How to Design Trusses in 7 Easy Steps
It seems most of us have a different methodology of what we are looking for when designing trusses. The intent of this article is to give you a solid, consistent method that you can use for any job of any type to gather information needed to complete the project successfully. Ready?
- CHECK THAT ARCHITECTURAL PLANS ARE COMPLETE
- Do elevations or sections include the necessary information to complete the project, such as heel heights, pitch, overhang, general loading, special loading and bearing elevations?
- Determine exterior veneer or cladding, i.e. brick, stucco, siding, etc.
- Check for windows that are arched or above truss bearing height and protrude into the truss area.
- Check for dormers & other architectural ‘specialties’ to determine if they are functional or decorative.
- Is there a foundation plan to determine interior bearing walls or points?
- Are all required dimensions supplied on the floor plan?
- Check for ceiling conditions, i.e. vaults, steps, trays, etc.
- Is there enough info to input porch entry beams?
If you can answer all of these questions to your satisfaction, proceed to step 2. If not, send out an RFI and put it back in the sales representative’s hands and start on the next project.
2. INPUT JOB-SPECIFIC SETTINGS INTO MITEK
- Start a new project and proactively set up your defaults.
- Are job, lumber & plate settings up to date?
- Be sure to verify client-specific templates and requirements are being considered and adhered to.
- Go into Sapphire and set up visibility switches and specific criteria for the job.
It is critical that you spend the time now ensuring that everything is set up properly so you don’t waste time later and have to go back and “re-do” something!
3. LAYOUT WALLS
- Determine if brick veneer or other cladding is to be shown on layout & how draftsperson has chosen to dimension the walls (either from face of wall or face of cladding).
- Determine exterior elevation height changes; “break” walls and dimension the breaks.
- Add exterior load bearing walls (watch for width changes of 2×4 & 2×6 walls).
- Add any interior bearing conditions or ceiling conditions and dimensions to properly location and confirm them.
- Determine if any beams are to be upset/upturned.
- Determine if any headers or LVL beams needs to be input and if they are upset/upturned.
- Are there any girder placement call-outs by builder that need to be considered?
- Add a wall height legend, hatch each different wall type and add notes calling out heights.
- Dimension all walls and confirm all dimensions with plans.
By doing these things in this order, you are building in a check of the previous items. Once completed, move to step 4.
4. INPUT PLANES
- Determine heel height, pitches & overhang length using your heel calculation tool.
- Determine of you need to hold back overhang for fascia board.
- Determine if a cantilever required over exterior veneer or cladding and input that cladding properly.
- If no raised heel height required, base nominal heel height on lowest pitch plane.
- Add planes to walls and solve each plane.
- Match up fascia lines to lowest pitch plane and double-check it to your manual calculation.
- Use 3-D view and review to confirm if planes match plans.
Take the extra time now to check these details out before proceeding. If you don’t, you will be sorry later!
5. LAYOUT ADD TRUSSES
- Determine truss layout and placement (i.e. commons, girders, hip setbacks, gable ends, piggybacks, etc.).
- Are any trusses a long-span, requiring a bigger chord size (and thus a potentially larger heel)?
- Re-calc top and bottom chords where necessary to match design criteria and shop preference.
- Study plans for attic access openings that need to be considered.
- Add in “containers” for openings such as attic, chases, etc.
- Use 3-D truss to confirm no trusses are sticking out of plane of roof or ceiling.
- Once all the above is confirmed, label trusses either in “build” order or in “walk-through” order.
Again, take the extra time now to check these details out before proceeding. Do yourself a favor and put as much as you possibly can into the layout properly so your trusses come in to engineering with the condition.
6. ENGINEER TRUSSES
- Get out a blank piece of paper and write down splice, webs and other lengths you want to use over and over. This will come in handy later, especially on a big job with over 100+ profiles.
- Do yourself a favor and turn on the analog view of your truss so you can see what’s happening. This will also show you when you mistakenly input a web as the wrong type, such as an “M-Block” as a web instead of a chord.
- Web, splice and plate symmetry where possible, not just in that specific truss, but throughout the job.
- Make sure all diagonal webs under hip flat chords are the same length.
- Ensure the bracing you have engineered the truss with is reality.
- Does the job require dropped top chords for purlins?
- Have piggyback loads been properly applied?
- Ensure plating does not protrude outside of truss (happens occasionally on high heels).
- Are all uniform and concentrated loads applied for girders, openings and attic conditions?
- Check trusses while engineering or on engineering sheets for TC/ BC CSI’s and deflection between trusses.
- Continue process until all trusses are designed.
- Perform a self-check using your “QA” tool (truss quantities, overhangs, loading, heels, profiles, etc.).
This is where you need to study your design criteria in detail. If you do not have a QA check sheet, get one! Chances are that you company is spending more on repairs each week that it would cost you to contact GDI and purchase the interactive version of this excel sheet.
7. FINISH DETAIL WORK ON LAYOUT
- Add spacing & setback dimensions.
- Truss span dimensions.
- Call out girder plies.
- Notate hangers/beams/hardware on layout with a legend.
- Hatch all stick-framed areas.
- Note on layout all questions, concerns or notable facts that need to be verified or considered.
- Call out any deviations from plan or plan errors.
- It is even helpful to put end details and any unusual conditions on the layout.
Note: Some designers choose to do some of this after trusses are input, which I totally agree with. It’s just another way to catch something you may have missed or were distracted away from. Either way, there will always be something you need to add after you are done so I listed it at the end.
I hope you have found this useful and it will allow you to increase your performance and productivity. Feel free to contact us if you would like some tools to enhance your design efficiency. We have many more than just the 3 represented here.
Christopher Gould – President
Gould Design, Inc.