Refined 3D finite element modeling of partially restrained connections including slip

The effect of partially-restrained (PR) connections on the behavior of steel frames
and their potential economical benefits is well recognized [18]. However, many structural
analysis and design approaches still consider connections as either fixed or
pinned. This assumption is mainly due to convenience and the lack of common
analysis and design approaches that address PR connections. Despite many full-scale
experimental studies that have been conducted to date, there is still a need for a
better understanding of the mechanisms that effect the non-linear behavior of PR
connections [8].
Non-linear moment rotation response of connections was recognized in the early
1930s. Standardized functions have been developed starting from basic linear and
bilinear approximations to more sophisticated models based on polynomials, cubic
B-splines and power functions fitted to available experimental data. Sherbourne and
Bahaari [13] have recently presented a review of these functions. Frye and Morris
[9] were among the first to incorporate these standardized moment–rotation functions
in steel plane frame analysis to investigate the effect of the connections on the
frame behavior.
Moment rotation functions can be useful for designers in practice. These usually
include small number of parameters taken into account from limited test data. The
lack of a large and parametrized experimental database does not allow for generating
standardized functions. Thus, there is a need to be able to analytically generate a
reliable moment–rotation response of PR connections that can be used in analysis
and design.
Non-linear finite elements are an attractive tool for modeling connections. Early
attempts to use finite elements for analysis of PR connections was by Krishnamurthy
[11]. As in many early studies using finite elements, many simplifications are made
due to the limitations of computational power. More recent studies using finite
elements in modeling connections have focused on end plate connections
[6,7,10,13,14]. In these studies 2D and 3D models are used with various simplifications
in the geometry of members, the bolts, and contact conditions. The effect
of friction on the response of end plate connections is usually neglected in these
models [7,10].
Azizinamini [3–5] preformed an extensive and detailed experimental study for top
and bottom seat angle connections with double web angles along with pull tests. In
addition, simplified 3D FE models for the pull tests are also studied. One quarter of
the top angle in the connection is modeled with 3D elements to simulate a pull test.
The force–displacement relation was converted to a moment–rotation relation in
order to examine the role of the top angle on the behavior of the connection and
approximate the overall response of the connection with a pull test. Different assumptions
and simplifications are made in order to avoid detailed modeling and reduce
the computational effort.
Yang et al. [19] consider a double web angle connection where the angles are
bolted to the column flanges and welded to the beam web. The bolts and angle are
modeled using 3D finite elements and wedge elements are used to model the weld
region. Contact is included between the bolt head and angle. However, the contact
between the bolt shank and hole is ignored.
In the studies on end plate connections and the double web angle connection, the
bolts are transferring the loads axially, thus eliminating the need for combined contact
and friction modeling between the bolts and members. These models are therefore
limited to these types of PR connections. The bolted connections tested by
Azizinamini et al. [4,5] are investigated in this study. These top–bottom bolted seat
angle connections transfer the forces by friction by clamping the parts together with
the bolts. Modeling such a mechanism requires the inclusion of contact and slip
between the connection members.
In this study, a refined 3D modeling of PR bolted connections are performed
recognizing contact and friction effects. The modeling approach is general and capable
of modeling various types of geometries of PR connections by using parametric
meshing techniques. Therefore the time of generating detailed 3D geometries is
almost eliminated. A calibration method for the pretension of the bolts is presented.
In this method, parametric solutions are first generated separately for a single bolt
clamping semi-infinite plates. These solutions are used to specify initial pretension
values for the bolts in the full connection. The correct pretension values are then
examined and corrected in the full connection model to achieve accurate final values.
It is shown in this study that the response of the bolted PR connections are sensitive
to the pretension of the bolts, thus correctly modeling the pretension and slip is
important.
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A constitutive model of concrete confined by steel reinforcements and steel jackets

Many strong earthquakes, such as the 1990 Luzon earthquake
(Philippines), the 1994 Northridge earthquake (USA),
the 1995 Kobe earthquake (Japan), and the 1999 Ji-Ji earthquake
(Taiwan), have occurred in regions of high seismicity
in the last decade. These earthquakes resulted in sgnificant
loss of life and property and caused infrastructure damage.
The columns are the most important structural members in a
structure, and the strength and ductility of columns significantly
influence the seismic capacity of a structure. Therefore,
the seismic retrofit of a column has become a very
important issue in countries subject to earthquake activity.
In the 1990s, the steel jacketing technique was developed
and experimentally verified to be effective in enhancing the
seismic capacity of columns. Therefore, the steel jacketing
technique has been widely applied in practical construction,
particularly in Japan, Taiwan, and the state of California in
the United States.
The steel jacketing technique was originally developed for
circular-sectioned bridge columns. Two semicircular steel
plates larger than the diameter of the column are formed in
the factory. The vertical seams between both half steel shells
are welded in situ to become a continuous steel tube with a
small annular gap between the bridge column and the steel
plate. The gap is filled with pure cement or epoxy matrix to
transfer the stress in the bridge column to the steel plates
(Priestley et al. 1996). Therefore, concrete confined by a
steel jacket can be seen as that confined by continuous lateral
steel reinforcement. Steel jacketing has proven to be effective
because none of the bridges retrofitted with a steel
jacket suffered damage during the 1994 Northridge earthquake
(CALTRANS 1994). In Taiwan, the steel jacketing
technique has also become a very popular seismic retrofit
technique after the 1999 Ji-Ji earthquake.
The steel jacket mounted around the column can increase
the compressive strength, shear strength, and ductility of the
column. By doing so, the constitutive behavior of the concrete
is changed due to the increase in the confinement stress
of the concrete (Moehle 1992; Priestley and Seible 1991;
Priestley et al. 1996). Therefore, it is necessary to develop a
suitable constitutive model for concrete confined by a steel
jacket in the structural analysis and also the retrofit design.
In this paper, a constitutive model of concrete confined by
both steel reinforcement and a steel jacket in the use of
retrofitting and strengthening reinforced concrete structures
is proposed, and test results are also recorded from 60 concrete
cylinders, 30 cm in diameter and 60 cm in length, confined
by steel jackets of different thicknesses and different
types of lateral steel reinforcement. The stress–strain curves
of the test results are compared with that of the proposed constitutive
model to show that the proposed model is effective.
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Beam element verification for 3D elastic steel frame analysis

In the past decade, there has been more widespread
use of 3D frame analysis programs in civil engineering
design offices to determine the buckling loads and the
member forces of steel framed structures. In most cases,
the use of 3D analysis has been necessitated by the
topology of the designed structure that does not permit
the use of 2D analysis, such as in the case of a sports
stadium. More recently, however, 3D frame analyses
have also been carried out on multi-storey multi-bay
rectangular frames such as high-rise storage rack frames.
The fact that this type of steel structure is generally
composed of open sections rather than tubular sections,
the latter normally used in space roof trusses and offshore
structures, has important implications for the
frame stability that are not generally well understood by
practising engineers. In design practice, either linear
buckling analysis or second-order elastic analysis is
performed to assess the frame stability.
The elastic buckling behaviour and the second-order
effects due to geometric nonlinearity of steel plane
frames are well understood and well documented in the
literature [1–4]. Commercial frame analysis programs
that can handle most or all of these two stability aspects
of planar (2D) steel structures have also been available
for many years. For the purpose of verifying a 2D beam
element or a 2D frame analysis program, there are many
well established and well defined benchmark examples
[5–7]. However, neither situation is true for 3D beam
elements or 3D frame analysis programs. Although 3D
linear elastic analysis is a fairly straightforward extension
of 2D analysis, at the member level there may be 3D
couplings between axial, flexural and torsional deformation
modes that control the buckling behaviour of
open sections. The comment of Springfield [8] that few
commercial frame analysis/design programs could deal
with out-of-plane buckling of beams or beam-columns
by other than empirical means is still largely true today,
except for the more expensive general-purpose finite
element analysis packages such as ADINA [9] and
ABAQUS [10].
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Full scale testing of space steel frame subjected to proportional loads

The second-order inelastic analysis enables designers
to directly evaluate the ultimate strength and behavior
of structural system. The direct use of second-order
inelastic analysis without member capacity checks is
expected to be allowed in future design codes. Over the
past 30 years, researchers have developed and validated
various methods of performing second-order inelastic
analysis on steel frames. Most of these studies can be
categorized into one of two types: sophisticated and sim-
plified second-order inelastic analysis. The sophisticated
analysis (plastic-zone analysis) uses the highest refine-
ment and is considered accurate [1,2]. However, this
analysis is not intended to be used in daily engineering
practice, because it is too costly and intensive in compu-
tation. The simplified analysis for practical design uses
the concentrated plastic hinge [3–7]. This analysis must
be verified by calibrating with plastic-zone analysis. The
plastic zone analysis also requires an experimental veri-
fication in order to confirm its validity, since experi-
mental results provide actual behavior and strength of
structures. Therefore, a realistic simulation such as full-
scale frame testing is quite necessary.
Two-dimensional two-bay full-size frames were tested
by Kanchanalai to verify the plastic-zone analysis [8].
All frames were bent with respect to the week axis in
order to avoid out-of-plane buckling. Two-dimensional
full-size frames were tested by Yarimci at Lehigh Uni-
versity [9]. The frames were sandwiched and supported
laterally by two parallel auxiliary frames preventing out-
of-plane buckling. All members were bent in strong axis.
A series of four tests were conducted by Avery and Mah-
endran [10]. Each of the four frames could be classified
as a two-dimensional, single-bay, single-story, full-scale
sway frame with full lateral restraint and rigid joints.
Two-series of tests were conducted by Wakabayashi and
Matsui for a two-dimensional one-story frame and a two-
story frame [11]. To prevent out-of-plane buckling, two
of the same specimens were set in parallel and connected
at the joints and at the mid-length of the members. Harri-
son tested the equilateral triangular space frame [12]. A
horizontal load (H) was applied on the top of the column
and a vertical load of 1.3H was applied at mid span of
the beam.
Although a number of large-scale frame tests have
been conducted in the past 30 years, the majority of
those are of only two-dimensional frames. Two-dimen-
sional frames are not a realistic representative model of
the behavior of real structures. The aim of this paper is
to conduct three-dimensional full-scale frame testing.
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Engineering Journal '93

The increasing use and reliance on probability based limit
states design methods, such as the recently adopted AISC
LRFD Specification,
1
has focused new attention on the prob-
lems of serviceability in steel buildings. These methods,
along with the development of higher-strength steels and
concretes and the use of lighter and less rigid building mate-
rials, have led to more flexible and lightly damped structures
than ever before, making serviceability problems more
prevalent.
The purpose of this paper is to focus attention on two
important serviceability limit states under wind loads;
namely, deformation (including deflection, curvature, and
drift) and motion perception (acceleration). These issues are
particularly important for tall and/or slender steel and com-
posite structures. A brief review of available information on
these subjects will be presented followed by a discussion of
current standards of practice, particularly in the United States.
Finally, proposed standards will be presented that, hopefully,
will focus attention, debate, and perhaps new research efforts
on these very important issues in design.
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Engineering Journal '92

The use of substandard and mismatched bolts continues to
be a major concern to bridge owners in the United States.
Based on FHWA-sponsored research at the University of
Texas, supplemental specifications were developed and issued
modifying fastener manufacturing, testing, and installation
procedures.
Nearly all bridge bolts are designed for dynamic loading.
They are designed to resist either tension forces and/or shear
forces. Fatigue concerns govern bolts designed for cyclic ten-
sion forces. Cyclic shear forces require slip critical connec-
tions. Both loading conditions require bolts to be installed
to a minimum preload.
The FHWA recommendations were developed in order to
assure the ability of bolts to achieve this preload. Minimum
nut strength is increased, maximum bolt strength is reduced,
thread fit tolerance is reduced, additional rotational-capacity
testing is required, and additional testing, documentation,
handling and shipping requirements are imposed. The ration-
ale for these new FHWA provisions are discussed.
Finally, slip critical joints depend upon friction between
faying surfaces to develop strength. Values of slip resistance
or coefficient of friction for various paints and coatings must
be determined by testing. Bolt design parameters depend
upon minimum values of tested coatings.
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Buckling of Prestressed Steel Girders 2

Prestressing of steel girders, in order to gain economy of
material, is starting to become popular in the United States.
An inherent danger in the stressing operation is loss of
stability of the girder between the points of attachment of the
tendon. The paper presents design charts for the elastic
buckling load induced by stressing an eccentric tendon, and
uses this to obtain a design buckling strength in accordance
with the LRFD Specification.
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Buckling of Prestressed Steel Girders

Prestressing of steel plate girders may lead to substantial
economies of material. Densford et al. quote savings of 30%
in steel tonnage and 27% in concrete tonnage gained by
prestressing a short composite steel-concrete bridge that was
designed by the Oklahoma Department of Transportation.
Similar savings may be obtained for steel girders. Although
the use of prestressed steel girders is relatively new in the
United States, the technology is well-established in eastern
Europe.Perhaps the easiest way to prestress a steel girder is to
use straight high-strength rods, which are anchored at the
ends of the beam as shown in Fig. 1, and stressed in a manner
analogous to that for prestressing concrete beams. These
"hard" anchorages may have a number of "soft" anchorages
between them, allowing relative movement of the tendons.
Other methods of prestressing steel girders are discussed in
Ref. 1. Stressing the high-strength rods induces substantial
compressive stresses in the bottom flange of the beam before
the external loads are applied, and raises the question of the
stability of the girder under this loading. If the attachment of
the tendon to the web is at large spacing intervals, then the
girder may buckle between the points of attachment in an
overall or lateral mode.
The designer must be certain that the
prestressing force is not large enough to cause buckling of the
girder. The use of LRFD design methods and the method in
this paper may be used to calculate the buckling strength.
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The Reinforcement of Steel Columns

There may be a need for a steel column to have load-
carrying capacity additional to that planned in the original
design. The column may be already in place and the
reinforcement may need to be carried out under load or with
the load temporarily relieved.
Columns may be reinforced by the addition of material in
the form of cover plates, or by changing the residual stress
distribution to a more favorable one, or by a method that
combines both of these effects. The effect of the addition of
material is obvious, and warrants no further consideration
here. This paper is concerned with those cases where welding
is used for the reinforcement, either alone or with cover
plates. The discussion is limited to rolled wide-flange shapes
as the shapes to be reinforced, and the loads are restricted to
static loads.
Reinforcement is usually understood to be the welding of
cover plates to the flange of the shape (Fig. 1). Figure 2
indicates the reinforcement of a shape by the laying of a weld
bead on the flange tip, which may be the only option
available in some conditions and which improves column
strength by changing the residual stress distribution.
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End Plate Moment Connections Their Use and Misuse

Up until about 1960, many field-bolted moment connections
were of the split tee-stub type (Fig. 1). These connections
were used in power house construction, in multistory office
buildings and other structures where moment capacity was
required between beams and columns. They served well, and
thousands are still doing their duty reliably in some of our
older structures. In later years, end plates (Fig. 2) have been.
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A Fresh Look at Bolted End Plate Behavior and Design

End-plate connections of the typical configuration shown in
Fig. 1 are increasingly used as moment-resistant connections
in framed structures. However, end plates designed by the
prying force formulas in the AISC Manual of Steel
Construction may be unrealistically thick. The prying force
formulas were proposed by Nair et al.,
based on their work on tee hangers. Previously,
Douty and McGuire
and later Agerskov4,5 have presented other versions
of the same basic
model, and/or suggested adjusted coefficients to reflect test
results. The research in the U.S.A. and abroad on this topic
has been summarized by Fisher and Struik.
In the prying force method, the end-plate region around
the beam tension flange is considered analogous to a tee
hanger, as in Fig. 2. Hence, the terms "tee flange" and
"plate" or "end plate" will be used interchangeably in this
paper; "tee stem" will likewise correspond to the "beam
flange". Figure 3 illustrates the dimensions and forces
involved in the application of the prying force method. The
section at or near the face of the tee stem at which the
applied force is transferred to the tee flange will be
designated the "load line", L. (All the notation used in this
paper is listed in Appendix A.)
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Bolt Tension Control with a Direct Tension Indicator

ACHIEVING THE MINIMUM required bolt tension in a high
strength bolted friction-type joint is a primary factor, since
the slip resistance of the joint is dependent on the bolt
tension. Current specifications
require that high strength
structural bolts be tightened to at least 70 percent of the
minimum required tensile strength.
For the past two decades installation has been primarily
controlled by either turn-of-nut or calibrated wrench
tightening. The turn-of-nut method depends upon strain
control as contrasted to the torque control of the calibrated
wrench method.
Since the turn-of-nut method is primarily strain control,
the effectiveness of the method depends on the starting
point and accuracy of the rotational measurements.
The variability of torque control is well known and was one of
the reasons for the development of. the turn-of-nut method.
Bolts tightened by the turn-of-nut method may have the
outer face of the nut match-marked with the protruding bolt
point before final tightening, so that an inspector can
visually note the nut rotation. If an impact wrench is used
for tightening, a slight peening of the bolt heads or nuts
gives an additional indication that the bolt has been
tightened. However, if the need for more inspection of bolt
tension is necessary, one must use a calibrated inspection
torque wrench with all its uncertainties.
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Use of Ultrasonic Testing in the Structural Steel Industry

IT IS IMPORTANT to note the distinction between a quality
weld and a reliable weld. Many of the discontinuities which
will be discussed may not be considered defective to a
particular weld joint or structural member. The
discontinuities may not make the weld less reliable, but
merely lower the quality of the weld. When does a
discontinuity become a defect? This is the question which one
must ask when separating quality from reliability. The
quality of a weld can be determined with ultrasonic tests;
however, the reliability of a weld is established from
destructive testing or welding history. Test specifications are
available based on these tests.
It should be mentioned, before proceeding further, that
radiography (still a primary weld inspection method),
magnetic particle, and penetrant techniques are powerful
inspection tools, and should not be overlooked when selecting
NDT methods. It is interesting to note, for example, that a
discontinuity orientated for maximum radiographic detection
is in its poorest position for ultrasonic detection, and vice-
versa.
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High Strength Bolting

JUST TWENTY YEARS ago Professor Wilbur Wilson, a
Professor of Structural Engineering at the University of
Illinois, played a major role in the founding of the Research
Council on Riveted and Bolted Structural Joints, the group
largely responsible for high-strength bolting as we know it
today. Since then, thousands of tests have been conducted on
high-strength bolts and bolted connections, specifications for
high-strength bolting have been issued by the Council, and
many millions of high-strength bolts have been used in
bridges and buildings.
The high-strength bolt rapidly took its place in the
structural field and has now become the "workhorse" fastener
for steel structures. Although this fastener has proven to be
extremely effective, occasionally trouble has been
encountered and questions have been raised concerning high-
strength bolting. Whenever such trouble has been
encountered, it has almost always been as the result of a lack
of understanding or improper application of the Council's
specification. To help counteract this problem, an attempt
will be made herein to provide some of the "whys" and
"wherefores" of high-strength bolting. Some of the questions
that are raised regarding high-strength bolting will be
examined, consideration will be given to some of the changes
that were introduced in the September, 1966 revision of the
Research Council's specifications for high-strength bolting,
and finally, some of the possible future changes and new
applications of high-strength bolting will be discussed.
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Small Scale Models for Steel Frameworks

THE ANALYSIS AND DESIGN of steel structures has become
increasingly sophisticated and efficient in the last decade.
For conventional structures, present methods of analysis can
be employed with confidence with the proportioning of
members based upon some prescribed criteria, such as
building and bridge construction codes. When these methods
can be employed, there can be no economic justification for a
model study of a structure. Some aspects of both member and
whole structure behavior, however, are still not well
understood for these conventional structures and as new
design criteria are developed for these situations, testing
programs will be required. For unconventional structures,
mathematical analyses that appropriately account for the
complex geometries and behavior may not be available. It is
in these areas, the development of design criteria for
conventional structures and the study of unconventional
structures, that model analysis can be useful.
The use of experimental methods has already been
demonstrated in tests carried out on full scale sections at
Lehigh University and other schools. These tests have
effectively contributed to the incorporation of plastic design
into building codes. On the other hand, full scale tests have a
number of limitations. To date tests have been restricted to
simple members and simple one, two or three story plane
frames. While more complex structures, in many cases,
would have masked the phenomenon under study there are
situations where tests on larger or more complete structures
would have been appropriate. Even in those situations where
full scale study is feasible, the same total investment in small
scale study would often permit a more comprehensive
investigation.
The purpose of this paper is to present the results of the
first phase of a study aimed at establishing a reliable small
scale ultimate strength modeling technique for wide-flange
steel frameworks. This paper will cover the
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Notes on Design Of Steel Parking Structures Including Seismic Effects 2001

The need for multi-story parking structures has grown considerably over the years and will
continue to grow as metropolitan densities increase. There are several key issues, which need to
be addressed in the design of multi-story parking structures. They are:
1. Site considerations, environmental and neighborhood impacts and traffic access
2. Number of parking spaces, car circulations, ramps and other architectural aspects
3. Security and safety
4. Structural aspects (particularly in highly seismic areas, seismic design aspects)
5. Cost and speed of construction
6. Life cycle cost of maintenance
7. Fire resistance and/or need for fireproofing.
The first three items in the above list, to great extent, are impacted by the decisions of
architects. Items 4 to 6 in above list, also are impacted by architectural aspects, however, these
three items are primarily impacted by the structural design and decisions made by the structural
engineers. Today, structural steel provides viable systems that address the above key issues. In
the past, a large percentage of parking structures throughout the country were designed and built
using reinforced concrete structures. However, since 1980’s in many regions of the US including
seismic areas such as California, more and more steel parking structures have been designed and
built. According to Emile Troup (1989), nearly three out of every five car parks for which
contracts were awarded in 1987 in New England were steel. He attributes this increase in use of
steel structures in open parking structures to the fact that as a result of research and testing done
in 1970’s the issue of fire-proofing of steel structures in car parks was put to rest and the use of
“unprotected steel“ in parking structures was accepted (Troup, 1989). Because of extensive
research and testing of bare steel structures subjected to fire, the fire codes no longer have very
stringent requirement for fire protection of steel car parking structures. This development, along
with education and dissemination of information on viability and economy of using steel structures
in car parking, may have been instrumental in visible increase in design and construction of steel
parking structures.
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Design of Small Base Plates for Wide Flange Columns 90

The 9th Edition• of the AISC Manual of Steel Construction
uses the Murray-Stockwell2 method for analysis of
small base plates, i.e., plates that are only slightly larger than
the column depth d and width bf. It combines this method
with the cantilever method of the 8th3 and earlier editions
for large base plates. The Murray-Stockwell method assumes
a bearing pressure of Ft,, the maximum permitted, over an
H-shaped contact area under the column cross-section
between the plate and the concrete. The cantilever method,
on the other hand, assumes a uniform bearing pressure, fp
< Fp, over the entire base plate surface of area BxN
(Fig. 1). Thus, the two methods assume very different bearing
pressure distributions and are difficult to combine into
a single method.
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SLOTTED BOLTED CONNECTION ENERGY DISSIPATERS 92

Various types of energy dissipating devices, utilizing friction as means of energy dissipation,
have been tested and studied by researchers [4, 6, 7]. Two of the common features of these
devices have been that their manufacture requires precision work or exotic materials and
that their installation demands specialized training. Consequently, the additional expense
in using such devices has prevented their wide acceptance in engineering practice. The
development of the Slotted Bolted Connections (SBCs) as energy dissipators represents an
attempt to overcome the abovementioned shortcomings of these systems. SBCs, as presented
in this paper, require only slight modification of standard construction practice, and require
materials that are widely available commercially.
In this paper a Slotted Bolted Connection (SBC), see Figure 1, refers to a bolted connection
where the elongated holes or slots in the main connecting plate, in which the bolts are
seated, are parallel to the line of loading. In addition a Belleville washer [8] is placed under
the nut. Two types of SBC specimen are discussed in this paper, one with brass insert plates
and one without. Upon tightening of the bolts, the main plate is "sandwiched" directly
between either the brass insert plates or the outer steel plates. The holes in the brass insert
plates and in the steel outer plates are of standard size. When the tensile or compressive
force applied to the connection exceeds the frictional forces developed between the frictional
surfaces, the main plate slips relative to either the brass insert plates in the case of the first
type specimen or the outer steel plates in the case of the second. This process is repeated
with slip in the opposite direction upon reversal of the direction of force application. Energy
is dissipated by means of friction between the sliding surfaces. Application of cyclic loads of
magnitude greater than the slip force results in approximately rectangular hysteresis loops.
The earliest investigations of SBCs as energy dissipators date back to 1976 when a series of
experiments were carried out at San Jose State University (SJSU) [1] on specimens similar
in concept to those presented here. The term SBC used here is adopted from the report
by T. F. Fitzgerald, et al. [3]. A number of other researchers have also investigated similar
devices [2, 5].
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Seismic Design of Steel Column Tree Moment Resisting Frames 97

One of the most common types of steel structural system is the moment resisting framing
system shown in Figure 1.1. Depending on their ductility, steel moment resisting frames
are divided into two categories of "Special" and "Ordinary". Figure 1.2 shows typical
behavior of Special and Ordinary moment-resisting frames under lateral load. Special
moment-resisting frames are designed to have higher ductility and be able to deform
inelasticly during earthquakes. Such inelastic ductile deformations increases damping and
reduces stiffness of the structure resulting in smaller seismic forces generated in the
structure. As a result, current codes allow special moment resisting frames to be designed
for smaller seismic forces than similar but ordinary moment frames.
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Seismic Design of Bolted Steel Moment Resisting Frames 95

Moment-resisting frames (MRFs) are structures that resist applied forces
primarily by bending of their members and connections. MRFs can provide large
open spaces without the obstruction usually caused by braces or shear walls. In
addition, because of their flexibility and relatively long period of vibration, MRFs
usually attract smaller seismic forces than the comparable braced or shear wall
systems.
Since the early days of riveting, steel MRFs have been very popular in
building construction. Many structures including the monumental high-rises of
the late nineteen and early twentieth centuries have been built using riveted steel
MRFs. On the west coast, many turn-of-the-century tall buildings in San
Francisco have riveted steel MRFs. Since the 1960's, with the advent of highstrength
bolting as well as welding technologies, bolted steel moment-resisting
frames (BMRFs) and welded steel moment-resisting frames (WMRFs) have been
one of the main structural systems used in office and residential buildings.
In recent years because of ease of fabrication and design and for
economical reasons, most of the steel moment-res]sting frames used in seismic
areas such as California have had welded moment connections. However,
welded steel moment-resisting frames are only one of the many possibilities of
steel moment frames.
The main purpose of this report is to present information on the seismic
design of steel rigid moment-resisting frames with bolted or bolted/welded
connections. Today, there is sufficient information and experience that bolted
and bolted/welded steel moment-resisting frames can be designed and
fabricated to provide safe and economical structural systems for seismic regions.
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