By Bruce MacKenzie – Sunday May 3rd 2015
ENGINEERS APPROVE THE DESIGN
Note: this is just one chapter in Solar on a Strata?
Before anyone installs equipment on the roof of our existing building, we have to know that it will be safe.
Of course, the wiring must be safe, but that is ensured by the Electrical Code, Canadian Standards Association (CSA) testing of the components,
the training and experience of our licensed electrician, and the local electrical inspector.
In this blog, I’m interested in three aspects of the structural strength:
- Is the roof strong enough to handle the ‘static load’ – the weight of the equipment just sitting there, in combination with possible snow, rain and people walking around ?
- Will the equipment resist ‘wind uplift’ – will it stay in place in a windstorm ?
- Will the equipment withstand ‘seismic acceleration’ – in an earthquake, will it slide around and damage other stuff up there, or will it fall off and hurt someone on the ground ?
The standard practice is to ask a registered Professional Engineer licensed to practice in BC to answer these questions. We have done so, and have received their answer.
1. Static load
Our building is from 1977, when it was still standard practice to build floors and roofs from dimensional lumber – i.e. boards cut from trees and nailed into place. Our roof is made of nominal 2″ x 10″ board that were installed at 16″ intervals – very common for that time period.
Our engineer told us that we are lucky to have this kind of roof, because newer buildings are designed with engineered trusses or manufactured H-beams that are designed only strong enough to hold the design loads. Our engineer has done the calculations and has certified that our 2 x 10s are stronger than they need to be just to hold up the roof itself and snow, so there is enough extra strength to hold the solar panels, mounting brackets, and ballast.
2. Wind uplift
Our proposed system only has the PV panels at a 10 degree angle. This is ‘flatter’ than most PV systems at our latitude (distance from the equator). The 10 degree angle makes it simpler to install, and it catches the wind less because all the pieces are closer to the roof.
The suppliers of our equipment provided a report on wind tunnel testing of their type of system and its mounting brackets. Our engineer is satisfied that our proposal will ‘resist wind uplift’ – safely stay in place on the roof – even in the maximum winds expected here in Victoria.
3. Seismic acceleration
The installation proposed for Central Park is ‘ballasted’ – the equipment just sits there, with weights to hold it down. This means that we don’t have to drill any holes through the roof, which can be a source of worry in multi-storey buildings. Ballasted systems are common in California, but not so much in BC. In fact, ours may be the first ballasted PV system in BC.
Until recently, building codes did not allow equipment to just sit on the roof, especially for calculating seismic acceleration. But in California, there have been so many installations of ballast mounted PV systems that the Structural Engineers Association of California struck a special ‘Solar Photovoltaic Systems Committee’, a subcommittee of the SEAOC Wind Committee, to determine how they could be safe without doing the same expensive testing over and over for each individual system. The results were two technical documents:
- PV1-2012 ‘Structural Seismic Requirements and Commentary for Rooftop Solar Photovoltaic Arrays’
- PV2-2012 ‘Wind Design for Low-Profile Solar Photovoltaic Arrays on Flat Roofs’
These reports give detailed guidelines and calculations for safely designing ballast mounted PV systems on flat roofs. Earthquake risk in California is similar to BC, and our engineers determined that our system meets the PV1-2012 seismic guidelines.
So it looks like we have the engineering approvals we need, and we can proceed to the next step.
Next in the series: Installation