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地震区预制/预应力混凝土结构的设计
Ned M. Cleland,博士,注册工程师
本文回顾了美国预制/预应力混凝土结构抗震设计的需求及背景。虽然预制混凝土
结构的组装和连接
工艺
钢结构制作工艺流程车尿素生产工艺流程自动玻璃钢生产工艺2工艺纪律检查制度q345焊接工艺规程
比钢结构施工更为普及,但预制混凝土工业仍作为钢筋混凝
土的一个分支在发展。这决定了预制混凝土的抗震设计
规定
关于下班后关闭电源的规定党章中关于入党时间的规定公务员考核规定下载规定办法文件下载宁波关于闷顶的规定
来源于现浇混凝土结构
的规定。回顾有关抗震设计规定的来源,有助于正确理解各种抗震等级下预制混凝
土建筑的设计与施工。
Design of Precast/prestressed Concrete for Earthquakes
by Ned M. Cleland, Ph.D., P.E.
In this paper, the background and requirements for the seismic design of
precast/prestressed concrete in the United States are reviewed. Although
precast concrete construction is jointed and assembled with details that
are more common to steel construction, the industry was developed as a
branch of reinforced concrete. This has resulted in the derivation of the
requirements for seismic design from monolithic cast-in-place construction
provisions. A review of seismic design requirements in this context is
important for an understanding of appropriate design and construction of
precast concrete building for all levels of seismic risk.
The history of seismic design precast/prestressed concrete in the United States
The use of precast/prestressed concrete as structural systems in buildings in the
United States has developed since the 1950’s. The greatest use during the early
progress of the industry was in regions of low seismic risk, or regions where
attention to earthquake risk was low. From the beginning, precast concrete has
been considered a branch of the broader field of reinforced concrete construction.
As such, the design rules for precast concrete were largely derived from the
building code developed for monolithic cast-in-place concrete. Although some
rules specific to the jointed and simple span nature of precast concrete were
developed, the dominant context for the concrete code is continuous, monolithic
construction.
In the United States, the authority to regulate construction is largely given to the
individual states, and not to the federal government. In the last half of the 20th
century, building codes were somewhat regional. Three different model codes
were developed and applied. There was also a difference in the philosophy of the
application of these codes. In the eastern region, the lack of a rule for some
feature was often taken as permission to design that feature in any way. In the
west, the lack of a rule for that feature was taken as a prohibition from using the
feature at all. It was in the west where earthquake experience was more common,
and the risk was more recognized. Where design for earthquakes was required,
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solutions for precast concrete were developed which followed the rules intended
for monolithic cast-in-place concrete. This is an approach called emulation.
In regions of low seismic risk, the controlling requirements for lateral forces come
from wind. Except in coastal areas exposed to the high winds from hurricanes,
the wind force required for design can be relatively low. Since concrete is heavy,
stacks of precast concrete components often have sufficient weight to resist the
required wind forces without additional strength from mechanical connections.
This fact, and the permissive application of some building codes, allowed early
precast construction to be constructed with few connections and with little
redundancy in the gravity load paths from roof to foundation.
In 1960, a disastrous accident of a gas explosion on the 18th floor of a 22 storey
apartment building constructed in London, England, with precast concrete floors
and large wall panels caused the entire end of the building to collapse. This
accident showed that precast concrete design requires consideration of the
possibility of abnormal loads and structural integrity, even when the seismic risk
is low. There is additional experience from around the world with poor
performance of precast concrete systems in earthquakes when the structures are
not designed for seismic risk or integrity. This experience has reinforced the
lesson that the design for an appropriate level of seismic risk and for structural
integrity in precast structures is required.
In the early 1990’s , the U.S. federal government took a stronger role in ensuring
that appropriate consideration of seismic risk in building construction would be
applied throughout the country. Building codes that before had only weak seismic
guidelines were updated by 1994 to meet minimum provisions of the National
Earthquake Hazard Reduction Program (NEHRP). Precast concrete construction,
which had largely been ignored in earlier seismic codes, was included directly in
the new recommended provisions, but it took until 2002 for many of these
provisions to be adopted in ACI 318, the code for reinforced concrete.
At the same time the regional codes were adopting improved seismic provisions,
code official were also working to merge these codes in to a single national
standard. In 2000, an organization created from the formerly separate code
groups published a single national code that included strengthened seismic
provisions based on the NEHRP 1997 Recommended Provisions and on the
Uniform Building Code, which had previously been the west coast code with the
most stringent seismic requirements. This code made the time horizon for
seismic risk 2500 years as the basis of system design. Since that first unified
code, two additional updates have been made, in 2003 and 2006.
In early recognition that the requirements in the building codes for seismic design
would increase, the precast/prestressed concrete industry in the United States in
1990 began a program of research focused on the seismic design of precast
concrete based on its unique properties as jointed concrete systems. This
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PRESSS (Precast Seismic Structural Systems) program culminated in 1999 with
the pseudo-dynamic test of a five-story precast concrete building, which
incorporated innovative precast frame and wall systems. Results from this
program have been incorporated into the concrete code by reference to new
standards.
As a result of poor performance of some precast concrete structures in Los
Angeles in 1994 during the Northridge earthquake, additional research was
undertaken by the industry with a focus on the design of the floor diaphragms in
precast concrete buildings. The test phase of this research has recently been
concluded with the shake table test of a half scale three story building
representing floors with both cast-in-place concrete topping slabs over precast
and with bare precast.
Today, most precast concrete structures in the United States have system
designs which are governed by the requirements for earthquakes, considering
both the acceleration risk and the soil conditions at the site. Limitations on
systems that are permitted and the level of seismic detailing required are
determined by the assignment of the building to seismic design categories (SDC).
These, in turn, are determined from the acceleration parameters and the
importance of the structure. Seismic Design Categories A and B are low risk.
Seismic Design Category C is moderate risk, and Seismic Design Categories D,
E and F are high risk. Most of the design in regions of high seismic risk continues
to be based on emulation of monolithic concrete construction, but with some
recognition of the distinctive features of jointed precast construction. In regions of
low and moderate seismic risk, jointed precast with appropriate levels of strength
and ductility and with detailing provisions for structural integrity are common.
Principles of precast concrete
There are some fundamental characteristics of precast/prestressed concrete
construction that promote superior quality and economy. These basic attributes
include the following:
• Precast concrete buildings come more from manufacturing than from
custom construction. The components are made in factories with higher
efficiency, higher quality control, and better safety.
• The work required for precast buildings is off-site more than in the field.
This should be seen in reduced field-applied materials and parts
• The efficiency of manufacturing is promoted by design with greater
repetition in the fabrication process and in the resulting components.
• Repetition is promoted through modular construction. Systems should be
developed with regular dimensions based on the limits of structural
performance, manufacturing capacity, and the plant-to-site delivery
conditions.
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• Standardization of building systems promotes the efficient delivery not
just a single project, but of multiple projects.
• Simplicity is important in all phases of design, manufacturing and
erection. It is important to reduce the number of steps or features that
can be done wrong so that the process can be managed with success.
Some of the ways to promote simplicity include:
o Simple spans. Statically determinant horizontal components
reduce the chances for errors.
o Provide the lateral force-resisting system elements that are
required, but not more.
o Include integrity measures in the standard details
o Allow for ample dimensional tolerance to account for the inevitable
variations permitted by economical processes.
• Limit field assembly operations to only what cannot be accomplished in
the manufacturing plant.
There are more measures that can be applied to improve efficiency, economy
and safety that will depend on the local requirements and environment.
Seismic design principles and their application to precast concrete construction
“Ground motion resulting from earthquakes presents unique challenges to the design of
structures. The forces that a structure must resist in an earthquake result directly from the
distortions caused by the motion of the ground that supports it. The response – magnitude
and distribution of forces and displacements – of a structure resulting from such ground
motion is influenced by the properties of the structure and its foundation, as well as the
character of the exciting motion.”1 The ground motion does not produce a static
load, but develop from the inertia of the structure and its resonance with the
motion. The effect is short in duration, but the magnitude can be large. When a
structure responds elastically to the motion, the resulting accelerations can be
several times the maximum ground acceleration. Most structures need not be
designed for elastic behavior.
Rather than responding elastically to ground motions, structures are designed to
withstand the large deformations by yielding in a predictable and controlled
fashion to dissipate the energy. This behavior requires a level of ductility that is
achieved by detailing. The building code requirements in the United States are
based on matching the level of detailing in the system with the code-prescribed
seismic forces used for the design. The building code requirements are simplified
to achieve performance levels without explicitly requiring analysis and design for
these levels.
Most structures are designed using the method of equivalent lateral force. This
method relies on three basic seismic parameters to achieve acceptable levels of
performance. These parameters are the response modification factor, R, the
deflection amplification factor, Cd, and the system overstrength factor, Ωo. The
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primary design of the seismic force-resisting system is based on the
accelerations expected from 2/3 of the maximum considered earthquake reduced
by the response modification factor, accounting for the natural period of the
building and the ductility provided by the prescribed detailing. This level of force
is generally associated with life-safety, where there are damaged components
but a remaining margin against partial or total collapse. The margin against
collapse is intended to be sufficient so that the performance of collapse
prevention is achieved for the maximum considered earthquake.
The seismic design parameters are not prescribed in the code for reinforced
concrete. They are listed in a table of such values in the design load standard,
ASCE -05, which is a primary reference of the building code. Only recently have
precast concrete systems been recognized separately from monolithic cast-in-
place concrete systems in ASCE 7, and their values have been derived from the
background as a branch of concrete design. It is important to respect the
derivation of the seismic design parameters to ensure that adequate inelastic
behavior is achieved in the design and detailing of precast systems.
The vertical lateral force-resisting systems for concrete, including precast
concrete, with assigned seismic parameters are walls and moment-resisting
frames. Although trusses or braced frames could be constructed using reinforced
or precast concrete, these systems not common in U.S. construction and are not
recognized in the load standard. The use of such systems requires either a
demonstration that the design and detailing is equivalent to another, permitted
system, or a design using nonlinear time-history analysis with an independent
peer review.
Building systems with precast concrete shear walls are often the systems chosen
by designers of precast concrete buildings. Systems which use structural walls
provide inherent stiffness that prevents damage in low and moderate
earthquakes. Since walls are often needed for the common uses of buildings,
systems that integrate them into the lateral support are usually efficient.
The ASCE 7 load standard recognizes precast concrete walls with three levels of
detailing requirements. Ordinary Precast Concrete Shear Walls can be used only
buildings with low seismic risk, and require only detailing for structural integrity.
These integrity requirements include at least two base connections for each wall
rated at a nominal force of 10 kips (44.5 kN) and floor or roof anchorage, based
on site acceleration with a minimum threshold force. These walls are assigned a
response modification factor, R, which is lower by a value of 1 than the
comparable ordinary reinforced concrete shear wall that is cast-in-place. For
moderate seismic risk, precast walls must be Intermediate Precast Shear Walls.
These walls have additional requirements for ductile connections. These walls
are assigned a response modification factor, R, which the same value of the
comparable ordinary reinforced concrete shear wall that is cast-in-place. They
may be used in buildings with low seismic risk to reduce the design force
required. In buildings classified with high seismic risk, shear walls must be
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special reinforced concrete shear walls. In this case, the load standard does not
distinguish between monolithic cast-in-place walls and precast walls. The
concrete code requires that when these walls are precast, they must meet the
detailing requirements for cast-in-place walls in addition to the ductile detailing
requirements applied to Intermediate Precast Concrete Shear Walls. Detailing for
these walls may include boundary elements with special confinement
reinforcement at close spacing that prevents bucking of the main flexural
reinforcing after yielding under cyclical loading. For special precast concrete
shear walls that do not emulate monolithic cast-in-place shear walls, there is
acceptance of walls which use unbonded post-tensioning tendons in accordance
with ACI ITG-5.1, which is the reference standard based on the walls tested
during the PRESSS research.
For moment-resisting frames, the ASCE 7 load standard does not make a
distinction between monolithic cast-in-place frames and precast frames and
assigns the systems seismic parameters based on the level of detailing for
inelastic behavior. The definitions in the ACI 318 concrete code define these
frames and point to the appropriate requirements for detailing. Ordinary moment
frames are required only to comply with the non-seismic provisions of the code,
except that the latest code adds a requirement for bottom steel continuity for
ordinary frames in buildings assigned to SDC B. These frames are assigned a
response modification factor of 3.
ACI 318 defines an intermediate moment frame as a cast-in-place concrete
frame complying with specific requirements that provide a higher level of ductility.
They are assigned a response modification factor of 5. Although not recognized
for precast systems, intermediate moment frames could be developed by strict
emulation of the comparable cast-in-place system. Since this requires frames
that are detailed as essentially monolithic, it is likely that increasing the detailing
to conform to special moment frame requirements to achieve the higher
response modification factor may be more practical.
ACI 318 defines a special moment frame as cast-in-place or precast concrete
with reference to the detailing requirements. For precast special moment frames,
these requirements include provisions for strong and ductile connections, in
recognition that these frames will not be monolithic. ACI 318 also recognizes
special moment frames that are not monolithic by reference to ACI 374.1. This is
the standard that provides for frames with unbonded post-tensioned tendons that
was developed from the PRESS research.
In addition to the vertical elements of the lateral force-resisting system, there are
requirements for the floor diaphragms both in ASCE 7 and in ACI 318. Although it
is an inherent assumption in the codes that the diaphragm should continue to
provide adequate strength and stiffness to mobilize the inelastic behavior of the
vertical systems, the current code provisions do not ensure this result. Although
certain key elements in the load path between collectors and the lateral elements
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are subject to higher design forces through the application of the system
overstrength factor to design forces, there are areas in the diaphragm subject to
high strain demand that must be protected with additional strength and ductility.
The Precast/Prestressed Concrete Institute, through the publication of the 6th
Edition of the Industry Design Handbook and through the Seismic Design Manual,
has provided interim guidelines for precast concrete diaphragm design which are
more stringent than the codes. These have come from research done
subsequent to the 1994 Northridge earthquake and a consensus determined by
the research and seismic committees in PCI. Research is currently underway to
provide a comprehensive design framework for precast diaphragms.
With precast concrete buildings, it is also important to make adequate
consideration in detailing for elements which are not part of the lateral force-
resisting system. Because the simple span and jointed nature of precast
construction, redundancy and integrity is not a natural outcome of the design for
code-prescribed gravity or lateral loads. These features must be designed into
the buildings. Code provisions for minimum levels of structural integrity meet
much of this requirement. In addition, the designer must consider the deformation
capacity of the assembly. Columns and walls must be able to tolerate the lateral
displacements anticipated from the post-yield displacement of the lateral force-
resisting system, determined from the displacement amplification factor, Cd, and
from flexibility in the diaphragm. It is also important to consider stiffness from
unintended load paths. Configurations which shorten the vertical spans of
columns or walls between floors may draw forces which exceed the shear
strength of those elements. Care in detailing is required to avoid or relieve such
conditions.
Design example
The applications of seismic design principles to precast/prestressed concrete
structures may be best shown by example. A building design