Effective Modelling in Fastrak Building Designer

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.

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.

Thursday 7 September 2006 16:50

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.

E:\CSC Doc\Building Designer\Engineers handbook\Building Designer Engineers handbook.fm

Chapter 2 : Building Effective Models in Fastrak Building Designer Building Designer Documentation page 445

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.



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.



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,



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.



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.

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Effective Modelling in Fastrak Building Designer

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