U.S. patent application number 11/031062 was filed with the patent office on 2005-06-02 for device for damping movements of structural elements and a bracing system.
Invention is credited to Mualla, Imad H..
Application Number | 20050115170 11/031062 |
Document ID | / |
Family ID | 34609952 |
Filed Date | 2005-06-02 |
United States Patent
Application |
20050115170 |
Kind Code |
A1 |
Mualla, Imad H. |
June 2, 2005 |
Device for damping movements of structural elements and a bracing
system
Abstract
The invention relates to the protection of structural systems
such as apartment houses against dynamic loading caused by
earthquakes, wind, traffic etc. When a frame structure is excited
by a horizontal external force, the girders starts to displace
horizontally. If such displacements are large or if they occur
periodically they may have serious impact on the conditions of the
structure and may cause severe damages or even result in a
collapse. The present invention relates to a very simply designed
and easily produced damper with two members being interconnected in
a rotational joint. By means of the arrangement of the damper in
the structural system, displacement occurring in the system is
transferred to the members being forced to rotate in relation to
each other. Frictional resistance between the two members is
dampening the rotational movement and thus the displacement in the
system. The invention further relates to a device wherein the
frictional resistance may be varied dynamically based on the
displacements occurring in the system.
Inventors: |
Mualla, Imad H.; (Rodovre,
DK) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
34609952 |
Appl. No.: |
11/031062 |
Filed: |
January 10, 2005 |
Current U.S.
Class: |
52/167.1 |
Current CPC
Class: |
E04H 9/0237 20200501;
E04H 9/028 20130101; F16F 7/04 20130101; E04H 9/02 20130101 |
Class at
Publication: |
052/167.1 |
International
Class: |
E04B 001/98 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 3, 1999 |
DK |
PA 1999 01087 |
Claims
1-33. (canceled)
34. A device for damping movements of structural and non structural
elements in civil engineering structures, the device comprising: at
least two members being interconnected in a rotational joint for
frictional damping of relative rotational movement between the at
least two members, the two members rotationally moving in opposite
directions, clamping means for clamping the at least two members
together, so as to maintain a clamping force and friction between
the at least two members in the rotational joint, means for
connecting each of the at least two members to respective ones of
the structural elements.
35. A device according to claim 34, further comprising a frictional
pad arranged between the two members so as to establish contact
between the members and the frictional pad so that the relative
rotational movement of the members is dampened by friction.
36. A device according to claim 34, wherein the clamping means are
adapted to vary the clamping force.
37. A device according to claim 34, wherein the joint comprises a
pin extending through each of the at least two members.
38. A device according to claim 37, comprising a bolt, at least a
portion of the bolt constituting the pin, the bolt having: a bolt
member with a bolt head, a nut with a nut head, the clamping force
being determined by the pretension of the bolt.
39. A device according to claim 38, further comprising means for
maintaining a substantially constant clamping force with time.
40. A device according to claim 39, wherein the means for
maintaining a substantially constant clamping force comprises at
least one spring arranged between the bolt head and a surface of
one of the members and/or between the nut head and a surface of one
of the members.
41. A device according to claim 40, wherein the spring comprises a
disc spring.
42. A device according to claim 41, comprising at least two disc
springs.
43. A device according to claim 42, wherein at least one disc
spring is arranged between the bolt head and a surface of one of
the members, and wherein at least one disc spring is arranged
between the nut head and a surface of another one of the
members.
44. A device according to claim 39, wherein the means for
maintaining a substantially constant clamping force comprise
hydraulic, pneumatic and/or electric means for maintaining the
clamping force.
45. A device according to claim 34, wherein the at least two
members comprise: a side plate and a central plate extending in
substantially parallel planes.
46. A device according to claim 45, wherein the frictional pad is
arranged between the side plate and the central plate.
47. A device according to claim 45, and comprising two side plates
arranged symmetrically around the central plate.
48. A device according to claim 47, comprising two frictional pads,
each frictional pad being arranged between a respective one of the
side plates and the central plate.
49. A device according to claim 45, wherein the central plate is
adapted to be connected to one of the structural elements in a
pivotal manner, so as to allow relative rotational movement between
the central plate and the structural element.
50. A device according to claim 45, wherein the central plate is
adapted to be connected to one of the structural elements in a
fixed manner, so as to prevent relative movement between the
central plate and the structural element.
51. A device according to claim 47, wherein the side plates are
adapted to be connected to one of the structural elements in a
pivotal manner, so as to allow relative rotational movement between
the side plates and the structural element.
52. A device according to claim 47, wherein the side plates are
adapted to be connected to one of the structural elements in a
fixed manner, so as to prevent relative movement between the side
plates and the structural element.
53. A device according to claim 34, wherein the friction pad
material comprises a MK101 asbestos free friction material by
Eurodeal A/S.
54. A device according to claim 34, wherein the at least two
members are made of steel, anti-corrosive steel, brass, aluminium
or any alloys comprising aluminium or any other steel material or
composite of steel and plastics or composites of plastics and
fibres of glass, carbon, Kevlar or similar or composites of any
ceramics materials and fibres of glass, carbon, Kevlar or
similar.
55. A device according to claim 34, wherein the clamping force
shows a variation of less than 5% in a 400 cycle test with 0,5
Hz.+-.0,1 Hz forcing excitement frequency and an displacement
amplitude of one of the at least two members of up to 10 mm at an
applied excitement force of .+-.2.5 kN and an initial clamping
force of 4 kN.+-.0,5 kN.
56. A device according to claim 34 wherein the frictional moment in
the frictional joint of the device shows a forcing frequency
dependent variation of less than 5% in the range 2-7 Hz at a
nominal frictional moment of 200 Nm.+-.20 Nm in 30-cycle tests at
each frequency.
57. A device according to claim 34, having a substantially linear
relationship between displacement amplitude of one of the at least
two members and energy dissipation in the frictional joint.
58. A device for damping movements of panel walls in building
structures, the device comprising: at least one member
interconnected to at least one panel in a first rotational joint,
the at least one member being further connected to another panel or
similar part of the building structure in a second rotational
joint, one or both of the first or the second rotational joints
providing a frictional damping of relative movement between the at
least one member and the panel or similar building structure, one
or both of the first or the second rotational joints further
providing a sliding movement of the at least one member in relation
to the panel or similar building structure to which it is attached,
clamping means for clamping the at least one member together with
the panel or similar building structure, so as to maintain a
clamping force and friction between the at least one member and the
panel or similar building structure in the rotational joint.
Description
TECHNICAL FIELD
[0001] This invention generally relates to the protection of
structural systems against dynamic loading such as earthquakes or
impact from oceanic waves, vibrations from traffic or impact of the
wind. More specifically the invention relates to damping of motion
or vibration in structures.
BACKGROUND OF THE INVENTION
[0002] When a frame structure is excited by a horizontal external
force, the girders starts to displace horizontally. If such
displacements are large or if they occur periodically they may have
serious impact on the conditions of the structure and may cause
severe damages or even result in a collapse.
[0003] Dampers play an important role in the protection of
structures such as buildings, and they exist in numerous variants.
Dampers are typically damping the motion by means of a frictional
force between two moving parts attached to the frame structure of
the building or by means of a fluid being pressed to flow between
two chambers through a restricted tube. Other similar well-known
methods of damping motion or vibrations exist. Some dampers are
actively changing the damping effect corresponding to external
conditions, and other dampers are passive dampers having a constant
damping characteristic. Typical dampers are costly to produce and
even more costly to assemble into a structural frame of a building.
Typically a building have to be designed for a specific damper,
either due to the bulky design of the existing dampers or due to
correlation between the structural characteristics of the damper
versus the characteristics of the building.
[0004] U.S. Pat. No. 4,409,765 (Pall) Relates to a building having
a pair of structural elements with a member connecting these
structural elements and where the member has a slip joint with
frictional surfaces. The slip joint furthermore has clamping means
for forcing the frictional surfaces together. The patent does not
disclose a rotational hinge like assembly of the damper, providing
a rotational friction.
DESCRIPTION OF THE INVENTION
[0005] It is an object of the present invention to provide a damper
that is based on a very simple design and comprising parts that are
easily produced. At the same time the damper must be easy to
assemble and flexible both for arrangement in different bracing
systems as well as in confined spaces and both for retrofitting in
existing structures as well as for new structures. A further
advantage of the present invention is a constant damping effect and
a price efficient and reliable system.
[0006] The objects of the invention are fulfilled according to the
invention by a device for damping movements of structural and non
structural elements in civil engineering structures, the device
comprising:
[0007] at least two members being interconnected in a rotational
joint for frictional damping of relative rotational movement
between the at least two members. The device comprises
[0008] clamping means for clamping the at least two members
together, so as to maintain a clamping force and friction between
the at least two members in the rotational joint, and
[0009] means for connecting each of the at least two members to
respective ones of the structural elements.
[0010] The structural element in civil engineering could be beams,
columns and slabs. The wall being dampened may comprise a
combination of structural elements as well as non structural
elements, and consequently the damper may dampen the movement of
both structural and non structural elements. The non structural
elements could be windows, doors, infill walls such as brick walls,
panels and partition walls.
[0011] Accordingly:
[0012] The damper device can be mounted in 2 or more directions
e.g. in a several storeys building.
[0013] The damper device can be mounted in reinforced concrete
frame structures with infill brick walls.
[0014] The damper device can be mounted in large panel walls to
reduce their sliding failure mechanism. The panels would typically
be made from concrete but they may be made from other material such
as timber, steel or composite materials.
[0015] The damper device can be mounted in elevated water tanks to
reduce their vibration response.
[0016] The damper device can be mounted in bridges and elevated
highways. It can be installed in two directions to reduce the
response. As an example a number of dampers may be arrange in a
first direction and a number of dampers may be arranged in a second
direction. The dampers in the first direction may be provided with
a damping structure, which is different from the damping structure
of the dampers arranged in the second direction.
[0017] The damper device can be used to reduce the vibration caused
by elevated machines, which are mounted on a frame structure.
[0018] The damper device can be mounted in many kinds of offshore
structures to reduce their vibration response due to wave loads,
e.g. from water or wind.
[0019] The damper device can be mounted in ready-made garages.
[0020] The damper device can be mounted in portable metal tents for
damping the movements of the carrying columns and beams of the
tent.
[0021] The damper device can be used to reduce the rotation of
joints in frame structures.
[0022] The damper device can be mounted in several storeys
industrial buildings.
[0023] The damper device can be mounted in timber frame
structures.
[0024] The damper device can be mounted in metal towers.
[0025] According to a preferred embodiment of the invention the
damper is adapted for damping the movement of prefabricated panels
or walls made of timber or light weight metal frames such as frames
made from a light weight steel alloy. The panels could as an
example be made in a panel factory and be pre-mounted with the
damper. The dampers could be either pre-adjusted for a specific use
of the panel or the dampers could be adjusted at a later stage when
they are mounted, e.g. in a residential structure.
[0026] The nature of the damper enables the use of the damper both
in existing structures as well as in new structures due the
simplicity of the concept.
[0027] The device may comprise a frictional pad arranged between
the two members in a sandwich fashion. The frictional pad provides
a dry frictional lubrication and intends to maintain a mainly
constant frictional coefficient. At the same time the frictional
pad intents to dampen the grinding noise prevailing from the
frictional movement of the members.
[0028] The device may furthermore comprise means adapted to vary
the clamping force. By varying the clamping force the frictional
force and thereby the damping characteristic is being changed and
can thus be adapted for a specific purpose, e.g. to match the
movement of a certain wind force, earthquake etc. The means for
varying the clamping force could be an electromechanic,
electro-hydraulic, pneumatic or similar mechanically or
electrically controlled device enabling dampers in a building to be
actively adjusted to actual conditions.
[0029] In a preferred embodiment of the invention the joint
comprises a pin extending through each of the at least two members.
The pin can act as the only member holding the damper together and
thus provide for a easy fitting of the damper and adjustment of the
damping effect. The frictional movement between the members or
alternatively between the frictional pad arise from rotation of the
members around the pin, which thus acts like a hinge pin.
[0030] In a preferred embodiment of the invention the device may
comprise a bolt, where at least a portion of the bolt constitutes
the pin, the bolt having:
[0031] a bolt member with a bolt head,
[0032] a nut with a nut head,
[0033] the clamping force being determined by the pretension of the
bolt. This is a simple and reliable embodiment of the invention,
where only simple tools are necessary for the assembly of the
device as well as for the adjustment of the clamping force.
[0034] The device may further comprise means for maintaining a
substantially constant clamping force over time. This is essential,
since the frictional force is a function of the clamping force and
since the frictional force is adjusted to match the damping
conditions.
[0035] The means for maintaining a substantially constant clamping
force can comprise at least one spring arranged between the bolt
head and a surface of one of the members and/or between the nut
head and a surface of one of the members. The spring can preferably
be a disc spring or more disc springs arranged in series or it
could be one or more disc spring(s) arranged between the bolt head
and a surface of one of the members, and another disc spring or
more disc springs arranged between the nut head and a surface of
another of the members.
[0036] In a preferred embodiment of the invention the at least two
members comprise a side plate and a central plate extending in
substantially parallel planes. The side plate could preferably be
arranged in either fixed or pivotal connection with one of the
braces of the bracing system, the brace being connected fixed or
pivotally to a member of the frame structure at the other end. The
central plate is attached fixed or pivotally to one of the members
of the frame structure, so as respectively to prevent or allow
relative rotational movement between the central plate and the
structural element. In this setup the frictional pad can preferably
be arranged between the side plate and the central plate and thus
provide a constant friction between these plates. In a further
preferred embodiment two side plates are accomplishing the central
plate, each being arranged symmetrically around the central plate.
Each of the two side plates are being connected pivotally or fixed
to one of the structural members or to a brace of the bracing
system, the braces being fixed or pivotally connected to members of
the frame structure at the other end. In this setup it is preferred
to adapt two frictional pads, each frictional pad being arranged
between a respective one of the side plates and the central
plate.
[0037] The frictional pad is intended to maintain a constant
frictional force over a period of time and even after many cycles
of movement. It has been found, that a friction pad material
comprising a MK101 asbestos free friction material by Eurodeal A/S
is suitable for the purpose. Furthermore it has been found, that a
device wherein the side plates and/or the central plate are made of
steel, anti-corrosive steel or brass is suitable but other
materials are adaptable such as aluminium or any alloys comprising
aluminium or any other steel material or composite of steel and
plastics or composites of plastics and fibres of glass, carbon,
kevlar or similar or composites of any ceramics materials and
fibres of glass, carbon, kevlar or similar.
[0038] Preferably, the clamping force shows a variation of less
than 10% such as 8% or even less than 7% such as 5% in a long term
test, such as a 200-1000 cycle test such as a 400 cycle test with
0,2-1 Hz forcing excitement frequency such as 0,5 Hz forcing
excitement frequency and an displacement amplitude of one of the at
least two members of 1-20 mm such as 10 at an applied excitement
force of +10 KN to -10 KN such as +/-2.5 KN. and an initial
clamping force of 1-8 KN such as 4 KN. In a specific test (cf. the
below discussion of experimental results, the variation was 5% in a
400 cycle test).
[0039] It is further preferred that the damping characteristic is
independent from the frequency of the force excitement. This is to
ensure that the damping effect is independent from the force
frequency of a specific earthquake, storm etc. It is preferred that
the frictional moment in the frictional joint of the device shows a
forcing frequency dependent variation of less than 10% such as 5%
in the range 2-7 Hz at a nominal frictional moment of 100-500 Nm
such as 200 Nm in e.g. 30-cycle tests at each frequency.
[0040] It is preferred that the relationship between displacement
amplitude of one of the at least two members and energy dissipation
in the frictional joint is substantially linear. This makes the
damper easier to model and thereby easier to design for a specific
purpose.
[0041] A further aspect of the present invention relates to a
bracing for a building structure comprising at least two structural
elements and a device for damping relative movements between the
structural elements, the device comprises:
[0042] at least two members being interconnected in a rotational
joint for frictional damping of relative rotational movement
between the at least two members,
[0043] clamping means for clamping the at least two members
together, so as to maintain a clamping force and friction between
the at least two members in the rotational joint,
[0044] means for connecting each of the at least two members to
respective ones of the structural elements.
[0045] Preferably the bracing system comprises any of the features
of the damper according to the present invention. The device for
damping could preferably comprise at least two side plates as
earlier mentioned and which are interconnected at at least one of
their ends by means of an interconnecting element, and wherein a
brace is mounted to the interconnecting element. In a further
preferred embodiment at least one of the side plates are being
interconnected to one of the structural elements by means of a
brace, and wherein the central plate is connected or mounted to
another one of the structural elements. Furthermore at least one of
the side plates could be connected to one of the structural
elements by means of two braces, the two braces being connected to
opposite ends of the side plate(s), and wherein the central plate
is connected or mounted to another one of the structural
elements.
[0046] The bracing system can be arranged with the side plates
being connected to one of the structural elements by means of two
braces and the damper being arranged in a V-shaped bracing. In some
technical literature this kind of bracing is referred to as being
an invert-V bracing or a Chevron Bracing. Similarly the bracing
system can be arranged with at least one of the side plates being
connected to one of the structural elements by means of two braces
and the damper being arranged in a D-shaped bracing, and similarly
the bracing system can be arranged with at least one of the side
plates being connected to one of the structural elements by means
of two braces and the damper being arranged in a K-shaped bracing.
The choice of arrangement may depend on the actual situation and
will be selected by a professional designer.
[0047] A further aspect of the invention relates to a damper for
damping movements of large concrete panel walls in building
structures, the device comprising:
[0048] at least one member interconnected to at least one panel in
a first rotational joint,
[0049] the at least one member being further connected to another
panel or similar part of the building structure in a second
rotational joint,
[0050] one or both of the first or the second rotational joints
providing a frictional damping of relative movement between the at
least one member and the panel or similar building structure,
[0051] one or both of the first or the second rotational joints
further providing a sliding movement of the at least one member in
relation to the panel or similar building structure to which it is
attached, the sliding being enabled by means of a tolerance
allowing movement in two perpendicular directions.
[0052] clamping means for clamping the at least one member together
with the panel or similar building structure, so as to maintain a
clamping force and friction between the at least one member and the
panel or similar building structure in the rotational joint.
DETAILED DESCRIPTION OF THE INVENTION
[0053] A preferred embodiment of the invention will now be
described in details with reference to the drawing in which
[0054] FIG. 1 is a perspective picture of details of the friction
damper device,
[0055] FIG. 2 shows a steel frame with a supplemental friction
damper device,
[0056] FIG. 3 shows the mechanism of the damper for damping
movement of a frame,
[0057] FIG. 4 shows the flexibility of using the friction damper in
different types of bracing systems,
[0058] FIG. 5 shows the possibility of having multiple unit
dampers,
[0059] FIG. 6 shows different settings for the damper within
structures,
[0060] FIG. 7 shows the effect of using different displacement
amplitudes on the hysteresis loops,
[0061] FIG. 8 shows the tested displacement in mm for two different
clamping forces,
[0062] FIG. 9 shows a force history plot for a 60-cycle test, a
Bolt Clamping Force history, a force (displacement hysterises) and
a Zoom of Force (displacement hysterises) for a devise with a brass
disc,
[0063] FIG. 10 shows the displacement effect for the highly
frictional material,
[0064] FIG. 11 shows the effect of displacement amplitude on the
amount of energy dissipation,
[0065] FIG. 12 shows the linear relation between displacement
amplitude and energy dissipation,
[0066] FIG. 13 shows the effect of using different clamping forces
(Ft) on the amount of friction force,
[0067] FIG. 14 shows the effect of clamping force (Ft) on the
amount of energy dissipation,
[0068] FIG. 15 shows the displacement history for 0.5 Hz frequency
and 3.3 kN clamping force and zooming for last 5 cycles,
[0069] FIG. 16 shows the force history for 400 cycles, Bolt
Clamping Force history, Force--Displacement hysterises and
Displacement history for the last 10 cycles,
[0070] FIG. 17-FIG. 20 shows results from measuring the coefficient
of friction,
[0071] FIG. 21 shows the experimental setup for a frictional damper
in a frame structure,
[0072] FIG. 22 shows the results of a dynamic test,
[0073] FIG. 23 shows the experimental setup for friction damper
response measurements FIG. 24 shows the effect of Different Forcing
Frequency 2, 3, 4, 5, 6 and 7 Hz on Moment--Theta Relation,
[0074] FIG. 25 shows the effect of forcing frequency 2, 3, 4, 5, 6
and 7 Hz on horizontal forces (Fh)--frame displacement
relation,
[0075] FIG. 26 show the exitation frequency tested zone,
[0076] FIG. 27 shows the effect of different displacement amplitude
on the frame response and energy dissipation--displacement
relation
[0077] FIG. 28 shows the effect of using clamping forces 3.77,
4.55, 5.55, 5.86, 6.63 and 6.89 kN on force amplitude. Effect of
different clamping forces on energy dissipation,
[0078] FIG. 29 shows the effect of forcing Amplitude on the frame
response and the effect of forcing amplitude on maximum frame
displacement,
[0079] FIG. 30 shows the effect of prestressing force in bracing
bars on frame displacement,
[0080] FIG. 31 shows frame displacement in left and right bar,
[0081] FIG. 32 show time history test results of 3.0 Hz forcing
frequency and excitation force of 0.8 kN,
[0082] FIG. 33 shows a multi-storey building with a damper device
installed in two directions in at least one bay in each direction,
and
[0083] FIG. 34 and FIG. 35 shows pre-fabricated panel walls with
dampers, the frame structure of the panels either made of timber or
lightweight steel.
[0084] As seen in FIG. 1 the friction damper comprises several
steel plates 1 and 4 rotating against each other. The damper may
further comprise a number of circular disc shims of friction pad
material--in FIG. 1 having numeral 3--placed in between the steel
plates, in order to have dry friction lubrication in the unit. The
frictional pads are ensuring a stable frictional force acting on
the steel plates and at the same time they reduce noise developed
by the sliding movement between the plates.
[0085] In a preferred embodiment of the invention the damper
comprise a central plate and two side plates. In FIG. 1 plate 1 is
the central plate and plates 4 are the side plates. As seen in FIG.
2 the vertical plate pivotally connects the damper device to the
girder of a frame structure through a hinge. The hinge is best seen
in FIG. 1 and has numeral 8. The hinge is adapted in order not to
introduce moment in the girder. This is essential e.g. when the
damper is being retrofitted in buildings designed without the
damper. Additionally the hinge increases the amount of relative
rotation between the central plate and the side plates, which again
increases the amount of energy dissipated in the system.
[0086] The two side plates 4 connect the damper to a bracing system
such as a Chevron bracing--as seen in FIG. 2--or similar
arrangement of braces e.g. in a D shape or a K shape. Various
bracing systems are shown in FIG. 4. The bracing system could
comprise bars 13 being pretensioned in order to prevent them from
buckling from compression force but could also comprise structural
members capable of absorbing compression. The braces are preferably
pivotally connected at both ends 14 and 15, by having plain
bearings to the damper 16 and to the column base connection 17, as
shown in FIG. 2. 18 is the upper frame column.
[0087] The reason for using two side plates is to increase the
frictional surface area and to provide the necessary symmetry to
obtain plane behaviour of the device. All plates and the frictional
pads have a centred hole for assembly with a bolt 2 with a nut 5 or
similar kind of confining hinge pin. The bolt or similar hinge pin
compresses the three plates 1 and 4 of the damper and the
frictional pads 4 in a hinge like connection. At the same time the
bolt 2 is used to control the normal force applied on the friction
pad discs and the steel plates, whereby the damping characteristics
of the damper is being changed.
[0088] When the damper is installed in a structural frame, as seen
in FIG. 2. it follows the horizontal motion of the frame--as seen
in FIG. 3. Due to the hinge connection between the central plate
and the upper column and hinge connections between the side plates
and the braces, again being pivotally connected to the base column,
the forces of the movement of the structural frame is being
transferred rotatively to the damping parts--as can be seen in FIG.
3. When the displacement of the structural frame starts, the
frictional forces developed between the frictional surfaces of the
steel plates and the frictional pads will resist the rotation of
the plates in the damper. This phase, wherein no sliding takes
place, is referred to as a sticking phase. When the applied forces
in the damper exceed the frictional forces, a sliding between the
central plate and the frictional pad takes place. The plates now
slides in a circular movement around the hinge pin or bolt. Due to
the tensile forces in the bracing a sliding between the frictional
pad and the side plates also starts but the side plates rotates in
the opposite direction relative to the central plate. In the
sliding phase, the damper will dissipate energy by means of
friction between the sliding surfaces. This phase will keep on and
change to the sticking phase when the load reverses its
direction.
[0089] This process of moving from phase to phase is repeated upon
reversal of the direction of the force application. FIG. 3 explains
the mechanism of the damper device under an excitation force in
different directions.
[0090] In order to keep a constant clamping force when the damper
is in operation, a spring 6 is preferably mounted between the bolt
head and the side plate, between the nut and the side plate or at
both sides. The spring could be of any kind but in a preferred
embodiment of the invention a combination of discs springs 6 and
washers 7, such as Belleville Washers, are used. These springs are
initially cone shaped annular disc springs that flatten under
compression. The washers are placed in order to prevent any marks
on the steel plates due to the disc springs when they are in
compression.
[0091] The damper is based on a very simple design and comprises
only parts that are easily produced. At the same time it is easy to
assemble and very flexible in arrangement. As seen in FIG. 4 and
FIG. 6 it can be arranged both in different configurations as well
as in different types of bracing systems.
[0092] The simplicity of the damper design allows constructing a
device with multiple units, based on the requirements of the
designed friction force and the space limitations. FIG. 5 shows an
example of multiple unit dampers, which give the designer the
ability to build a damper comprising several units. The damper in
FIG. 5 comprise disc springs 9, central plates 10, side plates 11
and frictional pads 12.
[0093] Experimental Results
[0094] In order to verify the frictional component of the proposed
friction damper device, a number of qualification tests have been
performed in the laboratory in order to evaluate the theoretical
studies of this damper.
[0095] The experimental program included two phases:
[0096] 1. Testing the damper device with three different types of
friction materials
[0097] 2. Testing a scale model steel frame with implemented
friction damper device.
[0098] Testing the damper with different friction materials done
with an Instron machine to verify the parameters which is affecting
its performance. These include cyclic tests of the damper. The
proper found material were used in the tests of the scale frame
model incorporating friction damping device, which were performed
by a shaker.
[0099] These experimental studies were carried out in the
laboratory of BKM department, DTU.
[0100] Phase 1, Damper Testing with Instron Machine
[0101] The damper specimens, described above, was placed within an
Instron hydraulic testing machine type 8502. The actuator of this
machine is capable of applying 250 kN dynamic load. Both
displacement, forcing frequency or applied force control were
possible through a controller unit. The test control was done from
the PC running Instron software; "Max 0.2". All testing was done
under displacement and forcing frequency control and all the
resulting data transferred to Data Acquisition Board System which
is integrated with system controller and in conjunction with a
PC.
[0102] For immediate visual observation of the results,
force--displacement curves were drawn on the PC monitor.
[0103] The damper fixed to the Instron machine by a frame holder
designed especially for this case, the frame being connected
rigidly to the machine. The damper connected to this holder by two
small plates fixed rigidly to the holder. Each of these plates
connected to side plates by a hinge. These two plates were used
later to connect the bracing bars to the damper. Inside these
plates, a ball bearing was fixed in order to reduce friction
through the damper activity with scaled frame model.
[0104] The applied load was measured by a dynamometer having two
strain gages fixed on it. This dynamometer was connected by a
bearing hinge at both ends to prevent any kind of bending.
[0105] The clamping force in the bolt was measured using two strain
gages embedded inside the bolt. The required clamping force can be
applied by tighten the bolt head with a spanner and getting the
reading directly from a multimeter.
[0106] In the beginning of the damper tests, several displacement
devices were tried. Linearly Variable Displacement Transducer
(LVDT) was used but because of the rotation of central plate around
the bolt, LVDT can not follow that and its needle was bend which
did not give a correct measurements. Extensometer (CAT NO.2620-602,
travel 5 mm, type dynamic) was another device been tried to measure
the displacement. This device gave much better resolution, but it
was not good enough for large displacements, specifically for more
than 5 mm amplitude.
[0107] All these setting devices were calibrated before and after
the tests or whenever there was an accident through the tests, the
setting was recalibrated.
[0108] When the central plate rotate through the tests, the
Potentiometer head, which stand on the edge of the central plate,
try to bend little bit because of the horizontal projection of
rotation. In order to solve this problem, a special roller had
fixed on the head of the potentiometer. This solution was tested
several times under several conditions and it went well.
[0109] Cyclic Tests of Friction Damper Device
[0110] In order to evaluate the damper performance, a series of ten
dynamic cyclic tests were performed with three different types of
materials; brass, highly frictional material and friction pad
material.
[0111] The performance of the friction damper is, in general,
affected by certain parameters. These parameters were studied in
tests, which are:
[0112] 1. Displacement amplitude.
[0113] 2. Clamping force.
[0114] 3. Long run test.
[0115] Brass Discs
[0116] Brass is cheap and widely available commercial material.
It's been used for long time for their known behaviour. Popove,
1993 had suggested to use them for their good and stable
performance instead of steel, in his damper. In this work brass was
chosen because steel and brass are known that they have a good
combination in friction and for economic reasons.
[0117] Before studying the above mentioned parameters, another one
was tested, which was the distribution of the disc spring washers
on one side and two sides of the steel plates.
[0118] Displacement Effect
[0119] The damper tested with displacement amplitudes of 5, 10, 15
and 20 mm with 0.3 Hz forcing frequency. FIG. 7 shows the applied
displacement and resulting hysteresis loops. It's clearly shown
that the amount of the area increased with the increase of the
displacement and the friction force was almost constant without
showing any fluctuation or disturbances.
[0120] The higher forces that can be observed at the end of each
cycle is because the relative velocity of the plates reach its
minimum value.
[0121] Clamping Force Effect
[0122] The bolt was clamped with different forces to study its
effect on damper as shown in FIG. 7. In this tests the clamping
force were increased from 13.36 to 27.32 kN, due to that the
friction force was increased from 0.65 to 1.5 kN. The result can be
seen in FIG. 8.
[0123] Long Run Test
[0124] A test of 60 cycle were done with 0.3 Hz forcing frequency
and 5 .mm displacement amplitudes, in order to see any degradation
in clamping force. The noises were high and heat generated also
remarkably. The results can be seen in FIG. 9.
[0125] Test results shows that the amount of change in friction
resistance was not much, at the end of 60 cycles, indicating that
the initial bolt tension was still effective. During the first ten
to twenty load cycles, the bolt connections emitted a roaring noise
or chatter vibration from faying surfaces. The load level were not
affected much by the presence of these vibratory noise & after
approximately twenty cycles, the noise in the faying surfaces
emitted a stable rubbing sound similar to metal milling work. The
force was increased from 0.76 kN to 0.98 kN. This increase is
probably due to many interacting phenomena:
[0126] 1. Increase of the plate thickness due to heat generated
from friction.
[0127] 2. Softening of the metal due to the rise in
temperature,
[0128] 3. Variation of the bolt clamping force
[0129] 4. Wear.
[0130] 5. Growing contribution of the roughness and plowing terms
to the slip load as more surface damage was taking place.
[0131] Considering that the brass discs have few abrasion, the
friction damper with use of brass, can be judged as having
sufficient durability under practical condition.
[0132] Highly Frictional Material
[0133] A material with high friction coefficient was tested. This
material is coated with abrasives, which have a trade name called
Felxovit, used to grain steel. This material is a fibre disc of 0,3
mm. thickness. It was glued on discs in order to have the required
thickness in the damper.
[0134] The material gave high frictional forces.
[0135] Displacement Effect
[0136] The damper tested with displacement amplitudes of 2 and 5
mm. As shown in FIG. 10 the friction force was not constant at
sliding and the hysterises are clear that they are not stable.
[0137] The thickness of the connected plates is changing upon
cyclic loading. It increase due to temperature rise because of the
generated heat caused by friction. On the other hand, wear at the
contact surface will reduce the thickness.
[0138] If the plate thickness increase due to heat, more than wear
effect, it will increase the bolt clamping force, which increase
the sliding force. An example was the brass test of 60 cycles, FIG.
10. If wear effect is larger than heat effect, then the bolt will
loss its clamping force, which cause sliding to occur at lower
design level and this will reduce the amount of energy dissipation
in the system. FIG. 10 is an example for such a case.
[0139] For the above mentioned reasons there is a need for looking
to another materials;
[0140] Friction Pad Material
[0141] The performance of the damper can be much improved if
another material that can resist wear as much as possible, in the
same time not generate much heat, also perform a stable hysterises.
Such improvement was likely to be achieved by providing a more
suitable material combination.
[0142] An asbest free friction material (friction pad material) was
found after searching for one with a special requirement. This
material has a friction coefficient of 0.35-0.45. The max
compression strength is 1100 kg/cm..sup.2 and the working
temperature is 165.degree. C.
[0143] It's a composite material.
[0144] This was tested with the same previous parameters. No wear
or adhesive wear in steel or in the material was noticed, only
small amount of powder covered the steel plates.
[0145] Displacement Effect
[0146] The damper tested with displacement amplitudes of 2, 4, 6,
8, 10 and 12 mm As shown in FIG. 11 the area of energy dissipation
was increased in relation with amplitude with a constant friction
force. It's clearly shown in FIG. 12 that the damper energy
dissipation, which is the area of one cycle is linearly dependent
on this parameter which make the damper workable under different
displacement amplitudes if its modified to semi-active device.
[0147] Clamping Force Effect
[0148] The bolt was clamped with different forces (Ft); 3.3, 2.75,
2.2, 1.65, 1.1 and 0.55 kN to study its effect on the damper as
shown in FIG. 13. The displacement amplitude was controlled with 5
mm. By plotting the energy dissipation per cycle with respect to
clamping forces. It can be concluded that the relation is linear,
which make using of Coulomb Law quite sufficient in the relation of
the normal force.
[0149] The tests showed that the friction pad material have a good
and stable performance without showing degradation or fading as
clearly seen in displacement time history of 30 cycles test with
0.5 Hz forcing frequency and 3.3 kN clamping force, FIG. 15. These
good results were encouraging for long running tests to check their
performance.
[0150] Long Run Effect, 400 Cycle Test
[0151] In order to study the damper performance with the friction
pad material under long running cyclic test, the damper was
subjected up to 400 cycle with 0.5 Hz frequency and 4 kN clamping
force. The results were very encouraging, no fading was notice,
very small amount of heat was generated and that cause the slight
increase in bolt clamping force, the noise was much less than other
materials.
[0152] FPM tests showed negligible damage to their friction
surface.
[0153] The successful performance of FDD in providing stable
`rectangular` hysteresis loops is due to the use of FPM. Results
see FIG. 16.
[0154] Measuring Coefficient of Friction
[0155] According to the FIG. 17-20 it is evident that the test
frame restoring force characteristics are bi-linear (as calculated)
even if the steel frame is in the elastic range It can be seen that
while the damper's frictional force is slightly increased at the
beginning of sliding. It continues sliding at a stable frictional
force for the entire sliding length after starting to slide. The
figures show the damper frictional force--sliding Displacement
relationship as an example of their respective test results. There
is no fluctuation in the frictional force of the damper. The damper
absorbed energy tends to fall as the Frictional Forces rises. Using
Frictional Pad Material or Brass which are imbedded gave very
stable frictional force and prevented abnormal noise generation at
frictional movement.
[0156] Since the device can develop such a variable reaction, it
can be used as part of a motion control scheme as semi-active
device.
[0157] The friction damper can work under wide range of
displacement amplitudes with constant and stable frictional
forces.
[0158] The `frame holder` which was designed to hold the damper
through the tests, was not rigid enough to prevent the small
horizontal motion of the top part which caused by cyclic motion
when large forces been used.
[0159] Experiments of a Frame with Friction Damper
[0160] A single story, one bay, steel frame model was build and
tested statically and dynamically in order to experimentally verify
the effectiveness of the friction damper concept. These test of the
damper device implemented with steel frame was planed to ascertain
the damper performance under practical condition prior to putting
it into use of the building.
[0161] The overall dimensions of the model frame are 1.125 m.
height and 1.10 m span. The frame can be seen in FIG. 21.
[0162] The frame structure columns are steel stripes of 50.times.15
mm. The beam is a hallow rectangular steel section of
90.times.50.times.5 mm and rigidly connected to the column by all
around butt welding. The structure is fixed rigidly to the massive
floor of the laboratory. The ratio of beam moment of inertia (Ib)
to column moment of inertia (Icol). is 91.73 in order to assure
very rigid beam.
[0163] This frame model has the following properties
1 Lateral stiffness Mass Lowest Natural (N/mm) (kg) Frequency, Hz
Measured 40.667 23.03 6.8 Calculated 40.57 23.65 6.7
[0164] The steel frame was excited by static and dynamic horizontal
forces applied by means of exciter. This exciter is connected to
the steel frame by rigid bar. The natural period of the frame
without the damper was 6.8 Hz, measured by free vibration test. A
digital storage oscilloscope VP5730 .ANG. was used in this
test.
[0165] Dynamic Response of Frame Model
[0166] In order to check the dynamic proprieties of the frame
model; the stiffness and the mass, a harmonic loading was applied
by the exciter. The response was compared with analytical solution
of this SDOF system. The results were very satisfactory as shown in
FIG. 22.
[0167] Experimental Setup
[0168] The frame girder is excited horizontally with a force,
experimentally applied by rigid bar, fixed between the frame girder
and the exciter head. The oscillation of the exciter head is
generated by the Exciter controller and amplified by the Power
amplifier. The force transferred between the structure and the
attached rigid bar is measured by is measured by force a Force
Transducer. This measured force is continuously stored by the DAP
Program. The acceleration of the frame is measured by an
Accelerometer, which is mounted at the top of the frame and the
measurements stored continuously by the DAP. The position of the
frame was obtained using potentiometer with a roller head rigidly
mounted on external frame holder. The setup is schematically shown
in FIG. 23.
[0169] The relative rotation between the steel plates was measured
by a potentiometer, with a roller head, fixed on side plate. These
measurements divided by the distance between the potentiometer head
and the centre-line of side plate. The rotation of central plate
was measured by another potentiometer, and the readings were
divided by the distance between the head and the centre of the
hinge that connect the damper to the frame girder.
[0170] Dynamic Response of Frame Model with Friction
damper--Experimental Evaluation Parametric Studies
[0171] Several parameters were studied experimentally to verify the
performance of the damper and to study their dependency. These
parameters were as fellow:
[0172] Forcing frequency.
[0173] Displacement amplitude.
[0174] Clamping force in damper.
[0175] Forcing amplitude.
[0176] Prestressing forces in bracing bars.
[0177] Forcing Frequency In a Frame
[0178] One of the most important parameter in verifying the
friction damper devices is the velocity dependency. The frame were
tested by 2.0, 3.0, 4.0, 5.0, 6.0 and 7.0 Hz forcing frequency with
same value for all the other parameters. The results, which
represent moment and relative rotation between the plates, Theta,
show clearly that it is almost velocity independent as it is shown
in FIG. 20.
[0179] As it is shown, the effect of changing forcing frequency is
not affecting much the Moment--Theta relation plots, which cancel
the necessity to include a velocity dependency term in Coulomb Law
for modeling the friction force. Another relation was studied which
is horizontal force (Fh) with frame displacement. FIG. 25 clearly
show that there is no big influence on the response.
[0180] Displacement Amplitude
[0181] The frame was tested with different displacement amplitudes
in order to verify its influence on the damper behavior. In these
tests the frame displacement was controlled with 1.75, 2, 2.5, 3,
3.5, 4 and 4.5 mm as shown in FIG. 27. The energy dissipation,
which is the area of force--displacement curve, for each amplitude
was, plotted verse frame displacement in FIG. 27.
[0182] It is clearly shown that varying the displacement amplitude
is linearly changing the dissipation energy, which is match with
what is found earlier in the tests of the damper with instron
machine.
[0183] Clamping Force in Damper
[0184] In order to verify the dependency of clamping force,
different values were selected starting from 3.77, 4.55, 5.55,
5.86, 6.63 & 6.89 kN. In these tests the displacement
amplitudes were controlled so they have the same amplitudes but the
force amplitude required to produce a certain displacement, were
different because of the damper clamping force, as shown in FIG.
28.
[0185] In FIG. 28 it's clearly show that increasing bolt-clamping
force in damper is almost linearly increase the energy dissipation.
This conclusion agrees with that been reached in the 1.sup.st phase
of the damper tests. The linearity of this relation lead to use
Coulomb law for modeling the friction force.
[0186] Forcing Amplitude
[0187] Different force amplitudes were used to excite the frame.
The Clamping force, pre-stressing force in bracing and forcing
frequency were controlled and different forces starting from 0.40,
0.50, 0.60, 0.70, 0.75, 0.80, 0.85 and 0.90 kN were used as shown
in FIG. 29.
[0188] Because of the setting of these tests the damper did not
activate under 0.4, 0.5 and 0.6 kN and no sliding occurred, only
sticking but when the load increased to 0.7 kN, sliding were
started and the displacement was increasing whenever the force
increased. In FIG. 11B the frame respond with small nonlinearly
especially when a force of 0.8 kN were used and went larger for
0.85 and 0.9 kN.
[0189] Prestressing Forces in Bracing Bars
[0190] In evaluating this parameter two types of test were
performed;
[0191] 1.1. A--Large clamping force, sticking only.
[0192] 1.1. B--Small clamping force, sticking and sliding.
[0193] In both cases the frame was excited with 3.0 Hz forcing
frequency and 0.8 kN force amplitude with large value of clamping
force which prevent sliding to occurred. Results see FIG. 30.
[0194] 1.1. A--Large clamping force, sticking only.
[0195] In these tests, the bracing bars were pre-stressed with
1.02, 2.2, 4.4, 6.9, 8.8 and 10.1 kN respectively. From figure (11)
its clearly seen that increasing pre-stressing force did not lead
to decrease the frame displacements, especially with forces of
1.02-6.9 kN. But when forces increased to 8.8 and 10.1 kN sliding
were started because the stiffness of the bracing system become
very large and prevent the horizontal motion of side plates, but
the applied forces will overcome the frictional forces which caused
sliding of plates.
[0196] 1.1. B--Small Clamping Force, Sticking and Sliding.
[0197] Four different pre-stressing forces 2.0, 4.0, 6.0 and 7.7 kN
were used in these tests. It is clearly shown in figure (11) that
increasing pre-stressing force did not affect much on the
horizontal frame displacement except the lowest value.
[0198] So from these tests of verifying the effect of pre-stressing
parameter it can be concluded that it is not playing a big part in
damper performance and increasing the forces dose not lead to
improve the performance and with the use of optimum pre-stressing
forces the same required response can be reached.
[0199] Long Run Tests
[0200] The damped steel frame model was tested with long running
tests under different frequencies and excitation forces. FIG. 16
show time history test results of 3.0 Hz forcing frequency and
excitation force of 0.8 kN. The frame response was very stable and
constant, 20 B, and the relative rotation of the plates was also
stable, FIG. 16D. So the conclusion was after more than 100 cycle
test that the damper performance was quite satisfactory.
[0201] It was also planed to subject the frame to hundreds of
cyclic test but the setup limitations prevents that. Results are
shown in FIG. 32.
* * * * *