U.S. patent application number 10/717216 was filed with the patent office on 2004-12-16 for rc building seismic reinforcement method utilizing steel portal frames without braces.
This patent application is currently assigned to Forest Engineering & Economics Co., Ltd.. Invention is credited to Imai, Katsuhiko.
Application Number | 20040250484 10/717216 |
Document ID | / |
Family ID | 32702261 |
Filed Date | 2004-12-16 |
United States Patent
Application |
20040250484 |
Kind Code |
A1 |
Imai, Katsuhiko |
December 16, 2004 |
RC building seismic reinforcement method utilizing steel portal
frames without braces
Abstract
A portal frame 3 made of wide flange sections without braces is
fixed to the external surfaces of an existing RC-column 5 extending
in the vertical direction and to the external surfaces of existing
RC-beam 8 extending in the horizontal direction of a reinforced
concrete building. The wide flange section column 4 fixed to the
existing RC-column 5 is assigned a bending rigidity roughly
equivalent to that of the existing RC-column 5, thereby reducing
the stress additionally occurring at the connecting part between
the existing RC-column 5 and column 4 by deforming the column 4
similarly to the existing RC-column 5 under the horizontal load
transmitted from existing RC-beam 8 and/or wide flange section beam
6 during an earthquake. Using a steel portal frame without braces
preserves the building appearance and avoids bracing which may
block windows while increasing earthquake resistance.
Inventors: |
Imai, Katsuhiko; (Osaka,
JP) |
Correspondence
Address: |
WILLIAM J. SAPONE
COLEMAN SUDOL SAPONE P.C.
714 COLORADO AVENUE
BRIDGE PORT
CT
06605
US
|
Assignee: |
Forest Engineering & Economics
Co., Ltd.
Hyogo
JP
Penta-Ocean Construction Co., Ltd.
Tokyo
JP
Neturen Co., Ltd.
Tokyo
JP
Kabushiki Kaisha Kanayama Koumuten
Osaka
JP
|
Family ID: |
32702261 |
Appl. No.: |
10/717216 |
Filed: |
November 19, 2003 |
Current U.S.
Class: |
52/167.7 ;
52/167.1; 52/167.8; 52/167.9 |
Current CPC
Class: |
E04H 9/028 20130101;
E04H 9/0237 20200501; E04H 9/02 20130101 |
Class at
Publication: |
052/167.7 ;
052/167.1; 052/167.8; 052/167.9 |
International
Class: |
E04B 001/98; E04H
009/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 22, 2002 |
JP |
2002-339274 |
Claims
1-7. (cancelled)
8. A seismic reinforcement method for existing reinforced concrete
structure having openings steel frame for stiffening the
RC-structure; fixing the steel frame to an outside of the
reinforced concrete structure; making said steel frame with wide
flange section without braces, fixing the steel frame outside of a
reinforced concrete column extending in a vertical direction and to
an outside of an existing RC-beam extending in a horizontal
direction of said building, the wide flange section column of said
portal frame having a bending rigidity roughly equivalent to that
of an existing RC-column, reducing the stress occurring at a
connecting part between the existing RC-column and wide flange
section column by deforming the wide flange section similar to the
existing RC-column under a horizontal load transmitted from the
existing RC-beam and/or wide flange section beam during an
earthquake, and increasing the strength in the horizontal direction
of the combination of the RC-column and the wide flange section
column by decreasing the deformation of the RC-column after
yielding so as to equalize the range of quasi-elastic deformation
of said combination to that of elastic deformation of the wide
flange section column.
9. The method according to claim 8, locating said wide flange
section column has an H-shape in cross section, and the web close
to said RC-column.
10. The method according to claim 8 further comprising fixing tie
hoops on the outer surface thereof and increasing the bending
rigidity thereof by placing cement mortar or concrete into a space
accommodating said tie hoops which is engaged with vertical
bars.
11. The method according to claim 8 wherein said wide flange
section column is made of a steel of low yield point, reducing
yield bending strength only without reduction of the bending
rigidity thereof, for reducing a response stress thereof during the
earthquake through plasticization hastened by yielding the
combination of the RC-column and the wide flange section column at
a bending strength of approximately 2 to 4 times as strong as the
existing RC-column.
12. The method according to claim 8 further comprising providing a
T-section, in the form of a T in plan view, extending over the
structure welding a leg of the T section on an outer surface of the
column at a tip thereof for three-dimensionally reinforcing the
building by the alignment of the T-section with interior RC-beams
or earthquake resisting walls extending perpendicularly to the
external walls and being united to the existing RC-columns to be
reinforced by the wide flange section columns.
13. The method according to claim 12, wherein; said T-section
projects outside said structure as wide as a verandah of each story
thereof.
14. The method according to claim 13, further comprising placing
additional beams made of high strength fluidized concrete or cement
mortar on both sides of said interior RC-beams or RC-beams over the
existing earthquake resisting walls for obtaining a desirable
bending moment based on post-tension generated by unbonded
prestressed steel bars buried in the additional beams for attaining
strength in a horizontal direction of said beams.
Description
TECHNICAL FIELD
[0001] The present invention relates to a seismic reinforcement
method for a reinforced concrete (RC) building utilizing rigid
steel frames without braces and, more particularly, to a method for
stiffening a reinforced concrete building of low seismic resistance
capacity by fixing steel portal frames to the external surfaces
around openings such as windows of the building.
BACKGROUND
[0002] A reinforced concrete building of low seismic resistance
capacity is often stiffened by enlarging the sectional area of
columns thereof. For example, the existing RC-column 50 in FIG.
12(a) is covered with additional reinforced concrete 51 as shown in
FIG. 12(b) or is wound by steel plates 52 as shown in FIG. 12(c).
Such methods are only applicable to a fully naked structure like a
bridge pier.
[0003] A brace is another means for reinforcing structures. Such a
reinforcement method is also applicable to schoolhouses, apartment
houses and/or office buildings having columns which can not be
accessed on all sides, i.e., the brace is used to stiffen the RC
portal frame using columns and beams bridged over said columns.
[0004] A portal frame is introduced into an existing RC-structure
since it is impossible to directly bridge a brace over an RC-column
and an RC-beam thereof. FIG. 13(a) shows an example of a steel
portal frame 54 with braces 53 buried in the wall and FIG. 13(b) is
an example of a steel portal frame 55 fixed to the outside of the
columns and beams. These days, this type of steel portal frame with
braces has been typical of the reinforcement used because of the
short time for reinforcement completion but also because it does
not increase building weight. In particular, a method utilizing
steel portal frames applied to the outside of an existing building
makes it unnecessary to remove windows and walls around the windows
thereof, and is increasingly used to remarkably shorten the period
for completing the building reinforcement.
[0005] JP11-193639A1 discloses a method using steel portal frames
applied to the external surfaces of columns and beams so that the
reinforcement may be carried out quickly and easily. A brace makes
such a portal frame for reinforcement more stable, i.e., the brace
of the steel portal frame combined with RC-columns and RC-beams
consequently reinforces the RC-structure.
[0006] Such a method can also reinforce the building while leaving
inhabitants therein. The frame however is made of wide flange
sections which introduce a sharp change in appearance to the old
apartment house, which were often originally monotonous. Coloring
the frames sometimes changes the original appearance of the house.
Further, use of a tube-in-tube type brace (see e.g., JP11-193570A1)
having an outer diameter much smaller than the members of the frame
provides the building with a lilting impression based on the small
appearance of the brace.
[0007] It has been recognized that the steel portal frame without
braces never contributes to the reinforcement of an RC-structure
because the frame is more deformative under the RC-column forces
and it is required to introduce braces into the steel portal frame.
The understanding that a brace is indispensable for the portal
frame reinforcing the RC-structure has already been widely and
deeply accepted by builders.
[0008] A portal frame with braces need not provide the frame
members with high rigidity, so that the sectional area of the
members can be rather small. On the other hand, it is important to
improve the characteristic of braces to be introduced into the
frame, therefore, research and development for the improvement of
the bracing has continued.
[0009] The above illustrates that the frame is merely an auxiliary
member as a toehold for fixing a brace to a building. It has been
believed that the steel portal frame being more deformative than
the RC-structure will not reinforce the RC-structure, and
therefore, a brace is always necessary to stiffen the frame.
SUMMARY OF THE INVENTION
[0010] The steel portal frame with braces, where the stress acting
on the brace is transmitted to the columns and beams of its own
frame, has the rigidity in the horizontal direction remarkably
higher than, e.g., 50 to 100 times as high as that of the steel
rigid frame. The horizontal load acting on the RC-structure 56
deforms mainly RC-column 57 as shown in FIG. 14(a) since the beams
of the building are generally locked by the building floor
slabs.
[0011] As shown in FIG. 13(b) the steel portal frame 55 fixed to
the RC-structure 56, having higher rigidity than the steel frame,
results in being loaded with much larger horizontal forces than
that of the RC-structure. FIG. 15 illustrates that the part except
the cross of column and beam, e.g., a spot fixing a gusset-plate
59a on the steel beam 58, gathers horizontal forces transmitted to
the RC-beam 60. In this case, the distribution of stress
transmitted from the steel portal frame 55 to the RC-beam 60 is
shown by arrows B departing from gusset-plate 59a, resulting in
applying an extremely heavy load on stud dowels, etc., mentioned
below.
[0012] The steel portal frame 55 is united to the RC-column 57 and
the RC-beam 60 with cement mortar or concrete placed into the space
between them accommodating chemical anchors and/or stud dowels. The
concrete formed in the space mentioned above results in being
overloaded because of the large difference of the allowable
strength in the horizontal direction based on the rigidity in the
horizontal direction of the steel column 61 of the portal frame 55
reinforced by a brace 53 from that of the RC-column 57 of the
RC-structure 56. In other words, the effect of reinforcement due to
the steel portal frames is gradually lowered with the increased
damage to the concrete in the connecting space caused by the
overload against the chemical anchors, etc.
[0013] The braces fixed close to windows for the purpose of
reinforcing an apartment house spoil the view from the windows even
if the braces are industrial refined products. The circumstance
that even an inhabitant who wants to reinforce the apartment house
does not always want windows with braces in his own living space
often delays the seismic reinforcement of the apartment house as
there is a conflict between the interest of those inhabitants who
are forced to have windows with braces and other inhabitants who
are not.
[0014] The introduction of a brace into the steel portal frame
requires the use of a gusset-plate 59, as there must be plenty of
steel for the reinforcement of the steel beam 58 to which the
gusset-plate 59a is fixed compared with the quantity spent for the
reinforcement of the crossing of column and beam holding the
gusset-plate 59b. The span of the old building requiring the
seismic reinforcement is generally so short that small steel portal
frames for the building are often used. This means that the amount
of the secondary steel, i.e., gusset-plates, stiffening plates,
etc., against the amount of steel used for columns and beams of the
portal frames will rise to uneconomic levels.
[0015] The object of the invention is to provide a seismic
reinforcement method for RC buildings utilizing steel portal frames
without braces in order to solve the problems mentioned above; the
first is to obtain a trim appearance having windows without braces,
though the steel portal frames are applied to the outside of an
existing building, the second is to maintain the combination of
steel portal frame and RC-structure as long as possible under the
horizontal load repeated during a big earthquake, thereby,
increasing the strength in the horizontal direction based on the
reinforcement effect due to the steel portal frame before the
building is destroyed so that the collapse of the building in the
early stage of the earthquake may be avoided even when it is a big
earthquake and the third is to save amount of steel used for
secondary reinforcement to successfully achieve a simplified
construction and reduced reinforcement cost.
BRIEF DESCRIPTION OF DRAWINGS
[0016] FIG. 1 is a schematic outside view of a part of an RC
building to which the seismic reinforcement method utilizing steel
portal frames made of wide flange sections without braces is
applied.
[0017] FIG. 2 shows schematic views of the composition fixing a
portal frame made of wide flange sections to the outside of
building; FIG. 2(a) shows a sectional view of the example using a
welded product of H-shape in cross section and FIG. 2(b) shows a
sectional view of the example using a steel column having high
bending rigidity.
[0018] FIG. 3 shows diagrams of deformation in the horizontal
direction of column vs. horizontal load acted thereon; FIG. 3(a)
shows load vs. displacement in the case that ordinary steel is used
for wide flange section column and FIG. 3(b) shows load vs.
displacement in the case that steel of low yield point is used for
wide flange section column.
[0019] FIG. 4 are plan views of existing apartment houses having
living spaces partitioned by earthquake resisting walls; FIG. 4(a)
shows an example of seismic reinforcement utilizing steel portal
frames without braces against the horizontal force in the
horizontal direction thereof and FIG. 4(b) shows an example of
seismic reinforcement utilizing T-section against the horizontal
force in the lateral direction thereof.
[0020] FIG. 5 is a plan view of the calculating model according to
the seismic reinforcement method utilizing steel portal frames
without braces.
[0021] FIG. 6 is a diagram showing load vs. displacement in the
horizontal direction of an RC-column and wide flange section column
having different bending rigidity from that of an RC-column.
[0022] FIG. 7 are plan views of combination situating the wide
flange section column close to existing RC-structure; FIG. 7(a)
shows an example of RC-column covered with the wide flange section
column, FIG. 7(b) shows an example of the wide flange section
column facing an external surface only of RC-column and FIG. 7(c)
shows an example of the chemical anchors fixed to wide flange
section column.
[0023] FIG. 8 is an elevation view showing the joining part of wide
flange section columns vertically in series arranged on a point of
contraflexure.
[0024] FIG. 9 is a plan view showing a composition fixing T-section
to the outer surface of web of the wide flange section column.
[0025] FIG. 10 is an elevation view showing a seismic reinforcement
composition fixing T-sections to both sides of living space.
[0026] FIG. 11(a) shows a detailed cross sectional composition of
the end of interior RC-beam taking along line A-A in (c), FIG.
11(b) shows a view taking along line B-B in (c) and FIG. 11(c)
shows a sectional view of the interior RC-beam supporting floor
slab.
[0027] FIG. 12 shows cross sections of a reinforced column, FIG.
12(a) shows original RC-column, FIG. 12(b) shows RC-column covered
with additional reinforced concrete and FIG. 12(c) shows RC-column
wound by steel plates.
[0028] FIG. 13(a) shows a schematic view of steel portal frame
buried in the wall and FIG. 13(b) shows a schematic view of steel
portal frame fixed to the external surfaces of columns and
beams.
[0029] FIG. 14 shows schematic views of the deformation of existing
RC-structure and steel portal frame fixed thereto under the
horizontal load, FIG. 14(a) shows the behavior of RC-column and
RC-beam in RC-structure and FIG. 14(b) shows the behavior of steel
column deforming together with existing RC-column.
[0030] FIG. 15 are schematic views of the transmission of stress in
RC-structure reinforced by steel portal frames with braces.
DETAILED DESCRIPTION OF THE INVENTION
[0031] The present invention is directed to a seismic reinforcement
method for RC buildings utilizing a steel frame for stiffening an
existing RC-structure by fixing the steel frame to the outside of
the reinforced concrete structure which has openings like windows
Referring to FIG. 1, the steel frame is a portal frame 3 made of
wide flange sections without braces, which is fixed to the outside
of an existing RC-column 5 extending in the vertical direction and
to the outside of an existing RC-beam 8 extending in the horizontal
direction. The wide flange section column 4 of the portal frame is
fixed to the existing RC-column 5 and is assigned a bending
rigidity roughly equivalent to that of the existing RC-column 5,
being much less than that of a steel portal frame with braces,
thereby not only reducing the stress additionally occurring at the
connecting part 9 (see FIG. 2) between the existing RC-column and
the wide flange section column by deforming the column 4 similarly
to the existing RC-column 5 under the horizontal load transmitted
from existing RC-beam 8 and/or wide flange section beam 6 during an
earthquake, but also increasing the resultant strength in the
horizontal direction of the combination 10 of the RC-column and
wide flange section column by decreasing the deformation of the
RC-column 5 after yielding so as to equalize the range of
quasi-elastic deformation of said combination to that of elastic
deformation of the column 4.
[0032] As shown in FIG. 2(a), the wide flange section column 4 is
preferably a welded product of H-shape in cross section. Web 4w is
arranged close to the RC-column. Referring to FIG. 2(b), the column
4 is fixed to the RC-column 5 is accompanied by tie hoops 22 welded
on the outer surface of the web 4w of the column Bending rigidity
thereof is increased by placing cement mortar or concrete 24 into
the space accommodating said tie hoops engaged with vertical bars
23.
[0033] The wide flange section column 4 is made of steel of low
yield point, reducing yield bending strength only without reducing
the bending rigidity thereof, thereby reducing the response stress
thereof in a big earthquake through the plasticization hastened by
yielding the combination of the RC-column and the wide flange
section column at the bending strength of approximately 3 to 4
times as strong as the existing RC-column 5.
[0034] Referring to FIG. 9, the wide flange section column 4 is
stiffened by a T-section 25, taking the form of T in plan view,
extending over all the stories, the leg 25a of which is welded on
the outer surface of the web 4w of the column at its tip,
three-dimensionally reinforcing the building by the alignment of
the T-sections with the interior RC-beams 26A or earthquake
resisting walls 26B not only extending perpendicularly to the
external walls but being united to the existing RC-columns 5 which
are reinforced by the wide flange section columns 4.
[0035] The T-section 25A projects outside as wide as a verandah 27
of each story.
[0036] As shown in FIG. 11(c) the interior RC-beams 26A or RC-beams
over the existing earthquake resisting walls have additional beams
made of high strength fluidized concrete 30 or cement mortar at
both sides thereof. This not only obtains the desired bending
moment based on the post-tension generated by unbonded prestressing
steel bars 31 buried in the additional beams, but also attains the
strength required in the horizontal direction of said beams.
[0037] The seismic reinforcement method for RC buildings utilizing
steel portal frames without braces according to the present
invention is disclosed below in detail referring to some examples.
The invention is a method for stiffening an existing RC-structure
by fixing a steel frame to the outside of a reinforced concrete
structure with openings like windows. Utilizing such reinforcement,
a building may be reinforced while leaving inhabitants therein.
What is notable in the invention is that the wide flange section
column of the steel frame is assigned a bending rigidity roughly
equivalent to that of an RC-column of the reinforced concrete
structure.
[0038] The steel frame column is equipped with a bending rigidity
of -70% to +200% as high as that of an RC-column. Accordingly, the
inventive reinforcement never compares to a steel portal frames
with braces which has a huge bending rigidity of 50 to 100 times as
high as that of an RC-column. This means that the bending rigidity
of a wide flange section column fixed to an existing RC-column is
of the same order as that of the RC-column, being much less than
that of a steel portal frame with braces.
[0039] FIG. 1 shows an outside view of a part of one RC building
story with windows 2 in the RC-structure 1 to which a portal frame
3 made of wide flange sections without braces mentioned above is
fixed. The steel column 4 of portal frame 3 is combined with an
existing RC-column 5 extending in the vertical direction along the
external wall of the building and the steel beam 6 thereof is
combined with an existing RC-beam 8 extending from the RC-column 5
to the right and/or left sides thereof, supporting the load
including the vertical load caused by the weight of the external
walls 7, etc.
[0040] The steel portal frame 3 composed of wide flange section
column 4 and wide flange section beam 6 has no braces, and is not
designed to have the high rigidity corresponding to that of a steel
portal frame reinforced by braces. The bending rigidity assigned to
column 4 fixed to the existing RC-column 5 is selected to be
roughly equal to that of the existing RC-column. It is noticeable
that the steel column deformation is approximately equivalent to
the deformation of the RC-column (being much more deformative than
the frame with braces) as applied to the reinforcement of the
RC-structure.
[0041] In brief, the wide flange section column 4 results in
deforming similarly to the existing RC-column 5 under the
horizontal load transmitted from existing RC-beam 8 and/or wide
flange section beam 6 in a big earthquake. The stress additionally
occurred at the connecting part 9 shown in FIG. 2(a) between the
existing RC-column and wide flange section column is remarkably
small in response to the decrease of difference of the bending
deformation of the existing RC-column 5 from that of column 4.
[0042] In addition, since the deformation of RC-column 5 is
restrained after yielding at the displacement in the horizontal
direction shown by ..sub.RC in FIG. 3(a), the combination 10 of the
RC-column and the wide flange section column 4 results in the
possession of a broad range S of quasi-elastic deformation
indicated by the line being slightly bent, though being regarded as
approximately straight, up to the deformation corresponding to the
elastic deformation H in the horizontal direction of the wide
flange section column, furthermore, providing a great increase of
strength in the horizontal direction thereof as described
below.
[0043] Since the yield bending stress of steel column 4 is much
bigger than that of RC-column 5 in the case that bending rigidity
of the steel column is of the same order as that of the RC-column,
the seismic resistance capacity of the combination of steel portal
frame and RC-structure results in an increase of at least 3 to 4
times as large as the RC-structure alone. The reinforcement
according to the present invention not only generates much higher
seismic resistance capacity than the existing reinforcement
achieving the strength in the horizontal direction at most twice as
strong as the existing RC-column, but keeps original openings
without braces. The cross-section of the steel portal frame made of
wide flange sections, being more narrow than the width of columns
and beams of existing reinforced concrete, simply blends well with
the appearance of an existing building, thereby making the
reinforcement of such buildings much more practical.
[0044] Referring to FIG. 2, the portal frame 3 made of wide flange
sections is combined with RC-columns 5 and RC-beams 8 by the use of
chemical anchors 11, stud dowels 12, spiral hoops 13 and high
strength non-shrink mortar 14. The existing building under the
horizontal load mainly deforms the columns because the beams are
always locked by floor slabs as described in FIG. 14(a).
[0045] The bending rigidity of the wide flange section column 4
being roughly equivalent to that of RC-column 5 under the condition
that wide flange section beam 6 is united to RC-beam 8 makes column
4 deform similar to RC-column 5 as shown in FIG. 14(b) so that the
difference of deformation of the former from that of the latter may
be very little, consequently, reducing the stress transmitted
through chemical anchors and/or stud dowels in the connecting part
9 (see FIG. 2) to a minimum.
[0046] The big additional stress in the connecting part will not
occur even under the horizontal load based on an earthquake,
accordingly, the horizontal loads are smoothly transmitted from the
steel portal frames to the RC-frames and vice versa. Such a steel
rigid frame provides stud dowel, etc. with the uniform load due to
the shearing force transmitted throughout all of the beams, being
different from the frame with braces described in the Prior
Art.
[0047] The steel portal frame 3 deforms similarly to the RC-frame 1
as shown in FIG. 14(b) though the former is originally more
deformative than the latter, simplifying the composition of the
connecting part as well as facilitating the rationalization of the
overall reinforcement, from design to construction. The inventor's
have determined that the deformation of existing RC-column comes to
much the same as that of the wide flange section column provided
that the bending rigidity of column 4 fixed to existing RC-column 5
is assigned to -70% to +200%, preferably -60% to +150%, as high as
that of the existing RC-column.
[0048] Although today's builders believe that braces are
indispensable for the reinforcement of RC-structure by reason that
deformative steel frames will not stiffen a reinforced concrete
building, the present invention dares to remove braces from the
steel portal frame. A tube-in-tube type brace is still an eyesore
even if it has a smart appearance. Needless to say, the steel
portal frame without braces according to the invention is
acceptable for seismic reinforcement, consequently, promoting the
seismic reinforcement of buildings which have not yet been
reinforced.
[0049] The steel portal frame without braces requires no
gusset-plates for braces. Since braces, being smart in appearance,
are often connected to frames by pin joints, the gusset-plates as
joints are inevitably required to be thick, besides, the web of the
wide flange section column must be reinforced by stiffening plates,
etc., for the sake of fixing the gusset-plates thereto. Adopting no
braces favorably results in requiring no reinforcements such as
stiffening plates.
[0050] Furthermore, many small steel portal frames for seismic
reinforcement are used for an old building, especially, for
out-of-date apartment houses 15 of short span shown in plan view of
FIG. 4. Applying the prior art portal frame with braces to the
building to be reinforced requires a high percentage of the space,
being not negligible, taken up by auxiliary reinforcements
mentioned above against wide flange section as main
reinforcements.
[0051] Though the portal frame with braces can not hide the
existence of secondary reinforcements applied to over all the
frame, the portal frame without braces itself becomes inconspicuous
due to few auxiliary reinforcements, approximately equal to zero,
and it may look as if the building was not reinforced.
[0052] The present invention makes the combination of RC-structure
and steel portal frame behave in one united body without acting
unfavorably to force applied on the connecting part of the
combination, therefore, the range of elastic deformation becomes
large by decreasing the deformation of the existing RC-column, and
besides, the strength in the horizontal direction of the reinforced
building greatly increases under the contribution of steel of high
original strength. Such a steel portal frame needs braces no
longer. The reinforcement of the RC-structure without braces has
not been achieved until the knowledge mentioned above was
discovered by the inventor.
[0053] The support for the wide flange section column having
bending rigidity equal to that of an existing RC-column being
practical is discussed below with the explanation of increasing the
strength of the combination according to an example calculation
based on the model 16 of FIG. 5. Provided the cross section of the
RC-column 5 is 60 centimeters square, the geometrical moment of
inertia thereof is given as follows:
[0054] 1. I=60.times.60.sup.3/12=1,080,000 cm.sup.4.
[0055] It is well-known in the design for earthquake-proof
buildings that 70% of original geometrical moment I of inertia of
reinforced concrete is available even at the allowable strength (at
the limit of elasticity) in spite of occurrence of little cracks
thereon, accordingly, I.sub.RC is obtained below:
[0056] 2. I.sub.RC=1,080,000 cm.sup.4 .times.0.7=756,000
cm.sup.4.
[0057] The yield bending strength (yield moment) of an RC-column
based on the geometrical moment of inertia is given below: Assuming
that the total sectional area at of reinforcing bars is 17.02
cm.sup.2 and allowable unit stress ft thereof is 2.4 tons/cm.sup.2,
the distance between centers of tension and compression resultants
j and the yield bending strength .sub.RCM.sub.y are obtained as
follows:
[0058] 3. j=7d/8=7/8 (60-5.0)=48.1 cm,
[0059] therefore,
.sub.RCM.sub.y=a.sub.t.f.sub.t.j=17.02.times.2.4.times.4- 8.1=19.65
t.m.
[0060] Next, provided the wide flange section column 4 is 40
centimeters wide and 40 centimeters deep, thickness t.sub.f of its
flange=2.5 centimeters and thickness t.sub.w of its web=1.6
centimeters, the geometrical moment Is thereof is as follows:
[0061] 4. I.sub.S=20
2.times.2.5.times.40.times.2.+-.1.6.times.40.sup.3/12- =88,533
cm.sup.4.
[0062] Since the ratio of Young's modulus of steel to concrete is
approximately 10, the geometrical moment .sub.eqI.sub.S of inertia
equivalent to reinforced concrete column is written as follows:
[0063] 5. .sub.eqI.sub.S=88,533.times.10=885,330 cm.sup.4.
[0064] This is approximately equal to I.sub.RC=756,000 cm.sup.4
calculated above.
[0065] Section modulus Z.sub.S, yield bending strength
.sub.SM.sub.y and .sub.SM.sub.y/.sub.RCM.sub.y of wide flange
section column 4 are respectively as follows:
[0066] 6. Z.sub.S=88,533/20=4,427 cm.sup.3,
[0067] .sub.SM.sub.y=4,427.times.2.4=106.2 t.m
[0068] and .sub.SM.sub.y/.sub.RCM.sub.y=106.2 19.65=5.4.
[0069] The strength in the horizontal direction of the combination
may theoretically increase by 5.4 shown above against 1.0 of
RC-column to 6.4 times in the case that stud dowels and chemical
anchors are enough for transmitting the strength of the wide flange
section column.
[0070] Since Young's modulus of steel is about 10 times as large as
that of concrete, the example calculation illustrates that a steel
column of bending rigidity similar to the reinforced concrete is
easily obtainable. Although the nature of reinforced concrete is
different from that of steel, their behavior is much the same
within the range of elasticity. Since the yield bending strength of
wide flange section column is higher than that of RC-column under
the condition that the bending rigidity is the same among them,
both the combination of RC-column and wide flange section column
basically results in being elastic.
[0071] As shown in FIG. 3(a) the strength in the horizontal
direction of a wide flange section column 4 made of ordinary steel
is 5.6 times, at least keeping elastic, as strong as that of the
RC-column 5 being regarded as 1. The steel is loaded up to
deformation H in the horizontal direction corresponding to the
limit of elasticity, whereby, the growth of damage of reinforced
concrete remains within the deformation in the horizontal
direction. If the yield bending strength of wide flange section
column is low, the load on the reinforced concrete column increases
accompanying with damage based on large deformation. According to
the present invention the steel portal frame contributes to
decrease the damage of reinforced concrete column also after
yielding.
[0072] Oppositely, the elastic behavior of the wide flange section
delays the advance in collapse of reinforced concrete. Though an
existing reinforced concrete structure absorbs seismic energy in
compensation for being damaged, the reinforcement according to the
present invention not only remarkably decreases the damage to the
reinforced concrete but lightens the repair works thereafter.
[0073] As described above, the bending rigidity of the wide flange
section column fixed to the existing RC-column is assigned to -70%
to +200% as high as that of the existing RC-column, i.e., 0.3 to
2.0 times. Above all, the bending rigidity of wide flange section
column had better be bigger than that of existing RC-column. In
consideration of using steel, 150% (=1.5 times) is enough to
restrain the occurrence of cracks in the reinforced concrete after
the RC-column 5 is beyond its elastic limit.
[0074] On the other hand, approaching the absolute value of % to
100 on the negative side means not to reinforce the building,
therefore, the value should be selected within at least -70%.
Because even the steel having bending rigidity of only 30% of that
of existing RC-column is expected enough for reinforcing an
existing slightly superannuated RC-structure. Any steel having much
higher bending strength than the reinforced concrete greatly
contributes to reinforce RC-columns, accordingly, transmitting the
load to the base of the building due to the decrease of damage to
the reinforced concrete as well as restraining the instability of
the building in the early stage of an earthquake.
[0075] Use of the steel of low yield point (.sub..y=1.5t/cm.sup.2)
for the column 4 reduces the yield bending strength thereof only to
about 1/2 as shown in FIG. 3(b) without reduction of the bending
rigidity thereof. According to the example calculated above the
yield moment of the wide flange section column may be reduced from
5.4 times to 2.7 times as large as RC-column 5.
[0076] Thus, the wide flange section column can yield at the yield
bending strength of 2 to 4 times as strong as existing RC-column,
not only promoting the plasticization of the wide flange section
column at a big earthquake but reducing the response stress against
the earthquake. In other words, the absorption of seismic energy
due to large plastic deformation against the excessive force
greatly restrains an instantaneous collapse without quick reduction
of strength of the wide flange section column.
[0077] Because of the excessively high yield bending strength of
wide flange section column, there occurs some problems in the
toughness-type reinforcement for high-rise buildings in spite of
almost no problems in the strength-type reinforcement applied to
middle- and/or low-rise buildings. It is significant to reduce the
yield bending strength for removing the problems described
before.
[0078] FIG. 3 shows diagrams of load vs. displacement in the
horizontal direction under the condition that the bending rigidity
of the wide flange section column is the same as that of an
RC-column. The column 4 made of ordinary steel elastically behaves
in a broad range as shown in FIG. 3(a) needs to largely deform
before the plasticization thereof based on a big earthquake.
[0079] On the other hand, FIG. 3(b) teaches us that wide flange
section column begins to yield at the horizontal strength of about
3 times as strong as that of existing RC-column so that the wide
flange section column may be plasticized in relatively early stage
during a big earthquake. It is well-known that moderate
plasticization of wide flange section column remarkably reduces the
response against the earthquake. Accordingly, small response stress
facilitates the design for not only the base of the building but
the whole structure thereof to support the economical design.
[0080] Selecting the bending strength of wide flange section column
equivalent to that of reinforced concrete column is the basis of
the present invention as explained above, then, giving rational
composition to the connecting part. FIG. 6 is a diagram of load vs.
displacement in the horizontal direction of RC-column 5
accompanying with the column 4 having different bending rigidity
from that of RC-column, being drawn correspondingly to FIG. 3. In
this case a combination of RC-column and wide flange section column
will not be formed, accordingly, RC-column and wide flange section
column deform independently. Therefore, the double-dotted chain
line of N made by piling load vs. displacement of RC-column with
that of wide flange section column, being drawn similarly to the
broken lines in FIG. 3, is not obtainable in practice.
[0081] The idea of the present invention is expanded below: FIG. 7
shows the examples in which the column 4 is fixed to the chemical
anchors 11 driven into the RC-column 5. FIG. 7(a), which is an
embodiment of wide flange section column covering the whole of the
projecting part of RC-column 5, and FIG. 7(b) is another of column
4 facing a flat external surface only of RC-column. Thus, the wide
flange section column is arranged so close to reinforced concrete
that only the narrow space 9A between existing RC-column and wide
flange section requires high strength non-shrink mortar.
[0082] Arranging the column 4 close to RC-column 5 requires no stud
dowels. The steel rigid frame structure can decrease the
consumption of secondary materials in compensation for the increase
of main supporting materials in the case that the bending rigidity
in the lateral (horizontal) direction of wide flange section column
equivalent to that of reinforced concrete column can be obtained in
spite of the reinforcement due to the wide flange section column
slightly projecting from the external surface of the reinforced
concrete. The simplification of steel portal frame contributes to
not only the decrease of construction materials but to a great
reduction of total cost for the reinforcement.
[0083] Incidentally, the position for joining columns 4 vertically
disposed in series to each other in the direction along the axis of
column is assigned to the point where the bending moment M.sub.5 of
RC-column 5 in FIG. 8 is equal to 0 (being called a point of
contraflexure). Because the middle point between upper beam 6u and
lower beam 6d gives the bending moment of M.sub.4=0 to column 4 as
well as to RC-column 5. The steel plate 17 welded on the lower end
of column 4 is joined to the steel plate 17 welded on the upper end
of another column 4 by high tension bolts 18.
[0084] Referring to FIG. 7, a hole 19 having an annular clearance
19a shown in FIG. 7(c) allows for an inaccurate position of
chemical anchor 11 against the wide flange section column 4 which
is formed in the web 4w of column so as to fix the column 4 to
chemical anchors driven into the RC-column 5. Welding the washer 21
used for a nut 20, locks the column 4 to the RC-column, on the web
4w to prevent the column 4 from moving.
[0085] Both the wide flange rolled section with web 4W drawn by
double-dotted chain lines in FIG. 2(a) and the welded product
consisting of three long steel plates are available for any portal
frame 3 explained above. The latter with the web 4w optionally
designed taking into consideration the position and/or size thereof
may advantageously provide products made out of a standardized
rolled section.
[0086] Though the standardized products of rolled section with wide
flange are used in general on the ground of mass-production, welded
products of H-shape in cross section are greatly available if the
web can be designed so as to obtain columns of bending rigidity
equivalent to that of RC-column 5. The web 4w close to the
RC-column as shown in FIG. 2(a) makes a narrow space 9A between a
wide flange section column and RC-column, decreasing the
consumption of expensive high strength non-shrink mortar 14.
[0087] A short distance between wide flange section column 4 and
RC-column 5 is preferable for increasing not only strength but the
bending rigidity of the connecting part as well as for aligning the
web of wide flange section column with the web 6w of wide flange
section beam 6, therefore, transmitting the horizontal force to
other wide flange section without unfavorable shearing force and/or
moment. In the case that the bending rigidity of wide flange
section column is insufficient it can be increased not only by
welding tie hoops 22 engaged with vertical bars 23 on the outer
surface of web 4w of the wide flange section column as shown in
FIG. 2(b) but by placing cement mortar or concrete 24 over
them.
[0088] The reinforcement mentioned above is applied to the building
being loaded by the horizontal force in the direction of x-axis
shown in FIG. 4(a). However, the old buildings to be reinforced
often have small rooms divided by a wall extending in the direction
of the y-axis. It is impossible to remove earthquake resisting
walls for the purpose of enlarging the rooms as far as interior
RC-beams or the walls are designed so as to resist the horizontal
force in the lateral direction, i.e., in the direction of the
y-axis.
[0089] Increasing the number of earthquake resisting walls and/or
the thickness of existing walls to reinforce the lateral direction
of a building reduces the living space. According to the present
invention, T-sections 25, taking the form of T in plan view, are
arranged so as to extend over all the stories along the outer
surface of the web 4w of the wide flange section column 4 as shown
in FIG. 9. The leg 25a of T-section is welded on the wide flange
section column 4 so that the T-section is aligned with an interior
RC-beam 26A and/or earthquake resisting wall 26B extending
perpendicularly to the external wall and being united to existing
RC-column 5.
[0090] Such T-section 25 provides the building with seismic
reinforced columns forming an H in the lateral direction (in the
direction of y-axis in FIG. 4(a)) because T-section 25 is united to
the web 4w of wide flange section column 4. Both wide flange
section column 4 itself being originally seismic reinforced column
in the directions of right and/or left sides (in the direction of
x-axis) and T-section 25 extending from the first story to the top
story together achieve three-dimensional reinforcement, i.e., in
the directions of x, y and z-axes of building. T-section 25 is
fixed not only to the front surface but to rear surface of building
as shown in FIG. 4(b) if necessary.
[0091] The verandah spreading in series on each story provides the
apartment house with space for reinforcing in the lateral direction
thereof, then, utilizing the whole width of verandahs at the
boundaries of neighboring houses. As shown in FIG. 4(b) the leg of
T-section 25A is projected up to the handrail 27a (see also FIG. 9)
on the line extended from the earthquake resisting wall, etc., by
equalizing the length of leg 25a of T-section 25A to the width of
verandah 27.
[0092] In such a case both interior RC-beam 26A and earthquake
resisting wall 26B should be reinforced as well as RC-column 5
combined with wide flange section column 4 on its external surface.
FIG. 10 shows an example of living space 28 on one story, to not
only the front side (see the left-hand side in the drawing) but
rear side (see the right-hand side in the drawing) of which
T-sections 25 are fixed. In this case the interior RC-beams 26A
supporting floor slab 29 may be reinforced as shown in FIG.
11(c).
[0093] FIG. 11(a) and (b) show the sectional views taking along
line A-A and line B-B in FIG. 11(c) thereof, respectively. Not only
placing high strength fluidized concrete 30 or mortar on both sides
of interior RC-beams 26A as shown in FIG. 11(c) but generating
post-tension in interior RC-beams by unbonded prestressed steel
bars 31 buried in additional beams after curing the cement which
provide the interior RC-beams 26A with desirable bending moment at
their ends, resulting in attaining the strength required in the
horizontal direction of the beams. Such reinforcement enables
removal of a part of the existing earthquake resisting walls for
the purpose of enlarging the living space.
[0094] According to the present invention, the wide flange section
column fixed to existing RC-column is assigned a bending rigidity
roughly equivalent to that of an existing RC-column so that one
column deforms similarly to another, thereby reducing the
additional stress occurred by the difference of the deformation of
the RC-column from that of the wide flange section column in the
connecting space faced thereto, and besides, remarkably decreasing
the force transmitted through chemical anchors, etc.
[0095] The instantaneous collapse of buildings during a big
earthquake can be favorably avoided since the increase of strength
in the horizontal direction based on the reinforcement due to steel
portal frame before the collapse of the building keeps the
combination of steel portal frame and RC-structure as long as
possible. In addition, the amount of damage to the RC-column which
should have yielded is restrained before the yield of wide flange
section since the range of deformation due to elastic behavior of
wide flange section column is wider than that of RC-column. Thus,
not only the damage of RC-structure is decreased but the repair
works thereafter is lessened due to delaying the absorption of
seismic energy while avoiding collapse of the reinforced concrete
structure.
[0096] Non-use of braces keeps the original view from windows and
use of a portal frame made of wide flange sections being narrower
than the width of existing columns and beams hardly changes the
appearance of building. All of inhabitants in an apartment house
and tenants in an office building may easily agree with the seismic
reinforcement structure out of self-interest, therefore,
accelerating the reinforcement process.
[0097] In the case of connecting a brace to a gusset-plate via a
pin joint the gusset-plate is required to be rather thick. On the
other hand, the steel portal frame without braces according to the
invention need not use such a gusset-plate itself, resulting in no
reinforcement for the web of wide flange section beam. The removal
of the steel materials secondarily used at a high rate for
reinforcing old buildings being short in span promotes an
inexpensive reinforcement operation.
[0098] Arranging the web of wide flange section column close to
RC-column facilitates to approximately equalize the bending
rigidity thereof to that of RC-column. A narrow connecting part
enables the alignment of the web of wide flange section column with
the web of wide flange section beam having a cross section smaller
than the wide flange section column, resulting in the transmission
of horizontal force in high efficiency on the portal frame made of
wide flange sections.
[0099] The narrower the connecting part is, the shorter the
distance for transmitting force so as not to apply excessive force
on the connecting part, resulting in saving very expensive high
strength non-shrink mortar and the secondary materials, e.g.,
anchors, etc. Accordingly, not only the simplification of the
connecting part but decrease the costs for construction are
attainable through the reduction of the reinforcement materials and
the rationalization of the reinforcement structure.
[0100] Both welding tie hoops engaged with main reinforcements on
the outer surface of web of the wide flange section column and
placing cement mortar or concrete over them increase the bending
rigidity of the wide flange section column so that it may become
approximately equivalent to that of RC-column even if the wide
flange section column results in having low bending rigidity by
reason of arranging the web thereof close to RC-column.
[0101] Assigning wide flange section column to the steel of low
yield point reduces the yield bending strength only, without
reduction of the bending rigidity, yielding the wide flange section
column at the bending strength of approximately 2 to 4 times as
strong as the existing RC-column, consequently, the response stress
is reduced by the plasticization of columns during a big
earthquake.
[0102] Both welding the end of leg of T-section on the outer
surface of the web of the columns wide flange section and extending
T-section all over the stories together achieve the complete
reinforcement of three axes of x, y and z, i.e., reinforcement in
the horizontal direction of the external walls of the building, in
the lateral direction perpendicular to the external walls and in
the vertical direction of the external walls.
[0103] Projecting a T-section as wide as the verandah of each house
more strongly reinforces the building in the lateral direction
thereof by utilizing the space of the verandah.
[0104] The additional beam formed at both sides of the existing
RC-beam connected to earthquake resisting wall, which is made of
high strength fluidized concrete or cement mortar and unbonded
prestressed steel bars under post-tensioning, provides the ends of
reinforced beam with the desirable bending moment, largely
increasing the horizontal strength in the lateral direction
perpendicular to the external walls of the building.
* * * * *