The Mote Method

Shimmy shims

February 3, 2010

I discovered that engineers in Alberta do not spend enough time on site, understanding the reality of construction practices.

For example, I recently challenged the two major general contractors doing the construction work, about their shimming practises. I was told that all structural steelwork was shimmed, bolts were tightened  and then grouted much later. This flew in the face of specifications, commonsense and ignorance. Engineers in the engineering office do not know this little piece of information. The QA managers told me this was their practice for as long as they have been working in the oilfields of Northern Alberta. They really thought I came from another planet.

For the most part this is not a problem for shallow structures and modules with substantial loads but it begs a different approach for tall structures, high wind loads and critical details such as shear keys.

I spent ten minutes to engage their intellect in the art of baseplate designs and engineering assumptions and pointed out the differences their practices raised. After this little alignment and agreement,we were able to proceed.

So the rules are; baseplate anchors bolts are snug tight (by hand) on shims, the baseplate is grouted and then after three days, the shims are removed and the bolts are tightened to specification. In a case of three steps forward and two steps backwards, head office ruled they could leave the shims in. Sigh…

If the centrally place shims are not removed and structural steel is bearing on these shims permanently then the grout will serve no purpose in the final design. Shims do not deform to allow full bearing on grout in time. Unless grout is positively loaded into compression, it is decorative and may decay in time. This durability issue is likely in the extreme conditions of Northern Alberta. The potential rotation of the column will now be about the edge of the central shim and not the extreme edge of the baseplate. The reduced lever arm will increase potential anchor bolt forces.

Design engineers need to allow for the construction practice. It is a blindspot when both sides consider it is obvious and clearly they diverge!

Even when pump foundations are grouted with epoxy, shims are being left in! The consequences in vibration are overlooked. So how is the epoxy grout able to dampen the vibrations if the finger point tips of shims are in the way? The hard spots of these shims will reflect the vibration back into the skid, pump and pipe system. Do the mechanical vendors know this is happening?

The whole question of shimming practices in Northern Alberta has been an eye opener and so very different to what I have seen and done in the past. My European and Middle East experience showed bigger effort to design for, and ensure, removal of shims. The only engineering attribute I have learned is that engineers spend more time on site, outside Canada, as part of their training, experience and learning curve to becoming licensed.

In Canada, I found engineers are not encouraged, welcomed or encouraged to participate in the construction team, so a very different culture evolves. The construction coordinators and managers see engineers as pedantic, erratic and inconsistent. These are the managers who want productivity on the board and cheerlead the contractors, ignore lessons learned and do not know how to improve specifications on future jobs. Yet if there is a problem on site, the engineers are blamed. It is the mentality of a school playground with bullies, layered with more borderline personalities. I cannot think of many engineers who can compete in this environment but we must! It is one thing to become licensed engineer but you have to be able to stand up and say whoa! Stop! You need the site experience of watching, learning, meaning of practicality and discovering the reality of specifications by others.

Shimming practices is high on that list.

Filed under: Uncategorized