Five Ways to Annoy Contractors
It is surprisingly easy, sitting in the office, to forget that the bunch of design calculations and drawings you’ve been pondering over all morning will one day have to be built by someone. Consideration of constructibility is a CDM requirement nowadays, and the dream of designing something that doesn’t cause a contractor to baulk is not yet dead.
So here are five, of the many, deadly sins that consultants often commit, which cause contractors to die a little inside (so I’m told)…
Rebar for Superman
Although no longer health and safety cannon the ’25 kg’ rule is still a good way of gauging what an individual can lift. This means that using 40 mm bars should be avoided, unless you’re planning to employ a gang of weight-lifters to build it.
Avoiding the largest rebar is only the start, however. The following table gives the weight of the maximum 12m length bar, and the length of a 25 kg bar for each diameter. Note that you need a small camel to lift a full H40 length.
|Bar Dia. (mm)||12m Weight (kg)||25kg Length (m)||Bar Dia. (mm)||12m Weight (kg)||25kg Length (m)|
Amazon Wish Lists
Not everything on a civils project can be designed using the fundamental elements of concrete and steel. Eventually you’re going to need some proprietary assistance, and the time comes to start specifying products. Often this selection will have some influence on the design and, if you’re lucky, you’ll be able to ask the contractor for a preference. More often than not, though, you’ll have to make the first guess.
In these situations, I have found, its better to consider product selections as assumptions. To be competitive contractors have to be able to shop around, and (unlike a lot of architects) suppliers don’t pay commission to designers. The problem is that the contractor cannot safely just pick anything. For example, the breaking load of steel parapets vary wildly between manufacturers; the wrong product could cause the structure to fail.
Whatever product is selected needs to meet both the specifications of, and the assumptions behind, the design. The following almighty three words, however, are our saviour of compromise: ‘Or Similar Approved‘. Tagging this note onto any named product gives the contractor scope to find the best deals (as most suppliers can provide similar products to their competitors) while still ensuring the consultant gets a chance to check it against the design.
Digging to the Centre of the Earth
A few years ago the rules for excavations were fairly simple, and trenches shallower than 1.25m didn’t need support. Now it’s much more complicated, and digs require support if they pose a risk. Regardless of this, many engineers still start with a 1.25m is safe attitude; and then work from there.
Digging, especially around buried services, is a difficult and laborious task. It follows that with almost every drainage or services design I’ve been involved with, the contractor has always asked ‘can we make it shallower?’.
There’s no ‘one-size-fits-all’ solution here, but with trench like excavations you can consider using oversized pipes to lessen long gradients- or use engineered, rather than nominal, cover requirements to bring buried aspects of the design back up to ground level.
Equal Opportunity Design
On the surface, the existence of the term ‘Contractor Designed Detail’ is a little bewildering. This is especially true when you consider that the basic definition of a engineering consultant is someone who designs. Of course there are times when you simply cannot design something until the contractor is involved. Typically this is when the project requires specific construction methods, or employs specialist techniques that require input from sub-contractors.
But with great power comes great responsibility, and abuse of this note will win you no friends.
The Ikea Instruction Manual
Every designer should aim to create economical engineering solutions, working with high utilisations and no wasted parts. There comes a point, however, where the finish product becomes too meticulous to be reasonably built.
Creating connections, for example, made out of seven different bolt sizes for maximum efficiency is, at best, likely to come back with a plea for rationalisation or, at worst, result in some serious construction mistakes.
Providing a single, worst case, pile design and repeating it for hundreds of piles will also, however, rub your contractor up the wrong way; contemplating all the money lost on a truly uneconomical design.
The art is to aim for enough division to provide the perfect balance between economy and buildability; which is about as close as Civil Engineering gets to Zen Philosophy.