Benches, Sidewalks, and Construction Tolerances

I have a project in which one of the elements we are furnishing and installing is a bunch of large precast concrete benches. They appear on a landscape sheet that also calls for aligning bench ends with sidewalk scores.  Simple in theory; prone to problems in real life.

Bench layout and detail

Bench layout and detail  (click to enlarge)

To avoid those problems I pushed on the GC and we met: me, the GC, his flatwork sub, the architect, and the landscape architect. Together we worked out an approach that had a good chance of avoiding problems with alignment. On my native planet (Earth) all measurements, dimensions, and manufactured items have tolerances. Tolerances (or accuracy if you prefer)  for car engine parts are a few thousands of an inch, in precast it’s a few 16ths, when tossing  hand grenade  it can exceed several yards.  Here on planetcommercialconstruction there is not uniform awareness of such tolerances.    As part of managing expectations and avoiding problems with our installed material, I asked the two architects about their assumed tolerances. Without hesitating, both said “none” – it should all fit perfectly. If you build, rather than design, you will of course be flabbergasted by this blatant disregard for the nature of material reality (or, disregard for the reality of materials). If you have years of experience, this reality will not be surprising  For those with interest in tolerances you can buy the book, or just go there and read the excellent introduction: )

Here I will note but not address the disconnect between that attitude (“no tolerances, everything fits perfectly”)  and the specifications that are full of stated tolerances and are an integral part of the contract.  I’d like to use this witnessed scenario as a springboard to speak to the intimately interconnected issues of:

  •  inefficiencies in how we build structures on planetcommercialconstruction
  • why progress with BIM and other software solutions to the challenges and cost of design is much slower and tougher than many people hope will be the case

Efficient movement of a design concept through all the steps that will result in a physical reality requires a lot of knowledge about many materials, products, and processes. Most folks who have not actually tried building complex modern structures cannot imagine how much knowledge is involved among all the specialists. In terms of the first issue above (inefficiency), because we involve people with knowledge post-design, post-bid, there is a lot of backtracking and meetings just like the one today. A lot of revision and re-design occurs.  The injection of relevant knowledge into the process earlier would be much more efficient in terms of all participants’ resources.

BIM relies on intelligent objects (doors, walls, benches, etc.). But if these objects do not have embedded within them full knowledge of their own tolerances they cannot be part of an automated design engine that delivers hoped-for results. Ultimately what they really need is the deeper,  more complex, and difficult to acquire and to  incorporate  knowledge of how tolerances of separate materials/products/assemblies interact. This is tough stuff to round up and harness.

You don’t have to trust the pronouncements here of a quirky precaster.  Check out Joshua Lobel’s  master’s thesis. His title is wonderfully playful if you know the vocabulary:
Building Information: Means and methods of communication in design and construction.  He distinguishes between design information and construction information and notes that BIM may lead to loss of knowledge and further disconnect between the two.


About Leo Schlosberg

Graybeard with experience in commercial construction and IT, and an interest in information flow and process. Aware and respectful of the enormous complexity, technical, legal, and other, embedded in every structure that is part of the built environment.
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