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Transcript
Design Considerations for Precast
Prestressed Concrete Building
Structures in Seismic Areas
Alfred A. Vee, P.E., Dr. Eng.
President
Applied Technology Corporation
Honolulu , Hawaii
Long -ti me PCI Professional Member
Alfred A. Vee has been in engineering
practice since 1953, specializing in the
design of precast and prestressed
concrete structures . Most of these
structures have been located in the
Pacific Rim - one of the most
seismologically active areas in the
world . In recognition of his innovative
work in advancing concrete tech nology, he was awarded an Honorary
Doctorate Degree in engineering from
Rose-Hulman Institute of Technology
in 1976. That same year Dr. Yee was
elected to the prestigious National
Academy of Engineering. He obtained
his bachelor's degree in civil eng ineering from Rose-Hulman Institute of
Technology and his master's degree
from Yale University . Dr . Yee has
lectured all over the world and is the
author of numerous published papers
including some that have appeared in
the PCI JOURNAL.
40
Discusses the major design considerations necessary in the
successful construction of precast, prestressed concrete
building structures situated in seismic areas. The importance
and interaction of stiffness, strength, toughness and,
especially, fail-safe connections are emphasized. Such
connections are needed not only to transfer loads but to
provide continuity and overall monolithic behavior in the entire
structure. Suggestions are given on how to increase the
stiffness of the structure as well as advice on good and poor
design practices. Examples of precast concrete buildings
drawn from the author's 40 years design experience are
shown and commented upon.
ound principles, attention to
detail and good judgment are
necessary in designing precast,
prestressed concrete structures in seismic areas. These ingredients, of
course, apply also to the design of all
types of structures, whether they are
made of concrete, steel or composite
materials. However, in the case of precast and prestressed concrete structures , particular attention must be
given to stiffness, strength and toughness requirements, and especially to
connection detai ls. Their function is
not only to transfer loads but to develop continuity and monolithic behavior
in the entire structure.
Strength, stiffness and toughness
minimize second o_rder effects resulting from drift, deflection and rotation.
S
It is important that movements and rotations be restrained in an earthquake
because they can cause both structural
and nonstructural damage to buildings
and in severe cases can induce co llapse. Shear walls combined with each
other through deep beams or deep wall
gi rders or trusses to form giant moment frames is one good method to increase the overall stiffness of a structure.
Soft floors, seismic joints and eccentricities (non-symmetry) in building frames should be avoided regardless whether they can be justified by
mathematical calculations. If seismic
joints become absolutely necessary
due to unusual structure size, it is suggested that suc h joints be semi-restrained by a system of tension ties in
PCI JOURNAL
combination with the insertion of a
cushioning material between building
components to reduce possible damage from pounding action.
The purpose of this paper is to present the major design considerations
that are required in the safe construction of precast, prestressed concrete
building structures situated in seismic
areas. Another purpose of the paper is
to present real life examples of buildings that have withstood major earthquakes and to discuss some of the design features and practices that do and
do not work well in seismic areas.
A list of articles pertinent to this
paper is given in References 1 to 12.
This paper is drawn from the author's 40 years experience designing
precast, prestressed concrete structures. Most of these structures are situated in the Pacific Rim which is one of
the most seismologically active areas
in the world.
CONNECTION DETAILS
The single most important ingredient in the design of precast concrete
structures is the connection details.
Connections between precast building
elements such as columns, beams,
slabs and shear walls must effectively
integrate the individual structural
components in full continuity with
each other so that the overall building
structure behaves monolithically. In
this manner, the structural analysis
and behavior of a building frame
would be identical to that for a cast-inplace structure except that the framing
system now uses precast concrete
components which are assembled together to act monolithically.
As an example, take the case of simple precast bearing wall panels supporting precast concrete floor slabs.
The most effective system we have
found uses composite in-place connections to develop full continuity between adjacent slab spans as well as
the precast concrete wall panel units.
Fig. la illustrates a precast bearing
wall panel supporting a precast floor
or roof slab. The joint detail is arranged so that dimensional errors in
construction would have little influence on the structural integrity and
performance of the composite system.
May-June 1991
The precast floor slab panels are integrated with a composite in-place
structural concrete topping which provides a uniform leveling surface and a
horizontal structural diaphragm to resist seismic forces_ Utilities such as
electric, telephone and television conduits can be buried within this topping. The precast concrete slab panels
are detailed with extended reinforcing
bars bent in a diagonal pattern, crisscrossing over the precast bearing wall
panels. Negative moment steel is
added in the composite topping to develop continuity between adjacent slab
spans. The precast concrete wall panel
of the upper floor is integrated in full
continuity with that of the lower floor
by means of grout-filled steel sleeve
mechanical connections_
This particular joint does not require
a specified minimum bearing area between the precast floor slab and wall
panel in order to achieve adequate vertical support. The extended diagonal
reinforcing steel from the ends of the
precast concrete slab panels is designed to provide full shear resistance
to the anticipated vertical dead and
live loads on the slab span. Therefore,
no reliance is placed on the compres-
sion bearing area between the floor
slab panel and the top of the precast
wall panel after this joint is in place.
The wall panel thickness or top of
the wall need not be widened to provide support tolerance for the precast
concrete slab. Negative moment steel
in the topping not only develops full
continuity between slab spans but also
serves as a tie to resist horizontal tension between the slab units. In effect,
the reinforcement prevents the slabs
from pulling apart over the wall support when the concrete undergoes volume changes or if an earthquake occurs.
In the event that this particular joint
is severely damaged during an earthquake and the joint deteriorates such
that the concrete in the top of the wall
panel as well as the bearing ends of
the slab panels are spalled off, the
floor slab will not be in any danger of
collapse. This is because the extended
overlapping diagonal bars in the bearing ends of the slab are designed to
fully resist anticipated vertical dead
and live loads_ No reliance is placed
on a bearing element or shear in the
concrete. Therefore, this detail is practically speaking fail-safe.
GROUT-FILLED
STEEL SLEEVE
CONNECTION - - - - - - ,
PRECAST BEARING
WALL PANEL
GROUT BED -------.
TUBES FOR GROUT
INJECTION
NEGATIVE MOMENT
CONTINUITY STEEL
COMPOSITE IN-SITU
CONCRETE TOPPING
EXTENDED DIAGONAL
REINFORCING STEEL
PRECAST SLAB
PANEL
PRECAST BEARING
WALL PANEL
Fig. 1a. Precast bearing wall panels support precast concrete slabs with in-place
composite structural topping.
41
Fig. 1b. Precast slab panel erroneously manufactured short of wall support.
Fig. 1c. Extended reinforcement from slab panels diagonally crossing over wall
supports to provide slab shear resistance. Supplementary negative moment steel
in the topping area will develop full continuity between adjacent composite slab
sections.
ADDED CONTINUITY REINFORCEMENT
CAST IN PLACE CONCRETE
COMPOSITE
BEAM
PRECAST CONCRETE
SOFFIT-- -----'
PRECAST/
PRESTRESSED
CONCRETE
JOISTS - - - '
EXTEND STRANDS
INTO BEAM - - - '
(a) INTERIOR BEAM
CONNECTION
(b) EXTERIOR BEAM
CONNECTION
Figs. 2a-2b. Connection detail for interior and exterior beam.
42
Fig. l b shows an actual project
where the floor slab was erroneously
manufactured short of its intended
support on the precast wall panel. Although this mistake is undesirable, the
extended reinforcing steel would prevent a catastrophic failure in the event
of an earthquake.
Fig. lc shows how the extended slab
reinforcing bars are bent in a diagonal
pattern, criss-crossing over the bearing
wall panel. Added negative moment
continuity steel is also shown. Once
the composite concrete topping is
placed and cured, adequate resistance
in the slab shear support is ensured
even though the precast concrete slab
did not initially rest directly on the
bearing wall panel.
Figs. 2a and 2b show a connection
detail developing full continuity between the precast concrete joists and
beams.
In Fig. 2a, a precast concrete soffit
is utilized for a composite beam supporting the precast concrete joists. Inplace concrete of the composite beam
is also used to develop full bearing
and continuity between the precast
joists and beam elements. Added continuity reinforcement placed in the
composite structural topping is utilized to develop negative moment for
the joists over the beam element.
In Fig. 2b, a cast-in-place beam is
used to support the precast concrete
joists. In all cases, reinforcing steel
and/or strands in the precast concrete
joists are extended into the in-place or
composite beam elements to develop
additional bearing and shear friction
which may be necessary in the event
that the precast joists are inadvertently
manufactured too short. This type of
joint has been used in thousands of
building structures without one single
instance of spalling, cracking or any
other signs of distress.
For developing fully continuous
splices in reinforcing steel bars, mechanical connections specially designed and tested are more reliable
than conventional lapped splices as
commonly used in cast-in-place construction. Figs. 3a to 3e illustrate some
of the more common types of mechanical connections.
Fig. 3a is a steel sleeve connection
that utilizes a special high strength
PCI JOURNAL
GROUT
MATRIX
METALLIC
MATRIX
THREADED
COUPLER
(c)
(b)
REINFORCING
STEEL BAR OR
STRUCTURAL
STEEL PLATE
OR ANGLE - - - - - - i
SWAGED
COUPLER
I
(I
!Ill
if11
LJ
r•
,,
il\1
II
(d)
(e)
STEEL
SLEEVE
(a)
COUPLER
WITH
TAPERED
THREADE
1
''\ill' REBAR
WELD
WELD ----------~1
LAP WELD
BUTT WELD
Figs. 3a-3e. Typical mechanical connections .
Fig. 3f. Butt and lap weld connections.
STRUCTURAL TESTS ON FULL SIZE COLUMNS WITH NMB SPLICE SLEEVES
(Test Report NPD-024)
n TES~
~.._,___
__ -,~ ~
~
I
11,000mm 1620mm
(3'-3-3 / 8')
!
'6~m I 1,000mm I 4-=(5~::)
2,000mm
(1'·3-3 / 4 ' )
:rj.JJ¥
(6'-6-3 / 4' )
(1' -3-3 / 4' ) (3 '-3-3/ 8' )
LOADING TEST IN PROCESS
r==- R(
10
Ca8 = 345kgf / cm '
RaY= 3,810kgf/ cm '
WaY = 4,010kgf / em '
20
(Concrete)
(Main rebars)
(Tie hoops)
X
10 ' )
40
6 or
30
4or
r
60
50
I
70
IOor
u
13
I
I
I
I
I
/
/
/
/
/
/
8
I
-[QJ~
I
I
I
0.2801 c
1
0
~!-m 0 8
~1008
4-D41(SD35)
f' c = DESIGN STRENGTH
40
10
20
R( X 10· ' )
--==:J
Fig. 3g . Summary of results of tests on full size columns with splice sleeves.
May-June 1991
43
non-shrink cement grout matrix which
forms a grout wedge anchoring the reinforcing steel to the external sleeve.
This type of mechanical connection
has been tested extensively for repetetive loads in both tension and compression and is widely used throughout the world.
Fig. 3b shows a splice connection
utilizing a metallic matrix which engages the deformations of the reinforcing steel bars to a serrated inner surface of the sleeves.
Fig. 3c shows a threaded steel coupler engaging threads cut into the ends
of the reinforcing bars. In some couplers the surface deformations of the
bars are used as threads which in tum
engage the coupler.
In Fig_. 3d, the threaded coupler is
tapered to accommodate the tapered
threaded ends of the steel bars. By tapering the threaded ends of the steel
bars, insertion into the threaded coupler is facilitated.
In Fig. 3e, the ends of the reinforcing bars are engaged by a steel sleeve
hydraulically swaged under pressure
to the surface deformations of the reinforcing bars.
Typical butt and lap welded connections are shown in Fig. 3f.
Fig. 3g is taken from a report on
repetitive loading tests on columns
with splice sleeve connections using a
special grout. In this test the steel
sleeves are located at the floor level of
a column-to-beam specimen. The assembly is loaded so that bending moments attain a maximum intensity in
the specimen. The column-beam connection is then wracked repeatedly by
a jacking arrangement to simulate
seismic forces on a column-to-beam
frame. Results of these tests show that
the mechanical connections enabled
the precast concrete to perform in both
strength and ductility equal to that of
monolithically cast columns with fully
continuous reinforcing steel.
In Fig. 4 the connection details between the column units and between
the column and beam or girder units
are illustrated. In this scheme a mechanical connection that can develop
full continuity between reinforcing
bars greatly enhances the flexibility in
the column-to-column joint locations.
When these connections are combined
44
GROUT JOINT
MECHANICAL CONNECTION
(GROUT FILLED SLEEVE)
IN-SITU CONCRETE
PRECAST CONCRETE BEAM OR GIRDER
t - - - - - PRECAST
:=;t;=~----GROUT
CONCRETE COLUMN
JOINT
MECHANICAL CONNECTION
(GROUT FILLED SLEEVE)
Fig. 4. Typical precast concrete frame.
~F-
71~
!!!-
BOUNDARY
MEMBER
REINFORCEMENT
H!-!-
(
I
!-!--~--
~
~
-~
HORIZONT AL
SHEAR
REINFORC EMENT
A
f-1!-
I
!-!!-!-
FLOOR LEV EL
GROUT JOINT
I
r--
-
-~~~
v
I
....
--~
--
FLOOR LEV EL
·-
I
-t
-H
~
~~
I)
~
-
CONFINEM ENT
REINFORC EMENT
11~
~
\
!J
lA
lMECHANICAL
CONNECTIONS
Fig. 5. Precast concrete shear wall with boundary members.
with adequate grouting in the joint
area, the vertical precast concrete column unit can be joined at the floor line
or at any other convenient location
along the height of the column to develop full continuity.
It should be noted that the mechanical connection between column units
can be "blindly" executed; that is, access openings are not needed, thus
eliminating the need for patching and
grouting after the connection is installed. The grout-filled steel sleeve
connection shown in Fig. 4 is classified as a "blind" connection that can
achieve full continuity in the steel. A
thin layer of grout inserted between
the column units, together with the
continuity provided in the reinforcing
steel by such connections, will result
in connected column units that act
monolithically as a fully continuous
member.
In a " blind" connection, the mechanical connection is basically a tapered steel sleeve which envelops the
PCI JOURNAL
ends of the connecting reinforcing
bars as shown in Fig. 3a. This sleeve
is made large enough to allow reasonable tolerance for erection and is completed by filling with a special grout to
develop the required anchorage of the
reinforcing steel.
Threaded couplers or sleeves filled
with metallic matrix can also be used
but these require access pockets to execute the connection and therefore
cannot be considered as "blind" connections. These threaded couplers or
metallic matrix sleeves have been used
mainly for cast-in-place construction
since the tolerances required wouio be
extremely close and access to the
sleeves must be provided before concrete pouring.
The joint location between the precast concrete beam and column can be
at a point of highest moment adjacent
to the column or at some distance
away for convenience in precasting
and erection. In the case of a horizontal joint, the joint area is cast-in-place
to develop full continuity. The mechanical connection is totally exposed
before installation with in-place concrete and, therefore, the reinforcing
bars can be spliced by grout-filled
sleeves, welding or other methods.
Welding is labor intensive and time
consuming. The heat generated from
welding can cause damage to bond in
the steel bars and cracking in the adjacent precast concrete. Furthermore,
high quality welding requires close supervision and inspection whereas mechanical connectors using sleeves with
grout can be installed quickly and reliably without the need for special skills
and supervision.
Grout-filled sleeves have sufficient
built-in tolerances and can easily be
installed at normal temperatures, thus
avoiding any damage to the reinforcing steel or adjacent concrete. In situations where epoxy coated reinforcing
steel is used, grout-filled sleeves are
particularly advantageous because no
heat or damage to the epoxy would result. Sleeves with a metallic matrix or
threaded couplers are difficult to apply
in precast concrete work since they
would require extreme precision in bar
placement to have all corresponding
reinforcing steel properly aligned.
To Clevelop continuity between the
beam and column elements, the longitudinal steel in the connection must
have sufficient anchorage in the column. Equally important, confinement
steel must envelop both the column
vertical steel and the horizontal bars of
the beam or girder in the high moment
region adjacent to the column. High
strength concrete sufficiently plasticized to completely fill all void areas
between the reinforcing steel and precast concrete contact surfaces will produce a fully continuous joint integrating the beam and the column in monolithic frame action.
Fig. 5 shows the connection between precast concrete shear wall panels with boundary members. The
grouted joint between the precast
panel elements in combination with
properly spaced mechanical connections to develop continuity in the reinforcing steel can produce a shear wall
frame acting monolithically throughout its height.
The precast concrete shear wall reinforcement is detailed in exactly the
same way as a cast-in-place shear
wall. Vertical wall steel spaced regularly throughout the length of the wall
panel is required to develop shear friction along the entire contact face between the precast components. Boundary elements are detailed to provide
the required tension and compression
resistance as would be analyzed in a
monolithic cast-in-place structure.
...
,
,.. -
....
, ... -·-· .....
···"
······,,.....,,.....
,......... ........ .
,, ... tlfl , ••••
.. . .
....... .......
, ..... .
..................
........
.
······
.....
............••••••
······•
, • .r.. ••
. ........ '
•••••
••••••••••
•••• •• •••••••••
················I
••••••••••••••••
••••••••••••••••
Fig. 6b. Lower portion of Ramon Magsaysay Building, Manila, Philippines.
Fig. 6a. Overall view of
Ramon Magsaysay Building,
Manila, Philippines.
May-June 1991
This 18-story (including basement) reinforced concrete structure utilizes precast,
prestressed concrete joists and composite in-place floor slabs. The structural
system to resist lateral forces due to seismic or wind loads is a shear wall system.
The shear walls are symmetrically clustered about the center of the building, thus
eliminating eccentric forces. This building experienced severe earthquake forces in
1968 and 1972 (Richter Scale 7.2) and in July 1990 (Richter Scale 7.7) without
suffering any structural damage.
45
Fig. 7. Hotel Liwayway, Manila, Philippines. Wide columns and deep spandrel beams make up this shear wall type structure
capable of maintaining rigidity even after cracking under repeated loads. This picture, which shows that the hotel suffered only
nonstructural damage on building's exterior, was taken shortly after the August 1968 earthquake which reached a magnitude
of 7.0 on the Richter Scale.
STRUCTURAL
FRAMING SYSTEMS
In the overall performance of a
structural concrete frame, primary
consideration should be given to the
development of stiffness, strength and
toughness.
In the past, some design engineers
believed that pure moment resistant
frame systems were more desirable
than shear wall systems for high rise
buildings in seismic regions. The justification for this philosophy is that moment resistant frames are flexible and
therefore can move, deflect and absorb
more energy, thus reducing the required calculated base shear forces . It
was also thought that, in the case of
shear wall structures, the base shear
forces would be greatly increased due
to the stiffness of such framing and
therefore would be considered undesirable for seismic areas.
However, our experiences during
46
the past 20 years have shown that flexible building structures that incur excessive deflection or rotation will suffer considerable damage in an earthquake. This damage could result from
repetetive stresses at the joints between framing members, which causes
progressive deterioration in stiffness.
This, in tum, would cause a further increase in deflections and rotations, resulting in considerable damage to nonstructural partitions, facades, utility
lines, building contents and other
building elements - including damage
to the structural frame itself. With progressive joint deterioration, such
building structures can become statically unstable and collapse completely. Increasing the lateral stiffness of
the structure will result in a decrease
in detrimental second order (P-Delta)
effects and thus increase the critical
load capacity and overall stability of
the building.
In the case of shear walls, these ele-
ments produce stiffness and, although
they may crack under repeated loads,
they are able to maintain a large degree of their stiffness without the significant deterioration displayed by
moment frame structures. Earthquakes
in the Philippines, Chile, Mexico and
elsewhere have repeatedly confirmed
the advantages of the stiff shear wall
building frame concept.
Figs. 6a and 6b show a high rise office building in Manila which utilizes
precast, prestressed concrete joists integrated with a cast-in-place structural
topping. Connections of the joists are
similar to that shown in Fig. 2b. The
main lateral resisting element is a central concrete shear wall core which
was cast-in-place. Main vertical reinforcing steel in the shear wall core was
joined by mechanical connections
using a metallic matrix.
In August of 1968, this building experienced a major earthquake with
epicenter immediately east of Manila
PCI JOURNAL
-~·
I
Fig. 8. Cast-in-place reinforced concrete structure with stiff upper floors and a soft moment frame ground floor showed partial
collapse of the flexible floor columns. The photograph on left shows the overall building, while the view on the right shows a
closeup of the column failure.
at a magnitude of 7.0 on the Richter
Scale. The only damage observed was
cracking in the nonstructural concrete
hollow block partitions of the toilet
and stairwell areas . Some loosening of
screws attaching metal partitions in
the toilets were also noticed.
The exterior surface of the building
is clad with marble slabs embedded to
precast panels which in turn are compositely attached to the building frame
by extended reinforcing bars . No
cracks or damage occurred on this facade despite reports from the night security personnel that the building did
undergo substantial accelerations and
movement during the earthquake.
Other nearby buildings suffered severe damage to both structural and
nonstructural elements. In some instances, partial or total collapse occurred, resulting in a loss of more than
300 lives. Virtually all moment frame
buildings of both structural steel or reinforced concrete suffered major
May-June 1991
structural or nonstructural damage
during the earthquake. Nonstructural
elements, such as marble or stone
claddings, nonbearing partitions, ceilings and utility ducts, were badly damaged or collapsed and required extensive repairs.
During this earthquake in Manila, a
hotel building structure (shown in Fig.
7) utilizing wide columns and deep
spandrel beams demonstrated its capability of maintaining stiffnes s even
after cracking under repeated loads.
The inherent stiffness of the framing
system prevented the collapse of the
building.
In another instance, a reinforced
concrete structure with stiff upper
floors and a soft ground floor moment
frame (shown in Fig. 8) suffered a partial collapse due to the flexibility of
the column beam framing system on
the lower floor.
A similar magnitude earthquake occurred in Manila in 1972 and the high
rise office building shown in Fig. 6
again experienced only minor damage
to the nonstructural concrete block
walls.
In the devastating earthquake of
July 16, 1990 (magnitude 7.7 on the
Richter Scale epicenter north of Manila), at least 25 major buildings were
heavily damaged in Manila, with a
loss of 34 lives. Again, this high rise
office building experienced only
minor damage to the nonstructural
concrete block walls in the toilet and
stairwell areas . No other structural
damage was observed.
In 1970, a 38-story building using
precast, prestres sed concrete floor
slabs and precast concrete column-tree
frames was built in Hawaii (see Figs.
9a-9d) . The lateral resisting framing
system comprised a cast-in-place
shear core and precast concrete column-tree frames. In addition to supporting vertical loads, the column-tree
frames functioned as a main lateral re47
UPPER
COLUMN
REINFORCING
BARS
LOWER
COLUMN
SPLICE
SLEEVES
Moana Hotel, Honolulu , Hawaii . This 38-story, 1260 room hotel was
constructed using cast-in-place shear walls with precast concrete moment frames
supporting precast prestressed concrete floor slabs. Built in 1970, today the
building is in very good condition and has required minimal maintenance over the
past 20 years.
Fig . 9c. Details for splicing upper and
lower columns used in building Ala
Moana Hotel, Honolulu , Hawaii.
Fig. 9b. Ala Moana Hotel, Honolulu , Hawaii . Precast moment frame unit being erected.
48
PCI JOURNAL
I I
I I
I I
I I
1 '
I 1
I 1
''
I
I
Fig. 9d. Ala Moana Hotel, Honolulu, Hawaii. Solid precast concrete walls were applied to transverse and longitudinal facade to
minimize drift between floors .
May-June 1991
49
'"""~/
,
tc) COMBINATION
OF (a} & (b)
(b) MOMENT FRAME
'
(a) CANTILEVERED
SHEAR WALL
:::::<
l\111\i
I/
m:ii l~ :::;:;:;
[:/
~::::
~
MOMENT FRAMES
tt
::;::
[:=:: t =:
[ : : :=:::
r/////r:
...
~'?.' ~ ~
-'\
MOMENT FRAMES
~:-:::ARW ALL
(d) SHEAR WALL - MOMENT FRAME COMBINATION
Fig. 10. Schematic diagrams showing the interaction of shear wall and moment frames.
CONCRETE
SHEAR WALL
SEISMIC
FORCE
SOFT
FLOOR
SOFT
FLOOR
AT REST
SEISMIC FORCE
Fig . 11 . Schematic concept of soft floor action.
sisting element in the longitudinal direction of the building while the shear
core primarily resisted the lateral
forces in the transverse direction. This
structure, designed for Seismic Zone
II, experienced seismic forces of magnitude 6.0 on the Richter Scale in
April of 1973 without any visible
damage.
The precast concrete column tree
frames were spliced at midheight with
grouted steel sleeves (see Figs. 9b and
9c). To minimize the drift and torsional effects in the longitudinal and trans50
verse directions, solid precast concrete
panels were inserted at the end walls
and extreme ends of the longitudinal
facade (see Fig. 9d).
In most high rise buildings, shear
walls combined with moment frames
are generally used to resist lateral
forces. As shown in Fig. 10, a pure
shear wall is stiffer at the lower levels
while a moment frame is stiffer at the
upper levels; the combination of these
two elements can result in minimizing
large deflections in both the upper and
lower floor levels.
Fig. 11 illustrates that a soft floor
can be particularly hazardous if it is
located in the lowest floor of a building where maximum lateral loads
could occur under seismic conditions.
An actual example of this type of failure is shown in Fig. 8. However, soft
floors have been known to fail under
seismic conditions even if located on
upper story levels.
If only the lower floors were to be
stiffened with shear walls and then not
continued at the upper levels, abrupt
change in stiffness could result in the
failure of the soft floor at the upper
level where a sudden change of stiffness occurs. Therefore, where practical, both shear walls and moment
frames should run continuously
throughout the entire building height
without allowing such soft floors to
occur.
A structural arrangement utilizing
shear walls connected by deep girders
and columns (where they are permitted architecturally) can create significant additional stiffness in the building
frame.
In Figs. 12a and 12b, a giant moment frame system can be created by
utilizing shear walls in combination
with wall girders that are floor-to-floor
in height. These wall girders could be
used wherever permissible, such as at
PC! JOURNAL
MECHANICAL ROOM
ELEVATOR PENTHOUSE
(a) GIANT MOMENT
FRAME SYSTEM
(b) GIANT MOMENT
FRAME SYSTEM
Fig. 12. Schematic elevations of giant moment frame system.
Ph3
Ph3
-~6_2=( 1 I 4) SEI --++--e:-_3~( 1I 16) SEI
-p
P-iP.. ~:'!'!':!'!'!:~:- . ~. "':
·:.
{::·::
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·Fig. 13. Relative stiffness of shear walls connected with deep wall girders (giant
moment frames).
mechanical room and elevator room
floor levels. As shown in Fig. 13,
when shear walls are coupled together
with deep wall girders the resulting
deflections are minimized considerably .
Fig. 14 shows a coupled shear wall
system utilized in the building shown
in Fig. 6. The vertical shear walls of
the elevator cores are joined together
at the roof penthouse level to enhance
the stiffness of the building.
In Fig. 15, shear walls located at the
end walls of the building are joined together with deep wall girders at the
roof level to form a giant moment
May-June 1991
frame. The deep wall girders at the
roof level are also used to support mechanical equipment rooms.
An effective and cost beneficial lateral shear resi sting framing system
could be attained by the use of super
diagonals as shown in Fig. 16. These
super diagonal s couple the internal
shear walls with the exterior column
frame to generate maximum resistance
and stiffness against lateral and overturning forces. The super diagonal can
extend several floors vertically from
each level in order to generate the
most efficient height to span ratio.
Super diagonals can be used effective-
ly where deep wall girders are inappropriate because of occupant access
problems.
Architectural and functional considerations will determine the most ideal
locations of these super diagonal elements. Generally, the roof area is free
from any functional restraints and
super diagonal frames easily could be
placed in this location to enhance the
lateral stiffness of the building.
Eccentricities .in the framing system
should be avoided wherever possible
regardless of justification by structural
calculations and detailing. Eccentricities generally produce exaggerated
movements that can be damaging. Indeed, many buildings with even moderate eccentricities have suffered total
collapse during earthquakes. Deflections, drift and rotations should be
minimized in every instance since they
are the prime cause of damage and
failure in building frames and nonstructural elements. Furthermore, a
symmetrical building would favor
economical precast concrete construction due to the uniformity in production and erection of the precast concrete components.
Seismic joints (Fig. 17a) between
building unit s s hould be avoided
wherever pos si ble since separate
building units have different periods
of vibration and under seismic activity
tend to pound against each other at the
joint areas. Considerable damage in
seismic joint areas due to pounding of
separate building units was witnessed
in the 1985 Mexico City earthquake
(Fig. 18). In many cases, total building
collapse was directly caused by thi s
pounding.
By eliminating seismic joints and
developing full continuity between
building frame units, pounding can be
prevented. However, in eliminating
seismic joints, floor areas now become
relatively large and the question of
shrinkage effects and floor slab restraints causing problems of slab and
wall cracking is of major concern. In
most instances, these problems can be
solved by the addition of continuous
reinforcing steel in both directions to
handle the shrinkage effects.
To reduce shrinkage stresses, these
large floor areas can also be cast in
smaller sections to establish temporary
51
shrinkage control joints which are then
connected structurally after a long period of curing and drying time during
construction. One method of connecting the smaller cast sections at the
shrinkage control joints is shown in
Fig. 19.
In other instances, however , the
floor area may be so large that seismic
(expansion) joints become absolutely
necessary. In this instance, the width
of the joint should be sufficient to prevent contact between the separate
framing units. However, in the case of
an extended period of seismic activity ,
resonance may build up considerable
movement beyond the magnitude of
the joint separation provided and severe damage may result. Installation
of a tension tie system in combination
with a suitable cushoning material between building units could be considered a partial solution in preventing
potential damage from pounding action (Fig. 20).
In this regard, it is again important
to emphasize the need for structural
stiffness in the separate framing units
since higher stiffness coefficients will
reduce the degree of lateral movement
and thus reduce the difficulty in developing a manageable seismic joint. The
stiffness can be increased substantially
by using shear walls , giant moment
frames or super diagonal systems.
PENTHOUSE
MACHINE ROOMI~
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............
For precast concrete construction in
seismic areas, all joints between precast concrete units should develop full
continuity and toughness. Floor slab
and beam units utilizing composite inplace concrete topping and jointing
can develop horizontal diaphragm action and fail-safe connections to resist
seismic forces .
With the availability of reliable mechanical splicing devices, precast concrete shear wall , slab, column, beam
and girder components can be installed at any convenient location in a
structure to develop full continuity and
monolithic frame action . The structural analysis for precast concrete construction would be identical to that of
a cast-in-place concrete structure.
Stiffness in the overall framing system is necessary to insure against
52
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:
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:
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.I I l l
:
:
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L
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ELEVATOR CORES
CONCLUDING REMAR KS
I I
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l
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:
GI ANT MOMENT
FRAME
::::·
:
~
ELEVATION
Fig. 14. Giant moment frame utilized in Ramon Magsaysay Building, Manila,
Philippines. The structure is 18 stories high (including the basement) .
structural and nonstructural building
damage by minimizing drift, deflection and rotation . To develop framing
stiffness, shear wall elements are extremely important. When these shear
wall panels are integrated with deep
wall girders to produce giant moment
frames , the stiffness in the structure is
greatly enhanced . Super diagonal
frames can also be an effective
method to develop stiffne ss in the
building frame . These frames can be
used where access or functional requirements require more open space in
a floor plan.
In seismic areas, the existence of
soft floors in the framing system can
lead to severe damage or collapse. Eccentricities (non -symmetry) in the
framing system should be avoided, regardless of mathematical justifications
and structural detailing.
Sei smic joints between building
units should be avoided wherever poss ible in order to prevent poundin g
damage. If such joints become necessary due to the overall size of the
structure, tension ties in combination
with a cushioning material can be included in the seismic joint design to
reduce possible damage due to pounding action .
PCI JOURNAL
GIANT MOMENT FRAM E
ELEVATION
moment frames utilized in the Municipal Office Building, City and County of Honolulu, Hawaii.
SUPER
DIAGONAL
--
ft--SEISMIC JOI NT
CORE WALLS
'/
///////,
/////
(a) BUILDING WITH SEISMIC JOINTS
////
'//
'/
'/ '/
'/
/.
(b) BUILDING WITHOUT SEISMIC JOINTS
Fig. 16. Super diagonal frame system.
May-June 1991
Fig. 17. Building with seismic joints (top) and building without
seismic joints (bottom) .
53
Fig. 18. Damage and collapse of buildings due to pounding action at seismic joints during Mexico City earthquake of
September 19, 1985.
54
PCI JOURNAL
REFERENCES
I. Englekirk, Robert E., " Overview of
PCI Workshop on Effective Use of
Precast Concrete for Seismic Resis---~----~o.f--2" TEMPORARY JOINT
tance," PCI JOURNAL, V. 31 , No. 6,
November-December 1986, pp. 48-58.
TUBES FOR
WIRE LATH
2. Englekirk, Robert E., "An Analytical
GROUT INJECTION
"STAY IN PLACE"
Approach to Establishing the Seismic
FORM
Resistance Available in Precast Concrete Frame Structures," PCI JOURNAL, V. 34, No. I, January-February
1989, pp. 92-101.
3. Englekirk, Robert E., "Seismic Design
STEEL SLEEVES AND
Considerations for Precast Concrete
JOINT TO BE GROUTED
Multistory Buildings," PCI JOURAFTER SHRINKAGE
SLABS OR BEAMS
NAL,
V. 35 , No.3 , May-June 1990,
OF SLABS AND
pp. 40-51.
BEAMS TAKE PLACE
4. Fintel, Mark, "Ductile Shear Walls in
Earthquake Re sis tant Multistory
Buildings," ACJ Journal, Proceedings
V. 71 , No.6, June 1974, pp. 296-305 .
5. Fintel, Mark, "Performance of Precast
Fig. 19. Cast-in-place joint detail for shrinkage control.
Concrete Structures During Rumanian
Earthquake of March 4, 1977 ," PCI
JOURNAL, V. 22, No. 2, March-April
1977, pp. 10-15.
6. Fintel, Mark, "Modem Concrete Structures Survive Romanian Earthquake,"
Civil Engineering -ASCE, V. 48, No.
-·+-~-1-~-------SEISMIC JOINT
10, October 1978, pp. 80-81.
7. Fintel, Mark, " Performance of Precast
and Prestressed Concrete in Mexico
,----=-~· -1
I
I
II
Earthquake," PCI JOURNAL, V. 31,
I
I
I
I I
tl
I
No. I, January-February 1986, pp. 18I 1
I II I
I I
I
I II I
42. (See also Discussion in NovemberI I
I
I rl I
I
December 1986 PCI JOURNAL, pp.
I II
II II
143- 144.)
I
I
I II I
8. Hawkins, Neil M., " State of the Art
I
I
I
I II
I
Report on Seismic Resistance of PreI II
stressed and Precast Concrete StrucI I I
TENSION TIES IN
I I
tures," PCI JOURNAL: V. 22, No. 6,
_! I
COMBINATION
I I
November-December 1977, pp. 80WITH A
I'
I
\
I I
110, and V. 23 , No. 1, January-Februi
CUSHIONING MATERIAL
ary
1978, pp. 40-58.
I
TO ABSORB IMPACT
I
9. Hawkins, Neil M., "Precast Concrete
I
I
I I
I
I
I
Connections," Proceedings, U.S.-PRC
1I
I I I I
I
Workshop, 1981 , pp. 28-42.
II
I
I
I
II
I II II I
10. Iverson, James K., " First Impressions
I
II
I I II
I
of Earthquake Damage in San FrancisI
II
I
I
I
I
co Area," PCI JOURNAL, V. 34, No.
I
II
I
I I II
I
I
6 , November-December 1989, pp .
II I
I
I
I
II
108- 124.
I
I
I
I
II.
Scanlon,
Andrew, and Kianoush, Reza
I
I
I
M., " Behavior of Large Panel Precast
I
Coupled Wall Systems Subjected to
Earthquake Loading," PCI JOUR NAL, v.. 33, No. 5, September-October 1988, pp. 124-151.
12. Seckin, M., and Fu, H. C., "Beam-Column Connection s in Precast Reinforc ed Conc rete Construction ," ACI
Structural Journal, V. 87, No. 3, MayJune 1990, pp. 252-261.
Fig. 20. Proposed detail for reducing damage due to pounding action at seismic joints.
t -f----1
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May-June 1991
55