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BUILDING EFFECTIVE DESIGN MODELS FASTRAK BUILDING DESIGNER
The most effective way to create design models in Fastrak
building designer is described in depth in chapter 2 of the
Building Designer Engineers Handbook a PDF documents which is
installed with your Fastrak software. Despite this, we see many
models which do not take our advice into account. To illustrate the
issues we have used a Fastrak model and provided design times on a
typical PC. THE KEY ISSUE Fastrak can carry out gravity and lateral
design all within the one model. Further Fastrak can also
automatically calculate all NHFs/sway results for you as well as
automatically calculating multiple wind load cases for you. It is
easy to create a very large model, with a lot of loading and simply
press the design key. This will still give correct answers, but for
designers looking to produce the most effective route to design it
is simply not a very effective way to work. An example is the issue
of Notional Horizontal Forces (NHFs). One simple click on the NHFs
button in the analysis attribute will add a significant amount of
work to the design process, where the NHF in +X, -X, +Y and Y will
be calculated for the entire factored vertical load. The software
will then automatically create a frame analysis and drift the frame
in X & Y with the NHFs applied. The design process will then
check the floor to floor drift of every column in the structure and
report the resultant lambda crit for all columns (with a summary of
the worst lambda crit in the X & Y). This is a wonderful
feature that will save you a huge amount of time, BUT, it does not
need to be applied all the time, and in fact it can be very much
more efficient in the design process to apply it only when
needed.
A TYPICAL FASTRAK MODEL A city centre retail development; Over
1600 beams (mostly composite) Over 160 columns 12 structural levels
11 gravity load cases Multiple lateral load cases.
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The lazy way to use Fastrak Building Designer Build the entire
model and simple press the button for an automatic design of the
structure. For the typical model with 2 gravity load combinations
(Construction & Dead + Live) and 4 Lateral load combinations.
Auto design whole model (all sections selected for you) time =
approx 20 minutes Check design whole model time = approx 8 minutes
NOTE individual beams/columns can be checked without running the
whole model. The reason that this is lazy is that it is almost
certain that a number of iterations will need to be applied to the
design process. The automatic inclusion of NHFs within Fastrak
while extremely useful and saving a huge amount of time is not
necessary when looking at the design of composite beams, further
there is absolutely no point in applying a huge range of lateral
loads to a building which is not stable. The effective way to use
Fastrak Building Designer The effective way is to carry out the
design as a series of stages and iterate at each stage. Stage 1
Gravity Design Use 2 combinations (Construction stage and Dead +
Live) only. Set the software for two design passes with NHF off.
Auto design time = approx 8 minutes Check design time = approx 4
minutes Iterate the designs until you are happy with the design of
the beams. Stage 2 Sway check Switch in NHFs and check the sway
stability of the building. Switch off the design checks in the
beams (you can recall the design result for any beam simply by
clicking on the beam). Sway check time = approx 3 minutes This is
critically important; many frames are sway sensitive and need to be
checked before you apply any lateral loading, it is often this
process that needs to be iterated a number of times. Ensure that
Lambda crit in X and Y is above 4 before you apply any lateral
load. 3 Lateral load check Now apply any lateral load (wind and
NHFs for lack of fit), and run a check on the complete model.
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Building Designer Documentation page 444 Chapter 2 : Building
Effective Models in Fastrak Building Designer
Chapter 2 Building Effective Models in Fastrak Building
Designer
Fastrak Building Designer is a design based structural
modeller.
Please remember that Fastrak Building Designer is a modelling
package, which dictates the design model and which creates analysis
models to accomplish this design. It is important that you
recognise that you must take ownership of the creation of the model
and the results that the software gives.
It is possible to create complete building designs quickly and
easily with Fastrak Building Designer, however as it is a design
based modeller you should take account of the following
issues1.
Grid LinesYou can define grid lines quickly and simply in
Fastrak Building Designer. Alternatively you can import them into
your model from a DXF file.
If you are importing grid lines from DXF files, please ensure:
that the grid lines you are using are accurate, that the DXF file
you are importing only contains grid lines.
If you are in doubt we advise you to use Building Designers
ability to import a DXF file and create a ghost image of the
structure. You can then add your Building Designer grid lines on
top of the ghosted DXF image.
AttributesIt is important to realise that the attribute sets are
used to set up defaults for the elements (beams, columns ) in your
model. The attribute sets are not linked to the elements once they
are created.
You can quickly make major changes to your model, for example
changing the grade of steel, quickly and easily, you need to change
the appropriate attribute(s) and then apply these attribute(s) to
the members.
IMPORTANT when you create any member it takes the current
default attributes. The default setting for a simple beam (unless
you change it) is that it is fully restrained. Please take care if
creating beams that are not fully restrained.
Loading Self Weight of Concrete SlabsBuilding Designer
automatically calculates the self weight of the structural
beams/columns for you and provides automatic calculation of items
like Notional Horizontal Forces (NHFs) and Wind Loads.
Footnotes1. This is particularly important if you are a new
user, until you become familiar with Fastrak Building Designer.
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Please note however that Building Designer DOES NOT
automatically calculate the dry weight, or the wet weight of the
concrete slab. When you calculate these values you should allow for
two key issues:
the slab loads you calculate should make some assessment of, and
allowance for, ponding that may occur,
when your model contains composite beams you need to create your
loadcases carefully to take account of the staged construction that
will actually take place. To this end Building Designer
automatically creates the Slab wet and Slab Dry loadcases. It is
important that you apply the relevant loads in the relevant
loadcase correctly.
Loading applied to slabsBuilding Designer applies floor loading,
area loads, line loads and point loads to the slabs in your model
and distributes them in the direction of span of the slab.
If you wish to apply loading directly to a beam, PARTICULARLY IF
THAT BEAM SITS AT THE EDGE OF THE SLAB, then you should use element
loads which apply the load directly to the member without involving
the slab.
To aid this Building Designer has a Create Perimeter Load
facility. You can access this from the Loading menu.
Staged modelling and designOur major piece of advice when you
are modelling in Building Designer is: DO NOT BUILD THE ENTIRE
MODEL BEFORE YOU VALIDATE AND DESIGN IT.
It is important that you build the model, validate and design it
in a staged process, for example: Validate and design ONE floor
before copying it up the building, Resolve the gravity design
before looking at the lateral design, Resolve the sway stability
before applying the wind loading.
There are often many nuances to creating your model, in
particular with composite design, and it is much more effective to
resolve any issues once (before you copy the floor to other levels
in your model) than it is to copy the floor to (say) ten other
floor levels, and then address the (usually simple) issues on each
copied floor (in this case ten times the work!).
Simple Construction is still the best methodThe most effective
design for a multi storey structure is still likely to be simple
beams and columns with bracing to resist the lateral forces. Simple
construction in BS 5950 implies certain types of modelling and
certain specific design rules (both inclusions and exclusions). We
assume your familiarity with these.
Use Simple1 beams and columns where possibleBuilding Designer
will happily design moment frames or continuous beams automatically
within a model, BUT, the design of these elements is much more
comprehensive (and hence takes longer). For this reason you should
only use such elements when your model specifically requires
them.
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If Building Designer gives warnings about braces on simple
beams, or intermediate floor levels on simple columns the answer is
not necessarily to make the affected elements into general
beams/columns. Look at the modelling and talk to CSC support if you
are not sure of the route that you want to take.
NOTE Simple Columns and Sway StabilityWhen you use simple
columns they are pinned at every floor level (except where they are
connected to a braced bay) to ensure that all lateral load is
transferred to the braced bay. This modelling is in line with the
SCI guidelines in the Steel Designers Handbook. You may pin or fix
general columns at each floor level as you wish.
Diaphragm ActionYou can switch diaphragm action on or off for a
given floor as you decide. However if you switch diaphragm action
on, then this applies to the entire floor.
Composite Beam DesignComposite design of beams is a complex
procedure when done rigorously. We assume that you are familiar
with the concepts of composite design before you use Building
Designer.
Building Designers composite design routines can automatically
choose the optimum stud layout and automatically select an
appropriate transverse shear reinforcing layout, as such the design
of any composite beam may have a range of possible solutions.
Example A typical 9 m composite spine beam can be shown to be
acceptable: with studs at 190 mm cross-centres and a 457x191x67 UB,
with studs at 200mm cross-centres and a 457x191x74 UB, which of
these solutions is better is up to you.
While it can sometimes be useful for Building Designer to
optimise a design, you might well take the view that you would
prefer to control the stud spacing and other critical design issues
rather than leaving Building Designer to choose a different layout
for every beam.
Please consider the following when you set up the attributes for
a composite beam.
It is important to realise that you can define attributes which
may make the design of composite beams impossible for example if
you set the stud spacing on a spine beam to 300 mm, but this does
not give the minimum amount of shear interaction then the selection
of a suitable beam size is not possible.
You should exercise care in the use of composite beams, if in
doubt design all beams as simple beams first and then simply select
those beams that you wish to be composite at a second pass.
Footnotes1. Pin type connections thus in this context a
composite beam is simple.
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Composite or simple beamComposite beam design is not a linear
process, and some beams are simply not suitable for design as
composite beams. You should take care when selecting beams for
composite design, and set appropriate design attributes such as the
critical e dimension.
The benefits of composite design are well known, however many
beams are not suitable for composite design, including:
beams with no slab, very short beams, beams with significant
eccentric load (for example a beam supporting a column close to
the support), beams with decking arrangements that will not
allow effective composite action.
Note Many engineers consider edge beams to be an ineffective use
of composite beams and specify simple beams to avoid significant
use of transverse shear reinforcing.
In short you should be diligent about the use of composite
beams. Exercise care when determining which beams are appropriate
for composite design, if in doubt design all beams as simple beams
first and simply select those beams that you wish to be composite
at a second pass.
Setting the appropriate level of deflection checksBuilding
Designer provides very comprehensive deflection checks on all
beams. You can set limits on the deflections for a variety of
conditions (dead load only, imposed load only and/or total load).
At the same time in order to allow for deflections with beams with
significant web penetrations Building Designer employs a
sophisticated integration based deflection check.
You should take care when setting the range of deflection
checks. You may consider the default deflection limits conservative
for some buildings.
Building Size and OrientationThe automatic calculation of NHFs
and sway checks are done on the basis that we are checking a single
building as outlined in section 2 of BS 5950-1. This effectively
says that each portion of the building between expansion joints
should be looked at separately for sway stability.
When using Building Designer we assume that you are following
this logical process.
Note NHFs and sway stability are calculated in the global X and
Y direction, you should take care to input the model with this in
mind.
Automatic NHF calculationsIf you switch the automatic
calculation of NHFs on, then Building Designer follows the process
below:
it automatically calculates all NHFs based on 0.5% of the
factored vertical load that passes through any beam/column
intersection in the structure,
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Note The values of the NHFs may vary for each load
combination.
automatically creates a sway analysis to calculation the drift
in X and Y of the structure under NHFs only,
automatically calculates the floor to floor drift of every
column in the structure and thereby establishes the worst lambda
crit in both X and Y directions,
reports this back for the critical sway stability values of
lambda crit in X and Y, if the crit value falls between 4 and 10,
Building Designer can (if the option is selected)
automatically apply the p effects, Building Designer can
automatically include the relevant NHFs in vertical load only
combinations to allow for lack of fit (if required).
Note This is not the same as sway and is a requirement of BS
5950-1.
On a typical model, this very valuable automated process will
significantly extend the design time, therefore we recommend that
you leave it switched off until you require it. For instance, there
is no point in switching this option on if you have no lateral load
resisting system in place or if you are making small alterations to
your model and you are iterating the design process.
Number of design runsThe default setting for Building Designer
is 3 design runs (passes) with NHFs on. This carries out:
initial load decomposition, initial design of all members,
automatic recalculation of the self weight of the structure,
automatic calculation of NHFs, redesign of the structure with NHFs
and self weight, automatic recalculation of NHFs allowing for the
revised self weight, a design check of the entire structure.
As a default this is a reasonable process that works for the
automatic design of moment frames as well as simple structures.
However if you are dealing with a large simple multi storey
structure we would suggest that it would be appropriate to perform
only two passes.
Number of load combinationsWhere you are looking at design
changes, for example to rationalise an area of floor, you can
switch off all the irrelevant load combinations.
For example if you are looking to redesign a series of composite
floor beams, then it is likely that only the construction case load
case and the dead + super load case are relevant. This may allow
the you to switch off all other load cases and concentrate on the
gravity design issues.
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Stepped Design ProcessIf you put some of these things together
you will see that there is a logical stepped design process. For
example pinned beams (such as the composite beams) will mostly be
unaffected by any lateral load, and hence you may design the beams
looking at gravity load only.
Using this stepped process to carry out the design you should
find the software provides detailed design very effectively.
ResultsUpon completion of the design process the Workspace
presents:
Model deflection results these are not a pass/fail for the
details of the model but simply an indication of the total
defection of the model under all the differing loads applied. If
the model suffers from excessive deflections then a warning will be
shown. In this case the remaining results could be incorrect as the
overall building analysis may be indicating that the building will
collapse. This may be irrelevant if you are looking at a gravity
design and are happy to ignore lateral load, but it may also mean
that the basic moment for which the building is being designed
could alter once the building is stabilised.
Sway Stability if the NHFs option in the analysis options is on
then Building Designer will carry out a full sway stability
analysis. Significant failure, may mean the lack of overall
stability such as the omission of diaphragm action or simply a
loose piece of the structure not connected into the main
bracing/diaphragm action. Use the deflection results to look at
this.
Lambda crit less than 4 BS 5950-1 says that if crit is less than
4 full a full second order analysis should be used. Methods for
rigorous second order analysis are incompatible with imposed load
reduction and therefore within Building Designer a crit less than 4
indicates that your model does not have sufficient lateral
stiffness.
Load in versus load out All loads are checked in and out of the
model it is essential that you check these results.
Building Effective Models In FTBD II.pdfBuilding Effective
Models In FTBD.pdf