Loading Trusses for Rooftop Mechanical Screens
Truss design involves the consideration of an incredible amount of variables and details; one of those details is the use of mechanical screens on roofs. What are these you ask? For more information, click here or here.
Typically mechanical screens are used as an aesthetic element to hide, in this case, roof top equipment from view. In high wind regions they can be used to reduce the load on rooftop equipment. Some screens are fairly lightweight while others need to be factored into the loading of the roof. Also, if you’re a secret agent and need to parachute onto any roofs consideration of these obstructions would be vital!
While going through the plans it’s important to highlight areas where something is anchored to the roof and may add additional dead load and uplift forces, such as (but not limited to):
- HVAC equipment
- Ventilation hoods
- Tie off anchors for construction and maintenance personal
- Rooftop screens
Along with highlighting the areas in question it’s a good idea to take notes of things to look into later so that you don’t bog yourself down with small details at the outset; just flag it and move on. Once you’ve completed your intake you can go back through your list item by item to find the information you need. As you do so you may find detail drawings or specification pages that are associated with that particular item, make a note of those as well so you can go back and find them easily.
Since nearly all the plans I receive to design from are in PDF format, I have created a method to assist in my determination of each project’s need. I find it helps my efficiency to create my own bookmarks as I do my first pass through the plans so that I can easily navigate later and find the information I need in order to design the building components. I do this on residential projects with 10 pages and on commercial projects with over 100 pages. It becomes more important to record which pages you need as the amount increases.
Regions that experience high wind events, like hurricanes, will be more particular about the loading and design of rooftop equipment and mechanical screens, so it’s more likely that you will find the information you need on the plans. Other regions may forget about the equipment altogether. Accounting for the dead load is only one component, the potential uplift and the sideways push/pull on the roof can lead to damage to the roof as the object is torn out and delays occupancy after the event. Rooftop screens in these regions are designed, in part, to reduce the loading requirements of the mechanical units on the roof as well as protecting that equipment from high winds.
Here I’ve highlighted the mechanical screen on a project I completed recently:
As I explored the plans I didn’t find any information regarding this screen. This was a “bid set” so most likely they didn’t know what they were going to use yet. It was in a region where the primary use would have been aesthetic. In this case I just noted on the layout that the additional loading was not included for the mechanical screen and that it should be looked into before any production trusses were considered. Had this been a “production ready” design, I would have taken the necessary steps and sent out the necessary RFI’s to gather the information.
The main object in this case, for this customer, is to let the decision makers know what has been and has not been taken into consideration in the loading of the roof to allow them to take the appropriate steps going forward.
What has been your experience with mechanical screens? Have you seen the design standards increase over the years? Let us know below!
Tim Hoke – Design Professional
Gould Design, Inc.