ T HY T HZ
T HY T HZ T HY T HZ
T HY T HZ T HZT HZ
T HZ T HZ T HY T HZ_{T HZ}_{T HZ}
T HZ T HZ_{T HZ}
T HZ_{T HZ}
T HZ
T HY T HZ T HZT HZ
T HZT HZ T HZ T HZ T HZ T HZ T HZ_{T HZ}
T HZ_{T HZ}_{T HZ}
T HZ T HY T HZ T HZ T HZ T HZ
T HY T HZ
Table 4. Peak StressResultant Response of the “new system” (Columns hinged at the base,
Radial Beams fixed with the columns at levels +7.7m and 5.3m)
(Design Spectral Acceleration equal to Rd = 0.087g )
Code number of the numerical analysis 5.3
Load combination Ey + 0.3 Ex
Peak Axial Force at the old columns due to earthquake loading
level +5.3m
±19.153(KN) (Μ=3.643KNm weak )
(substructure y  y 30ο)
Maximum Bending Moment at the old column * (level 5.3m)
Weak direction (Μz)
10.990KNm (Ν=3.780KN)
(substructure y  y +30ο) Maximum Bending Moment at the old column (level 5.3m)
Strong direction (Μy)
4.908KNm (Ν=2.082KN)
Peak Axial Force at the columns (level –0.93m) ±33.305KN (Μ= 0 ) (sub. y  y +30ο)
Maximum Shear Force at the old columns – Weak Direction (Qy) Level –0.93m / level +5.3m / level +7.7m
16.336KN / 1.788KN / 21.820KN (substructure y  y +30ο) Maximum Shear Force at the old columns – Strong Direction
(Qz) level –0.93m / level +5.3m / level +7.7m
0.734 / 0.734 / 6.132 (substructure x  x +30ο)
Maximum Axial Force at the old peripheral beams (level +7.7m) ±17.380 KN (Μz , My = 0.052 KNm)
Maximum Bending Moment at the old peripheral beams (level
+7.7m) Weak Direction (Mz) (maximum torque = 0)
0.699 KNm (Ν=14.170 KN)
0.609 KNm (at the column’s face)
Maximum Bending Moment at the old peripheral beams (level
+7.7m) Strong Direction (My) (maximum torque = 0)
1.264 KNm (Ν=5.364 KN)
1.093 KNm (at the column’s face)
Maximum Shear Force at the old peripheral beams (level +7.7m) Qy=0.945 KN Qz=1.371 KN
Max. Axial Force at the old R.C beams of central ring (level +7.7) (KN). (Maximum torque = 1.031 KNm)
±5.333KN (Μz =12.377 KNm, My =0.356
KNm) ( substructure y  y +30ο)
Max. Bending Moment at the old R.C beams of central ring (level+7.7) Strong Direction (Μz.) Weak Direction (Μy)
Μz =12.900KNm, N= 4.745KN (sub y  y)
My = 1.611KNm, N= 5.070KN (sub x  x )
Maximum Shear Force at the old R.C beam of the central ring (level +7.7). Strong Direction (Qy) Weak Direction (Qz)
Qy = 14.030 Qz =1.688
Peak Axial Force for the FRP Diagonal Bracing
External Bays / Internal Bays
Maximum Axial Force at the radial FRP beams (level +7.7m) ±23.890 KN (substructure y  y +30ο)
Maximum Axial Force at the radial FRP beams (level +5.3m) ±36.470KN (substructure y  y +30ο)
Max. Bending Moment at the radial FRP beams (level +7.7m)
Strong Direction (My) (Qy = 0.107 KN)
3.060 KNm (Ν= 16.748KN)
(substructure y y +30ο)
Max. Bending Moment at the radial FRP beams (level +7.7m)
Weak Direction (Μz.) (Qz = 1.777 KNt)
0.199 KNm ( N=0.339 KN) (substructure y y )
Max. Bending Moment at the radial FRP beams (level +5.3m)
Strong Direction (My) (Qy = 0.179 KN)
4.408 KNm (Ν=36.413 KN)
(substructure y y +30ο)
Max. Bending Moment at the radial FRP beams (level +5.3m)
Weak Direction (Μz.) (Qz = 2.391 KN )
0.343 KNm ( N=0.012 KN) (substructure y y ) Max. Axial Force at the FRP beams of the central ring
(level +5.3) ( Maximum torque = 0)
±3.573 KN (Μz = 16.620 KNm, My =
0.040 KNm) ( substructure y  y +30ο)
Max. Bending Moment at the FRP beams of the central ring
(level +5.3) Strong Direction (Μz.) Weak Direction (Μy)
Μz =16.840KNm, N=3.139 KN (yy 30ο)
My =0.191KNm, N= 0.594 KN(x x 
30ο)
Maximum Shear Force at the FRP beams of the central ring
(level +5.3) Strong Direction (Qy) Weak Direction (Qz)
Qy =18.330 KN , Qz =0.208 KN
* The peak maximum bending moment at the column * (foundation level of columns ) Weak direction (Μz) estimated for the “old system” were of the order of 200mm
Figure 15. Connection of the cocentric ring type beams and the columns
. DISCUSSION OF THE RESULTS – CONCLUSIONS
The presented results belong to an upgrading system that was selected from a number of alternative
 structural elements was dictated primarily by the construction
6

upgrading schemes that were studied in this investigation. The basic concept of all the alternative upgrading schemes was the same. They employ diagonal stiffening elements at the bays formed by the vertical R.C. columns as well as horizontal radial beams.
The location and number of the additional
 n was to check the capacity of both the old and new structural members
he numerical investigation, which was performed, demonstrated that the proposed upgrading of the old
e level
REFERENCES
. Provisions of Greek Seismic Code 2000 , OASP, Athens, December 1999.
December 1999.
that were considered, was based on its effectiveness in inhibiting the development of excessive displacement response for the earthquake loads.
An additional part of this investigatio
of the upgraded system to demands arising from the used load combinations (Table 2). Towards this goal the experimental results of the mechanical properties, presented in section 1, were utilized, together with established design principles and safety factors.
T
structural system, employing fiber reinforced plastic (FRP) components, is successful in the following:
 It limits the levels of maximum displacements that develop at the columns of the old system both at th
of 5.3m as well as at the top level of 7.7m. This decrease is quite significant (approximately 5 times). This reduction was achieved with the addition of the new structural elements made of FRP, as described in this paper.
1