7 Surprising Facts about Complex Truss Designs
“That’s a lot of trusses!” The wide eyes and stares at multiple tractor-trailer loads of trusses, slowly being slid onto the site of an emerging new home or commercial building, is a unique kind of compliment to a truss designer. It is true, large projects with a huge volume of material can sometimes be difficult or unwieldy for a designer to manage and successfully execute, and they certainly look impressive; however, an open secret in the design community is that sheer size of a project is not most reliable indicator of the challenges we face.
On a recent large, Canadian project, I got to thinking about real challenges of a complex truss design, and the surprising truth about crazy and complicated projects.
#1: Simpler is Stabler
Some rules are unsaid. In the truss design business, beyond the mathematical analysis, we try to maintain a larger view of a truss system which considers multiple factors:
- Ease of installation
- Stability during manufacturing
- Stability during transportation and delivery
- Erecting during the craning and installation process
- Longevity over time during years of weather and extreme conditions
It is an unspoken rule that complex truss designs need to have a good chance of surviving each of these events. Simple trusses with strong, triangular shapes are inherently stable; the triangle is the very strongest geometrical shape, and trusses leverage this axiom in every way.
With a complex truss, however, the odd and asymmetrical geometric shapes, as well as the frequent difference in height (elevation) between supports, means that the production, handling and installation of the truss can become extremely awkward. The challenge of successfully executing a truss like this can be met creatively, including changing the geometry of “base” and “piggyback” cap trusses, the use of ‘temporary’ truss members which are identified in the software as ‘non-structural’ and marked for removal on-site by the erection crew once the truss is safely in position, or perhaps designing one large truss in several pieces which can carefully be assembled in place.
#2: Crazy Trusses = Crazy Reactions
Sir Isaac Newton states in his 3rd Law states that for every action there is an equal and opposite reaction. This is utterly true of trusses! Wherever a bizarre-shaped truss rests on a bearing, it can throw off loads and reactions in awkward directions.
While a component designer is not always responsible for all the effects of this reaction, there is a level of wisdom and awareness of these reactions that can mean phone calls to the builder or architect, taking a second look at the parameters of the truss design such that these reactions can be reigned in, or perhaps the suggestion of creative solutions to counteract the problem.
On one very challenging set of non-symmetrical scissor trusses which were generating large “horizontal” reactions, we suggested we counteract the thrust of the truss pushing outward on the wall but building an engineered beam into the top of the wall framing… turned sideways. A notch was built into the “toe” of the truss, and the beam distributed this horizontal reaction to the side walls of the structure where it could be effectively counteracted.
#3: Trusses Might not be the Weak Link
Often with a very challenging truss design, the connections between the trusses, rather than the trusses themselves, are the weak link. On this particular project, the design of the foundation led to a situation where a tremendous amount of load accumulated in several critical places, to the point where no commercially-available hanger was available to make the connection. An unspecified, custom-designed and engineered connection detail was called for to successfully execute the design, even though the trusses were able to be withstand the stress without a problem.
#4: Coordination of Information is Exponentially Harder
Simple truss designs have only a few variables or “parameters” to consider. Resolving conflicting or contradictory information on such jobs is relatively simple, as there are only a few related variables which meaningfully interact. On a large and complex truss design however, a large set of inter-related variables come into play, such as:
- Bearing lines
- Load path
- Heel height
- Relationship of floor systems to roof systems
Any of these can threaten to turn the design process into a nightmare of unfinished conversations, partial solutions, or un-coordinated decisions which can slow the project to a crawl. This is a mathematical problem: as the number of variables increases in a linear fashion, the number of relationships between those variables increases exponentially. All mistakes and interferences are exactly that – a problem in the making – so the sigh of relief that follows a well-executed project is not unlike the first assembly of a Boeing 747, where the entire airplane comes together with no hiccups.
#5: Revisions Are the True Challenge
Related to the “exponential variables” problem from above is the further issue of revisions. This can be related to a number of factors, such as:
- The greater risk of encountering adverse conditions in the field
- The chance of ‘hiccups’ and mistakes in the plan
- The greater number of details that require approval from a homeowner or building committee
- The building owner changing their mind and wanting something else
- Or (my favorite) just plain bad luck
Large and complex truss designs often go through multiple rounds of revision and adaptations. Here is where the ability to manage information and be an effective team communicator becomes critical for the truss designer!
The process of helping to iron out details, implementing a constantly evolving design strategy, and keeping a cool head while encouraging a cheerful and positive outlook in everyone else, is the true challenge of a component (truss) designer. None of this grueling ordeal is visible in the end product, but managing and navigating revisions it is perhaps the one greatest variable in the success or failure of a complex truss project.
#6: Complex Trusses Still Require a Great Framer
Even after all the work required to fully detail a complex truss design is complete, the truth is, a great framer may still be needed to bring the project across the finish line. The old skills of calculating a challenging rafter layout with a framing square may be getting harder to come by, but mathematical ability, experience and a canny eye for detail are still the ‘secret sauce’ for a successful truss installation.
Many architectural details simply are beyond the ability of a truss to reproduce, or are simply not worth the trouble and risk of misalignment in the real world. Trusses cannot stand to be “tweaked”, and so a great framer is a great asset to take a detail like this flared skylight and build the proper interior pitches into the rough “box” left in the truss framing, calculated by the designer to allow for seat cuts and “wiggle room” for the framing crew:
#7: Lots of Trusses = Lots of Documentation
In the same way that successful managers often feel they do nothing but sign forms and push paper, a professional truss designer at the top of her game may often feel she does nothing but manage the flow of documents and information. The documentation can come is numerous forms, such as:
- Unintelligible emails
- Hasty texts
- Blurry photos
- Crude sketches
- Illegible scans
- Voicemail ping-pong games
- Contradictory plans
- Lost change-orders
- … The list goes on an on
All these are the lifeblood of a complex component project, as each of member of a large and diverse team of players try to bring their needs and knowledge to bear on the final product. Some of the most important players do not even seem to take much interest in how critical is their input! Publishing and extracting documentation from other people can make the life of a designer feel that they are “herding cats”, but they know the value of keeping their eye on a final product into which each member of the design and construction team has had their full say. The value inherent in a well-executed truss design does not lie merely with the designer; it lies in the accumulated insight and coordinated wisdom of the entire team, expressed in the final product.
At the end of the day, all of these factors have to be taken into account for a seamless finished product. So many component (truss) salesman and managers out there simply do not understand the critical factors involved in this process. This is unfortunate, but true. Ask any truss designer who has worked on a project of the size and difficulty as the one I have presented here and they will confirm this as fact. Hopefully articles like these and your comments below can help to open some eyes to this sobering reality.
Jonathon Landell – Project Manager
Gould Design, Inc.