Computer Aided Design

David French Partnership went over to CAD back in 1989 with AutoCAD release 10 running on the DOS operating system. Our first CAD computers were a Dell 386 20MHz and a Dell 386 25MHz machine. Back in those days, to run AutoCAD, you had to buy a ‘maths co-processor’ chip to do the calculations. This cost £750 for the chip on its own! It is amazing how things have moved on with modern high-speed computers about half the cost of that maths co-processor!

From the outset David French Partnership planned to make our use of AutoCAD as efficient as possible and planned our approach carefully. We wanted to have a system that was uniform across the whole company so drawings produced from each technician all looked the same with uniform layering and styles. We recognised the advantages of being able to customise AutoCAD and our systems have always had time saving customisation. Our drawing template has many predefined layers, customised line types, hatch patterns and text styles.

One of the first problems we found with AutoCAD was that (in those days) there was no way to work with multiple scales on a drawing. We found this to be a big issue when producing engineering drawings and details. Looking through CAD magazines I came across an advert for ‘MultiScale’ After trying a demonstration copy of the software we ordered it for all our systems. This enabled us to achieve exactly what we needed.

We were so pleased with ‘MultiScale’ that we asked the developers, ‘Computer Draft Systems’ (CDS) if they had any other utilities. Initially they were reluctant to talk about their software in development but eventually they admitted that they had a package ‘MultiSteel’ under development. We saw this software about 6 months before its planned release and we were so impressed that we immediately ordered it for all our systems and had a Beta copy installed a few months ahead of its release.

Initially there was only the steel package but we then persuaded CDS to develop a reinforced concrete detailing suite which they did… and called it ‘MultiRebar’. With both packages, we gave the developers input on the system operation and offered suggestions for possible improvements.

We now run with the full MultiSuite software on all our workstations and the developers have allowed us access to some of their special routines which allow our customisation to work seamlessly with their system. As well as Beta testing their latest versions we are a reference site for them.

Architects and CAD

The advantage of CAD drawings is they are more accurate and the big benefit is they can be shared between the design team. If we are given a hand drawing and asked to do the engineering often we have to re-draw it, as least in part, to enable us to show the structure and detail it. This takes time and adds unnecessary cost to the project.

Even a nice CAD drawing can still leave us with a lot of work before we can use it. The drawing below was a drawing we received from an architect. On the face of it, it looks fine and achieves what the architect wants to achieve, looking acceptable when printed.

What should happen is you have one line colour and one line type per layer. Different sections of the drawings should be on different layers so everything is easily identifiable and easily manipulated. British Standard BS1192 covers layering but as so often happens when bureaucrats get hold of things their layering convention is not intuitive.

The logic behind a mandatory layering system is to enable drawings from different disciplines to be merged while each remains separate. For instance, using the example above A244_D is the layer for information by the Architect about the stairs whereas S244_D is the information by the Structural Engineer. Merging a drawing between the Architect and Engineer should provide 2 sets of information on 2 separate layers that will match if they are right.

We have a standard set of layers within David French Partnership which are matched to line colours and pen thicknesses in order to produce drawings to our satisfaction. Within our MultiSuite software we have the ability to convert our layer system to any other layer system if required. We also have tools to enable us to decipher Architects drawings that are poorly layered but this costs us time and money.

Architects and BIM

A BIM model is normally a 3-dimensional model and in the perfect situation allows all the disciplines to add their information to ensure that everything is correctly coordinated. In the ideal world, a change from one discipline can be reflected in all interlinked disciplines.

For this to work efficiently the base computer model has to be accurate.

When we came to use the model, we found that he had cheated, allowing walls to overlap rather than joining correctly as shown by the yellow arrow. Once shaded and plotted this junction looked fine but when we came to add the structural beams we found it very hard to locate the correct intersection points for the beams.

Also, the lines are not square as shown by the slight joggle in the lines shown by the red arrow. This means when you draw a beam perpendicular to the horizontal wall it is not parallel to the wall that you want to support. These errors are small but slow up the following disciplines design process.

The whole benefit of BIM is accuracy so each discipline can accurately position their information relative to others and if the base model is not accurate everything else that follows will have errors defeating the whole advantage of the approach.

The biggest problems with the BIM approach is time and money. The client has to be prepared to pay more for producing an accurate 3-D model and to achieve this takes time. Most projects are on short design programmes and typically the engineering work has to start before the architectural design is complete and this cannot really be done with the BIM approach.

The costs of working on a 3-D model are also much higher as in 2-D it is fairly easy to show a beam but in 3-D the beam has to be modelled and the connections detailed all in 3-D. Although there are tools to enable this to be done for simple situations many architectural solutions require bespoke designs where the simple tools will not work.

At this stage, the 3-D software is not as advanced as it needs to be and it is very hard to produce drawings to suit different disciplines. The software seems to be setup more for architects than engineers. Typically, engineers want to show the structure below a floor level whereas architects want to show the room layouts above a floor level and the software is not as flexible as needed to achieve this easily. At the moment, the 3-D model is fine for the basic general arrangement drawings but it is still better to export to 2-D for the detailed drawings such as reinforced concrete detailing.

David French Partnership LLP have Revit Software and are committed to BIM where the client is equally committed and prepared to fund the extra design and detailing costs with the intention of minimise issues during the build.