Staggered Truss
Framing Systems Using ETABS
This article presents information on modeling,
analysis and design capabilities of ETABS for Staggered Truss Systems. The user
interface features that enable you to efficiently create computer models of
Staggered Truss Systems are outlined. The important technical aspects
associated with such systems that are addressed by the numerical capabilities
of ETABS are also presented. Details associated with the design procedures and
requirements for Staggered Truss Systems may be found in Reference [1].
Typical ETABS
Model of Staggered Truss
Building


Creating
the Staggered Truss Computer Model
There are various ways in which you, the design
engineer, can use ETABS to model staggered truss systems. The options allow you
the latitude to apply your own ways of creativity and engineering. The
numerical power and options that ETABS offers makes it possible to model important
aspects of the Staggered Truss System in one consistent threedimensional
model. New innovations allow you to create complex models and capture all the
important aspects of the structural behavior of the system very efficiently.
ETABS
Staggered Truss Template


2Dimensional
Model or 3 Dimensional Model
You can model the structure as a simple
twodimensional frame or a complete threedimensional system. The
twodimensional system will consist of a typical multistory Staggered Truss and
can be drawn very easily in a 2D Elevation using the drafting tools provided
in ETABS. A threedimensional staggered truss system can be created instantaneously
using the special template that is provided for such systems in the ETABS template
library. You can then, using the
template model as a base, incorporate any special conditions or complexities
associated with your particular building. ETABS is equipped with very powerful
editing options and drawing controls. The editing options allow onscreen operations
including object modification, replication, mirroring, trimming, aligning, extending
among others. The drawing controls allow precise creation and positioning of
structural elements.
You can also replicate your 2D model into the
third dimension and connect the frames together with spandrels to create a 3D
model very efficiently.
While using ETABS you can use any set of units and the units can be dynamically
changed at any stage of any session. It should be noted however that design
codes are usually based on one specific set of units.
Auto
Sections & Grouping
In the creation of the computer model you do not
have to come up with explicit preliminary member sizes for analysis. You can
apply an autosection property to all of the members. An autosection property
is a list of section sizes instead of a single size. The list contains all the
section sizes that you might want to consider as possible candidates for the
member. In the design optimization process for a particular member only the section
sizes available in the autosection property list will be considered. In
addition you can use design groups to enforce uniformity of member selection
across a group from auto selection lists during the design optimization process.
Effective use of autosection properties can save many hours associated with
establishing preliminary member sizes.
Special autosection properties for staggered
truss systems have been built into ETABS for the chord and web members based
upon the AISC Section Data Base.
Physical
Member Geometry
The ETABS model is based on a physical member
concept. For instance, the top chord of a staggered truss is created as one
contiguous member without any concern about breaking up the member into
segments to create elements to allow connectivity to the diagonal and vertical
members at the panel points. You are creating the physical structural members
not the analytical element model. The analytical model is created at analysis
time using the points of intersections defined by the geometry of the physical
model. At output time the information from the analysis elements is reassembled
to produce force and moment diagrams and design information for the original
physical member.
Similarly, in the modeling of the flexibility of
a floor plate, you will typically create only one large floor object that
represents a floor plank or the whole floor deck. The finite element model is
automatically created at analysis time, based upon the physical geometry of the
floor framing members and some userdefined properties associated with the
floor object.
Rigid
& Flexible Floor Diaphragms  Diaphragm Shears
ETABS offers you a wide variety of options for
modeling various types of floor systems. With ETABS you can model floor
diaphragms as rigid or flexible (semirigid).
In the case of rigid floor diaphragms model each
floor plate is assumed to translate in plan and rotate about a vertical axis as
a rigid body, the basic assumption being that there are no inplane
deformations in the floor plate. The disadvantage of such an assumption is that
the solution will not produce any information on the diaphragm shear stresses
or recover any axial forces in horizontal members that lie in the plane of the
floors.
The concept of rigid floor diaphragms for
building type structures was introduced nearly 40 years ago as a means to lend
efficiency to the solution process. See Reference [2]. And proved to be very
effective, especially for methods associated with structural dynamics. However,
as mentioned above it had serious limitations with braced frame structures and
buildings with diaphragm flexibility issues. With the recent advances in
numerical methods and personal computer technology the reasons that justified
the use of rigid floor diaphragm models may no longer be valid.
The floor system supported by the staggered
truss is usually made of precast concrete hollowcore planks connected with
shear connectors and then grouted, with or without a concrete topping. Other
options include concrete supported on metal deck with steel beams or joists.
Under the influence of lateral loads the staggered
truss geometry causes transfer of lateral shears that can generate significant
shear stresses through the floor system. These stresses need to be evaluated.
Therefore, it is important that the staggered truss floor system be modeled as
a flexible (semirigid) diaphragm so that the diaphragm deformations get included
in the analysis. These deformations are important not only for the evaluation
of the diaphragm shear stresses but also for the recovery of the axial forces
in the chords of the trusses and inclusion of the effects these deformations
have on the forces of the web members.
The Physical Member Concept allows the automatic
modeling of flexible floor diaphragms, each floor plank essentially being a
floor object. Opening objects can be placed over floor objects to punch holes
in the floor system. The meshing of the floor and opening objects for the
creation of the finite element model is automatic. For diaphragm deformation
effects to be accurately captured the mesh need not be too refined.
Staggered
Truss Diaphragm Shears Diagrams


Current numerical techniques allow you to
specify modification factors to stiffness components of the finite elements
that are created that will limit the way in which the elements behave. Firstly
the elements can be forced to act only in shear so that all in plane bending in
the concrete due to lateral loading is resisted by a C and T couple causing axial
forces in the longitudinal spandrel beams, with no action being taken by the
concrete. Secondly, the inplane behavior of the concrete will not contribute
any strength or stiffness in the vertical direction of the trusses.
You have the option to define section cuts
across a series of elements and have the forces integrated across the section.
This is a very powerful way to get shear forces across a section cut through a
floor or overturning moments across a series of columns of a frame.
Dead,
Live and Superimposed Vertical Loads
The Vertical loads are applied as uniformly
distributed loads to the floor plates.
These loads can be separated as dead load, live load or superimposed
dead load. The selfweight of the deck and members can be automatically included
in the dead load. The floor objects for the floor planks or the steel decking
are given a spanning direction for automatic load transfer to supporting elements. Live Load Reduction Factors are calculated
for each member based upon the tributary area that is being carried by the
corresponding member. Various code dependent formulations are available for
these calculations, however the values can always be overridden with user
specified values.
Wind
& Seismic Lateral Loads
The Lateral loads can be in the form of wind
loads or seismic loads. The loads are automatically calculated from the dimensions
and properties of the structure based upon builtin options for a wide variety
of building codes.
For rigid diaphragm systems the wind loads are
applied at the geometric centers of each rigid floor diaphragm. For modeling
multitower systems you can have more than one rigid floor diaphragm at any one
story.
The seismic loads are calculated from the story
mass distribution over the structure using code dependent coefficients and
fundamental periods of vibration. For flexible floor systems, where there are
numerous mass points, ETABS has a special load dependent Ritz vector algorithm
for fast automatic calculation of the predominant timeperiods. The seismic
loads are applied at the locations where the inertia forces are generated and
do not have to be only at story levels. Additionally, for flexible floor
systems the inertia loads are spatially distributed across the horizontal
extent of the floor in proportion to the mass distribution, thereby accurately
capturing the shear forces generated across the floor diaphragms.
ETABS also has a very wide variety of Dynamic
Analysis options, varying from basic Response Spectrum Analysis to Large
Deformation Nonlinear Time History Analysis. Code dependent response spectrum
curves are built into the system and transitioning to a dynamic analysis is
literally a few mouse clicks away once the basic model is created.
Frame
Design Procedures & Drift Optimization
The Design algorithms for member selection,
stress checking and drift optimization for various design codes are documented
in detail in separate technical notes. See References [3] and [4]. These
procedures involve the calculation of member axial and biaxial bending
capacities, definition of Code dependent design load combinations, evaluation
of KFactors, unsupported lengths and second order effects, moment
magnifications and finally utilization factors to determine acceptability.
You can generate displays of energy diagrams
that demonstrate the distribution of energy per unit volume across the
structure. These displays help in identifying the members that contribute the
largest to drift resistance under the influence of lateral loads. For drift control,
increasing the sizes of these members will produce the most efficient use of
the added material.
Along the same lines, you can activate an
automatic member size optimization process for lateral drift control based upon
lateral drift targets that you specify for any series of points at various
floors. The drift optimization is based upon the abovementioned energy method,
whereby extra material for drift control is distributed to members in
proportion to their energy per unit volume for the particular lateral load
case.


Deformed
Shapes of Staggered Truss
Building


Construction Sequence Loading
Implicit in most analysis software is the
assumption that the structure is not subjected to any load until it is
completely built. This is probably a reasonable assumption for wind and seismic
loads and other superimposed loads. However the dead load of the structure is
continuously being applied as the structure is being built. In other words, the
lower floors of a building are already stressed with the dead load of the lower
floors before the upper floors are constructed. Engineers have for long been
aware of the inaccurate analytical results in the form of large unrealistic
beam moments in the upper floors of buildings due to the assumption of the
instantaneous appearance of the dead load after the structure is built.
In many cases the analytical results of the
final structure can be significantly affected by the construction sequence of
the structure and the manner in which the structure is built and activated and
the incremental dead load gets applied and prestresses the partially built
structure.
Situations that are sensitive to the effects of
the construction sequence include, among others, buildings with differential axial
deformations, transfer girders involving temporary shoring and trussed
structures where segments of the truss are built and loaded while other
segments are still being installed. Staggered truss systems may be susceptible
to these effects.
ETABS has an option whereby you can activate an
automatic incremental storybystory construction sequence loading of the
building for a particular load case. This procedure will load the structure as
it is built. Typically you would to this for the dead load case and use the
analytical results from the sequence loading to be combined with the other load
cases for the design and stress check of the building.
AutoCAD
Plans & Elevations
ETABS has Options to create and
export plans and elevations in the form of DXF files to AutoCAD. You can
identify the plans and create elevations in the form of planar or developed
elevations to be exported.
About
ETABS
Dating back over 30 years, with the original
development of TABS, the predecessor of ETABS, it was clearly recognized that
buildings constituted a very special class of structures. See Reference [2]. It
has been demonstrated that recognition of the special characteristics of
numerical problems associated with building type structures can produce numerical
procedures that are very efficient and stable. Also, information received and
produced by such special purpose software will be in a form that targets the
special requirements of the Structural Engineer involved in the design of
buildings.
Today ETABS, continuing in the same tradition,
has evolved into a completely Integrated System for the Analysis and Design of
Buildings Structures. Embedded beneath the simple, intuitive user interface are
very powerful numerical methods, design procedures and international design
codes that allow you to be versatile and productive, whether you are designing
a simple 2dimensional frame or performing a dynamic analysis of a complex
highrise that is applying nonlinear dampers for interstory drift control.
ETABS works off of an integrated database. The
basic concept is that you create only one model consisting of the floor systems
and the vertical and lateral framing systems to analyze and design the whole
building. Everything you need is integrated into one versatile analysis and
design system with one user interface. No external modules to maintain and no
worries of data transfer between modules. The effects on one part of the
structure due to changes in another part are instantaneous and automatic.
The analysis methods include a wide variety of
Static and Dynamic Analysis Options. The integrated model can include, among
others, complex Composite Floor Framing Systems with Openings and Overhangs,
Steel Joist Systems, Moment Resisting Frames, Complex Shear Wall Systems, Rigid
and Flexible Floors, Sloped Roofs, Ramps and Parking Structures, Mezzanine
Floors, Trussed Systems, Multiple
Tower Buildings
and Stepped Diaphragm Systems.
The numerical methods allow modeling of steel
deck floors and concrete floor systems allowing secondary systems to
automatically transfer loads to main girders. The automated finite element
meshing of complex floor systems with automated displacement interpolation at
mismatched mesh transitions, coupled with Ritz analysis for dynamics, makes
inclusion of diaphragm flexibility effects in the analysis very practical.
Vertical Dynamic Analysis options allow you to
include the effects of vertical ground motion components in your earthquake
analysis. It also allows you to perform detailed evaluations of vertical floor
vibration problems in addition to the traditional empirical methods that are
also built into the software.
Special problems associated with building type
structures, such as, Calculation of Centers of Rigidity, Global and Local
PDelta Effects, Inclusion of Joint Panel Zone Deformations, Effects of Joint
Rigid Zone Ends and Member End Offsets due to Cardinal Points of a section have
been addressed with customized numerical techniques that allow you to include
these effects into your analysis effortlessly.
More advanced numerical methods include
sophisticated options for modeling Nonlinear Dampers, Pushover Analysis, Base
Isolation, Construction Sequence Loading, Structural Pounding and Uplift.
A wide variety of export options allow you to
transfer information from the ETABS database for use with other software
packages. Some uses of these export options, among others, are framing plans
and elevations using AutoCAD, foundation and slab analysis using SAFE and
information for detailing packages using CIS/2.
References
[1] Wexler, Neil and Lin, FengBao
Staggered Truss
Framing Systems, Steel Design Guide Series, American
Institute of Steel Construction, December 2001
[2] Clough,
R. W., King, I. P. and Wilson, E. L.
Structural Analysis of
Multistory Buildings, Journal of the Structural Division,
ASCE, Vol. 89, No. 8, 1963.
[3] Computers
& Structures, Inc.
ETABS – Integrated
Building Design Software, Steel Frame Design Manual, January 2002
[4] Computers & Structures, Inc.
ETABS – Integrated
Building Design Software, Steel Composite Floor Framing Design
Manual, January 2002