Document Text Contents
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12 LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
7.2-STEEL TWIN I-GIRDER AND SINGLE TUB
GIRDER SYSTEMS
7.2.1-General
F or potentially torsionally flexible systems such as
twin I-girder and single tub girder structural systems,
the designer shall consider:
• The out-of-plane stiffness of twin I-girders, prior to
becoming composite with a concrete deck, can be
significantly smaller than the in-plane, or vertical,
stiffness. This can lead to a lateral-torsional
buckling instability during construction.
• Single tub girders, prior to becoming composite
with a concrete deck, behave as singly symmetric
sections with a shear center below the bottom
flange. AASHTO LRFD Article 6.7.5.3 requires top
lateral bracing in tub section members to prevent
lateral torsional buckling of these sections.
• Prior to becoming composite with a concrete deck,
twin I-girders with bottom flange bracing will
behave as a tub girder and will exhibit the same
tendencies toward lateral-torsional buckling. Top
lateral bracing shall be provided as for tub sections,
or the stability shall be checked as a singly
symmetric member.
7.2.2-Lateral-Torsional Buckling Resistance-
Twin I-Girder
For evaluating the stability of twin I-girder systems
without a composite deck or lateral bracing, the
equation given by Yura and Widianto (2005) may be
used:
(7.2.2-1)
where:
Mn = nominal in-plane flexural resistance of one
girder (kip-in.)
Mer = critical elastic lateral-torsional buckling
moment of one girder (kip-in.)
s spacing between girders (in.)
E modulus of elasticity of steel (ksi)
L effective buckling length for lateral-torsional
buckling (ft)
C7.2.1
Several incidents have highlighted the need for a
careful evaluation of the stability of pedestrian bridges,
especially during the construction stages. Structural
systems consisting of two parallel girders can exhibit
very different behavior during construction, depending
on the bracing systems used. Lateral bracing contributes
significantly to the lateral-torsional buckling capacity of
the beam. For girders without lateral bracing during
construction, lateral-torsional buckling capacity should
be carefully evaluated. After the deck is cast, the section
is effectively a "C" shape, which is singly symmetrical.
Use of the appropriate lateral-torsional buckling equation
is critical, and reference should be made to Galambos
(1998). Further information is contained in Y ura and
Widianto (2005), as well as Kozy and Tunstall (2007).
Page 21
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES 13
Iyo =
Ixo =
out-of-plane moment of inertia of one girder
(in.4)
in-plane moment of inertia of one girder (in.4)
in-plane plastic moment of one girder (kip-in.)
Where a concrete deck is used, continuous twin I-girder
systems shall be made composite with the deck for the
entire length of the bridge.
7.2.3-Lateral-Torsional Buckling Resistance-
Singly Symmetric Sections
The lateral-torsional stability of singly symmetric
sections not covered in Article 7.2.2 shall be
investigated usmg information available m the
literature.
8-TYPE SPECIFIC PROVISIONS
S.l-ARCHES
Arches shall be designed in accordance with the
provisions of AASHTO LRFD with guidance from
Nettleton (1977).
S.2-STEEL HSS MEMBERS
S.2.I-General
The capacities or resistances of connections for steel
HSS members shall be in accordance with the Chapter
K of the specifications and commentary of AISC (2005)
or AASHTO Signs. Resistances for fatigue design shall
be in accordance with Section 2.20.6 of Structural
Welding Code-Steel ANSI! A WS D 1.1 or Section 11 of
AASHTO Signs. All loads, load factors, and resistance
factors shall be as specified by AASHTO LRFD and
these Guide Specifications. For member design other
than connections:
• Flexure resistance of steel HSS members shall be
according to AASHTO LRFD Article 6.12 as box
sections.
• Shear resistance of steel HSS members shall be
according to AASHTO LRFD Article 6.11.9 as box
sections.
C7.2.3
Equations for the determination of the lateral-torsional
buckling moment in singly symmetric sections are given
in the Guide to Stability Design Criteria for Metal
Structures by Galambos (1998), specifically in
Chapter 5. Equation 5.10 of that chapter presents the
general formula for singly symmetric members where
bending is in the plane of symmetry. Methods for
accounting for location of loading with respect to the
shear center are provided, as well as for determining the
appropriate buckling lengths considering rotational
restraints.
CS.2.I
AISC has partnered with ClDECT to publish a set of
HSS Design Guides. These guides are published
internationally and have not been reviewed by AISC and
are not necessarily in accordance with the AISC
Specifications. However, the documents are a good
resource on HSS connections and systems.
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Page 20
12 LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
7.2-STEEL TWIN I-GIRDER AND SINGLE TUB
GIRDER SYSTEMS
7.2.1-General
F or potentially torsionally flexible systems such as
twin I-girder and single tub girder structural systems,
the designer shall consider:
• The out-of-plane stiffness of twin I-girders, prior to
becoming composite with a concrete deck, can be
significantly smaller than the in-plane, or vertical,
stiffness. This can lead to a lateral-torsional
buckling instability during construction.
• Single tub girders, prior to becoming composite
with a concrete deck, behave as singly symmetric
sections with a shear center below the bottom
flange. AASHTO LRFD Article 6.7.5.3 requires top
lateral bracing in tub section members to prevent
lateral torsional buckling of these sections.
• Prior to becoming composite with a concrete deck,
twin I-girders with bottom flange bracing will
behave as a tub girder and will exhibit the same
tendencies toward lateral-torsional buckling. Top
lateral bracing shall be provided as for tub sections,
or the stability shall be checked as a singly
symmetric member.
7.2.2-Lateral-Torsional Buckling Resistance-
Twin I-Girder
For evaluating the stability of twin I-girder systems
without a composite deck or lateral bracing, the
equation given by Yura and Widianto (2005) may be
used:
(7.2.2-1)
where:
Mn = nominal in-plane flexural resistance of one
girder (kip-in.)
Mer = critical elastic lateral-torsional buckling
moment of one girder (kip-in.)
s spacing between girders (in.)
E modulus of elasticity of steel (ksi)
L effective buckling length for lateral-torsional
buckling (ft)
C7.2.1
Several incidents have highlighted the need for a
careful evaluation of the stability of pedestrian bridges,
especially during the construction stages. Structural
systems consisting of two parallel girders can exhibit
very different behavior during construction, depending
on the bracing systems used. Lateral bracing contributes
significantly to the lateral-torsional buckling capacity of
the beam. For girders without lateral bracing during
construction, lateral-torsional buckling capacity should
be carefully evaluated. After the deck is cast, the section
is effectively a "C" shape, which is singly symmetrical.
Use of the appropriate lateral-torsional buckling equation
is critical, and reference should be made to Galambos
(1998). Further information is contained in Y ura and
Widianto (2005), as well as Kozy and Tunstall (2007).
Page 21
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES 13
Iyo =
Ixo =
out-of-plane moment of inertia of one girder
(in.4)
in-plane moment of inertia of one girder (in.4)
in-plane plastic moment of one girder (kip-in.)
Where a concrete deck is used, continuous twin I-girder
systems shall be made composite with the deck for the
entire length of the bridge.
7.2.3-Lateral-Torsional Buckling Resistance-
Singly Symmetric Sections
The lateral-torsional stability of singly symmetric
sections not covered in Article 7.2.2 shall be
investigated usmg information available m the
literature.
8-TYPE SPECIFIC PROVISIONS
S.l-ARCHES
Arches shall be designed in accordance with the
provisions of AASHTO LRFD with guidance from
Nettleton (1977).
S.2-STEEL HSS MEMBERS
S.2.I-General
The capacities or resistances of connections for steel
HSS members shall be in accordance with the Chapter
K of the specifications and commentary of AISC (2005)
or AASHTO Signs. Resistances for fatigue design shall
be in accordance with Section 2.20.6 of Structural
Welding Code-Steel ANSI! A WS D 1.1 or Section 11 of
AASHTO Signs. All loads, load factors, and resistance
factors shall be as specified by AASHTO LRFD and
these Guide Specifications. For member design other
than connections:
• Flexure resistance of steel HSS members shall be
according to AASHTO LRFD Article 6.12 as box
sections.
• Shear resistance of steel HSS members shall be
according to AASHTO LRFD Article 6.11.9 as box
sections.
C7.2.3
Equations for the determination of the lateral-torsional
buckling moment in singly symmetric sections are given
in the Guide to Stability Design Criteria for Metal
Structures by Galambos (1998), specifically in
Chapter 5. Equation 5.10 of that chapter presents the
general formula for singly symmetric members where
bending is in the plane of symmetry. Methods for
accounting for location of loading with respect to the
shear center are provided, as well as for determining the
appropriate buckling lengths considering rotational
restraints.
CS.2.I
AISC has partnered with ClDECT to publish a set of
HSS Design Guides. These guides are published
internationally and have not been reviewed by AISC and
are not necessarily in accordance with the AISC
Specifications. However, the documents are a good
resource on HSS connections and systems.
Page 39
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