Scaffold Erection Drawings in Canada

[P205]

I have been retained to produce erection drawings for scaffolding that is over 50 ft tall for a contractor and I'm wondering if there are design guides and/or regulations for the design of the lateral system (wind). I'm not concerned with the gravity side of things, it's more the tension due to wind loads that I'm concerned about.

Detail:
- Masonry repair work on side of 55 ft brick building
- Repair work, so no large loads due to stacking brick
- Scaffold structure will be tarpped over, therefore wind loads
- Walk-through frame system

This is what I'm thinking:
1) Determine wind loads as per Fig. I-8 of structural commentary (wind loads on wall elements).
2) Determine number of tension ties to the existing building are required based on factored wind loads over tributary area of lag shield anchor.

 

[Lomarandil]

It won't be binding in Canada, but ASCE 37 should provide some of the basics related to temporary construction loading. Just don't expect any grand revelations. If you can design it to permanent wind loads without incurring too much extra cost, that will be pretty close to what you'd get with a more "refined" approach.

It will definitely be a flexible system.. so tributary area should be fine.

Make sure you consider some unbalanced loading conditions, workers leaning on rails, etc.

----
The name is a long story -- just call me Lo.

 

[P205]

Thanks Lomarandil

I was thinking of using the 1/10 year wind instead of the 1/50 year. But that only reduced the wind loads by roughly 30%.

 

[P205]

The way my calculations are working out, I'm gonna need a lag shield at every level and at 4ft o.c. horizontally. Does that seem right to you guys?

 

[CANPRO]

Designing your tie-backs for a 1/10 year wind might be difficult. Too many ties make it difficult to get around inside the scaffold and creating a lot more for the scaffolders and the masons. Ties every 4' c/c would not be acceptable where I practice.

Typically, the tarps are attached to the scaffold with break-away connections (short bungee cords usually) that have a specified breaking strength. The strength of the bungees and their spacing on the tarp will dictate your design load for the tie-backs to the building. On your drawing, it will be very important to specify the maximum and minimum spacing of the bungees - spaced too closely, and the tarp won't blow away in high winds as expected. You should also specify on the drawing the expected wind speed at which the tarp blows off - this is a bit of a balancing act...you don't want the tarp blowing off at 50 km/hr winds, but as you've seen with your 1/10 year wind, too high of a wind load will make the ties unreasonable. Depending on local conditions and duration of the job, I've found 75 km/hr to be a nice comprise.

If the contractor is experienced with enclosed scaffolds, they'll probably already have break-away connectors they want to use. I've asked contractors before what they use for fastening the tarp to the scaffold and they've answered "don't worry, that tarp isn't going anywhere"...that is a gigantic red flag. If you ask that question and they say "we have 90 lbs bungees", then your're on the right track.

For the ties into the masonry - I strongly advise specifying some percentage to be load tested on site. You have to use your judgement on the frequency of the testing, but I'd test one probably every 20'-30' along the length of the building at the base, and then depending on the consistency of those results, require some additional testing as you move up the building.

 

[P205]

Thank you for all the great information CANPRO. Just as you suspected, when I asked about how they plan to tie the tarps down, they mentioned the break-away bungee system. So now I just need to hunt down technical specs/documents from the supplier.

 

[P205]

I should note, the 1/10 year design pressure is lower than the 1/50 year because it's a return period. Less likely of a high wind event in shorter return periods.