Chapter 5
Overview of Analysis And Design Procedures
Controlling the design procedure
Under the Design menu there is a Design Options… setting which shows the dialog below.
The first page of the dialog provides an option to Start from beginning of order file on each pass, this is unchecked by default. For all or the vast majority of members in most models this setting will have no effect on the results, it simply serves to speed up the iterative design procedure. However in some models checking this option may achieve a lighter weight design result. This is discussed further in “Leaving the Start from beginning of order file on each pass option un-checked”.
The Perform check of fields provide a way to speed up the design process when you want to only tweak a particular part of the design of your structure. For instance if you have designed all the floors in your model, and are satisfied with the resulting beams, but you want to work with the columns, you can remove the check against the types of member with which you are satisfied, and Building Designer will ignore these during the design process. These options only affect the checking process. If a particular element needs to be designed, then this will happen irrespective of the settings you make here.
A full 3D analysis may expose small forces that are normally ignored in the design of
members. The options for ignore forces below on the second page of the dialog simply provide you with a way of setting negligible/nominal force levels with which you are comfortable.
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When the small forces from the 3D analysis are below the specified threshold levels they are ignored so that design can proceed automatically. If the forces are above these limits, then you will be warned during the design process.
The last page of the dialog provides similar functionality for the connections in your model. If forces below the threshold do arise, then they are ignored during the design process. Forces higher than these generate warnings during the design.
Why is it an iterative procedure?
Basically the procedure is as follows:
1. For the first analysis run Building Designer assumes (guesses) section properties for the members that are to be designed (as opposed to checked).
2. Building Designer constructs and analyses this model. 3. Building Designer designs all members.
4. Building Designer compares the member sizes that result from the design (step 3) with the member sizes which were used to construct the latest analysis model (step 2) – if any of these are different then Building Designer goes back to step 2 and constructs a new analysis model based on the sections resulting from the latest design and then again proceeds to step 3.
5. If the comparison at stage 4 shows no differences then Building Designer performs a final check design on all the members in the structure.
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The need for iteration on steps 2 to 4 above occurs when structures include moment connections between members. In such circumstances no member can be operated on in isolation, as soon as the stiffness of one member is changed it can affect design forces (principally the design moments) in lots of members. Hence the analysis model must be synchronised with the latest design.
Controlling the iterative procedure
Under the Design menu there is an Analysis Options… setting which shows the dialog below.
Building Designer can either perform a first-order or second-order analysis of the frame. If the sway of the frame is such that the amplification factor method for catering for sway is
acceptable, then Building Designer can automatically handle this for you.
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The number of Passes limits the total number of times that Building Designer will loop through the iterative analysis and design procedure. Points to note on this are:
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Iterative analysis and design will usually finish naturally in a few loops (repetitions of steps 2 to 4 detailed in “Why is it an iterative procedure?”). However, it is possible that it will enter an infinite cycle of loops. This setting is primarily intended to stop that from happening.•
There is no correct or wrong number to enter at this point, it is simply an upper limit.•
If you set it to 10 it does not mean that the analysis and design utilises all 10 loops, you willoften find that the design still converges in 2 or 3 loops in which case it would not have made any difference if you had set the limit to 5, 10 or 100.
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This setting has only been exposed because you may sometimes find it useful to set a low number to force faster design – refer to the section below.Speeding up iterative analysis and design
The iterative analysis and design procedure might be time consuming, but may be speeded up in different ways.
Limiting the iterations
If you set a low number (for example 2) as the limiting number of analysis and design loops the initial design will be faster with the following implications/possibilities:
1. Setting to 2 loops means that there is one design loop and one final (faster) checking loop. 2. The analysis will therefore be correct and synchronised with the selected members. 3. This does not guarantee that all members pass (there is never such a guarantee). When the
design status is checked you may find some members that fail. You could change and re-check such members interactively.
4. More difficult to spot is the possibility that the design of other members may not be as fully optimised as possible.
5. Although General Beams and General Columns are most likely to be affected by the above, any member could be affected.
We anticipate that many users will prefer to set a low number such as 3 (probably leaving the
start from beginning of order file option un-checked at the same time – see the next
subsection) and that this will generally produce a fully optimised design for the vast majority of members (in particular all the simple beams, composite beams and simple columns) in most structures.
You might then reset only the general beams and general columns to Design and rerun the design allowing more iterations. This avoids repetitive design of members such as simple and composite beams.
Leaving the Start from beginning of order file on each pass option un-checked
With this option unchecked the design process for each member at the second and subsequent iterations starts with the section that was found to work in the previous iteration, if this section fails then Building Designer will start to look at bigger sections.
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When this option is checked the design process for each member at the second and
subsequent iterations will start right at the beginning of the design order file working through until it finds the first section that works.
By activating the start from beginning of order file feature you get the advantage of a detailed iterative design that is likely to find a reasonable minimised-weight solution, but you also get disadvantages:
Slower — Each design iteration will be slower because every possible section is reconsidered for every member.
Slower — Since during each design iteration some members will get bigger and some smaller the model that needs to be re-analysed will differ more greatly. This can lead to more design iterations (as well as each iteration taking longer).
Non-Convergence — For the reason noted above it is more likely that the design could start oscillating between iterations where one group of members get bigger but another get smaller, and then vice versa in the next iteration.
Unless you have a highly indeterminate rigid framed structure where many alternative load paths can be imagined, it is quite likely that this setting will make little difference. A point to note is that there is no single correct answer for these indeterminate structures. (If beam A is made bigger can beam B be made smaller?). There may be lots of different safe configurations and hence different solutions may be found depending on how the software is driven. The examples in “3D Analysis Effects” illustrate this principle.
Overall our advice would be:
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If you would prefer to see faster design we would recommend leaving this un-checked. If you intend to limit the number of design iterations (to 2 or 3) then we particularly recommend that you leave this option un-checked.•
Conversely - if you want to activate this option then we recommend that you do not apply small limits to the allowable iterations.Chapter 5 : Overview of Analysis And Design Procedures Building Designer Documentation page 41
Before spending time looking at detailed design results it is always worth reviewing things at a broad-brush overview level. The most basic checks are made obvious within the workspace summaries shown above.
Maximum Nodal Deflections
Maximum deflections are identified and noted – if any of these are clearly much too high then there may be mechanisms developing or the defined structure is simply not capable of dealing with the loads being applied to it. If extreme deflections are being reported then the analysis results may be suspect.
If there are any issues of this nature, then you should investigate these before spending (potentially wasted) time looking at detailed design results.
Sway Sensitivity
Once again, before spending too much time reviewing detailed designs, it is also advisable to review the sway results and decide on the approach you will take to designing for sway. It is conceivable that any changes you make in order to deal with sway could affect the designs of many elements.
This topic is discussed in a little more detail in “Sway Resistance”.
Loading Summary
This is simply a mathematical double check – does the sum of applied loads equal the sum of the base reactions? If there is a discrepancy identified by this comparison (but the maximum deflections noted above seem reasonable) you will probably need to contact your local support department for help in assessing the problem.
Review of Selected Sections
It is always been worth spending a little time reviewing the results to see if they are in line with expectations. Where you have rigid framing this is even more essential. A quick review does not need to look at the design detail, simply look for things such as:
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do the typical beam and column sizes look reasonable?•
where you expect to see a hefty beam or column have you got one?•
are there big beams or columns where you did not expect them?•
where you expect similar sizes have you got similar sizes?•
where you expect symmetry is there symmetry? and•
1have you limited the use of composite beams to situations where composite beams arepractical in reality? For further information see “Controlling Composite Beam Design” in the extended Quick Start Guide.
Footnotes
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Review Analysis Results
Within Fastrak you can review the analysis results for your entire model quickly and easily. It is always worthwhile doing so as this gives you important information on how your structure is working. You can view deflected shape diagrams, axial load, shear force, bending moment and foundation load diagrams for all member types in your structure, or limit the views to just those particular member types of particular interest.
In some cases this may also help you to identify issues with the analysis model.
Note The model can also be exported to S-Frame where all the same sorts of results for the static analysis can be reviewed and there is also access to more advanced analysis options, e.g. Buckling Analysis (analytical assessment of lcrit), P-Delta Analysis, Vibration and Response Spectrum Analysis, etc.
3D Analysis Effects
Traditional design approaches tend to involve idealisation and simplification of the analysis model. Very often this would have meant simplification of the structure into discrete 2D planes, which could be analysed either by hand or in a simple 2D analysis. Engineers working with 3D analysis packages sometimes encounter unexpected results, which only make sense after some careful consideration. For the purposes of this document we are calling these 3D Analysis Effects.
Since Building Designer allows you to model rigid frames and uses a full 3D analysis to generate design forces we anticipate that you may encounter these sorts of effects. The following two subsections illustrate two simple examples.
Continuous Beam Example
The above model is not intended to be highly realistic, it does however illustrate a 3D Analysis Effect quite clearly.
Continuous beams (spanning 6 m then 9 m then 4.5 m) run from right to left of this floor area. These are supported on simple steel beams spanning front to back which are in turn supported by the columns. There are 3 internal lines of continuous beams which all receive the same loading.
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Designing by hand most engineers would probably consider that the analysis of a single 2D continuous beam line with pinned supports (as shown below) would be an adequate idealisation.
A more accurate analysis would attempt to model the spring effect at each of the supports – that is the supports are not completely fixed against vertical translation.
This spring effect is inherently modelled in a full 3D analysis and the results after analysis and design in Building Designer are shown below.
Notice that different sections are chosen for the central continuous beam line on grid 3 when compared with the beam on either side of it.
A first reaction to this sort of result might be to suspect that the design is wrong. However, a closer examination shows that the design is correct, and that it is correctly based on differing design forces.
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Compare these with the equivalent diagrams for one of the adjacent beams shown below.
Note The maximum sagging moment has reduced from 255.3 kNm to 245.0 kNm. This sort of variation is enough to force the selection of a larger beam on the central beam line where the moments are higher.
Interestingly, we achieved the above result by limiting the initial design to 2 iterations and then redesigning only the continuous beams using the method covered in “Speeding up iterative analysis and design”.
If we completely redesign the same model allowing up to 20 iterations and we also set the start from beginning of order file option, then the design does not converge (the design completes the maximum 20 iterations and then checks whatever sections it has at this point). The result is as shown below.
The continuous beams are actually heavier. Of the other beams, some are heavier and some are lighter. At this point all the beams pass so this is a second alternative acceptable design.
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To see a third alternative you might drive the design a different way. For example you might decide to use nothing larger than a 914 deep section, so you change the two cross beams on grids B and C to 914 305 UB 224 and put the 3 continuous beams back into design mode. This time the design converges quickly (because the conflict between the stiffness of the supporting beams and the stiffness of the continuous beams is removed). The result is shown below.
Once again, all member designs pass so this is a third alternative and completely acceptable design.
Braces Carry Gravity Loads Example
This is probably a simpler example of a 3D analysis effect, however it does initially seem to fly in the face of traditionally accepted design practice. In traditional hand calculations the load chase-down puts all gravity loads into the columns. Where columns are also part of a bracing system providing sway stability, brace and column loads for the sway case are assessed in isolation and are added to the column loads for column design checks as necessary. The possibility that braces carry gravity loads is never considered in this traditional hand calculation approach.
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Consider the simple model shown above. It is braced on all four sides and in this example the initial sizes of the braces have been made very large to exacerbate the analytical effect.
When reviewing the results after design you might wonder why the column at C1 which is part of a braced panel is smaller than the column at B1 which supports the same floor area.
You can review the summary design results and design forces within Building Designer, or you could export the two columns to the simple column module where you can see the results as shown below.
The top capture shows the details for the column at grid intersection C1 which is part of the braced bay. It shows that the column loses load to the brace at second floor level.
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Another good way to review these sorts of effects is through by exporting the model to
S-Frame. The view below shows the axial load results for the front elevation.
The large brace forces are clear to see. If we change the brace section size to a smaller, more realistic section, then Building Designer finds that the same section size is adequate for both columns. When this model is exported to S-Frame the effect still occurs, but is less significant in this instance.
When you define braces in Building Designer you must always specify their section size, and
Building Designer checks these for you (they are not designed automatically). This means that this is not an effect that can become exacerbated by an iterative analysis and design procedure. As a closing point of guidance on this topic, we suggest that you keep brace sizes to a realistic minimum during building design. This realistic minimum is likely to be driven by sway considerations.
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Interactive Design
You can export General Beams and General Columns to their respective stand-alone design packages. However, these members never truly stand-alone, they always interact with other members in the structure. This interaction is demonstrated in:
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“Continuous Beam Example”, and•
“Braces Carry Gravity Loads Example”.When you export elements to General Beam and General Column you lock in the interaction effects. If you do not change anything you can check the same beam or column and see the