@scribldogomega

Saturday, March 30, 2013

TRUSS ME



Revit 2014 is coming up quick, here are a few links to what’s new. I am not going to rehash these posts. There are some awesome enhancements to the product.

Truss me, I know you are busy people.

A year or so back I was sitting around a table with some of the brightest minds at Autodesk and in the BIM world at an event on the east coast. I remember distinctly being asked by Autodesk, “what do you see us needing to work on structurally with Revit”. At the time, I didn’t have an answer. Maybe I was bit under the weather ;), maybe a bit sad at the ending of the event and saying goodbye to new friends, maybe a bit intimidated at sitting at the same table with the likes of Havard Vasshaug (http://revitnorge.blogspot.com/), Aaron Maller (http://malleristicrevitation.blogspot.com/), Michael Sullivan, Graham Stewart and so many others. If you have had the pleasure of being in a round table discussion with the likes of Aaron then you will know how difficult it is to convince yourself that you should interrupt his thought stream/conversation with what now seem like small kindergarten ideas forming in your head.

So I digress, at the time I couldn’t form a sentence. Now, a year and some months later, after having been immersed in daily Revit work, and a six month stint trying to train multitudes of people to not fear but embrace the technology I may have an answer. Trusses! The specter of having to model 1.6 million square feet of an existing industrial facility and setting the modeling standard for all consultants that will be using the model in the future has my heart skipping beats, or maybe it’s the coffee.

 Anyway I started this document to point my finger at trusses in Revit and say …see, see…this is what you need to fix, this sucks, waaaaah (see there I go reverting to childlike behavior again). But it’s turned out to be a document on dealing with the pitfalls. Hopefully Autodesk will pay some attention and address some of these issues and It will be as easy as creating my profile in the truss template, loading it into project, assigning the members, and having everything look beautiful and not break when I go to present my model to the client. 


The problems (as I see it) with Revit trusses:


  •   Plan representation for trusses with overhangs:
This is a big one for me. It may seem petty and I have found workarounds online (below). The symbology breaks back at bearing points making it difficult from plan to tell where the two conditions pictured below may stop and start. On a massive project this would be a true pain in the butt to deal with

Condition A: a truss sitting on top of two bearing walls.

Condition B: a single member outrigger and a joist. 

To me, this leads to basic missing functionality that could be added to the truss family templates
·         There is no plan representation for the family(except in the family template you are building it in what looks like a plan view.(see screenshot below)
 

This could probably be fixed with a small amount of code (sorry programmers, I know that is a way oversimplified statement and probably likely to anger a lot of you)

But basically what we need is a toggle. Plan Symbol representation breaks at bearing or not.




  • Visibility parameters set for members don,t work in the project environment.


I created a viz parameter for my overhangs, there are some conditions where there isn’t one or the other. A workaround is to simply create separate trusses for all the conditions, and I suspect that the embedded nature of the members being added in the project environment is what screws this one up. 

viz parameter in family for overhangs
 
This doesn’t work in project environment. 


Everyone knows and bitches about the next one.

  • Symbology does not show if there is a roof in your plan.
 Quick fix: set roof transparency to 1. 




  •   Truss height calculations

The truss height dimension in Revit is between references for members set in the family. So depending on what member you use for your top/bottom chords you will have to do some work to get it to fit correctly under your deck. For the example below my truss height at the peak was 15’-0”, my deck thickness was 1 ½” but measured at the ridge point it is 1 9/16”

There are two methods I use to get my truss to be all snug under the deck:

·         Unpinning the top chord and aligning it with the b/deck. Unpinned members will remain a part of the truss system when moved, copied or arrayed.

Or

·         If you don’t like the thought of unpinning members this can be completed using some math. (which If you have multiple trusses with variable heights/corridor heights/ridge distances from bearing points this quickly becomes a large amount of work)

In a perfect world you could subtract ½ of top and bottom chord and be done.
But Revit calculates that peak point right at the peak, not in a plane perpendicular with your top member.


 

(deck bearing is at 14’-10 7/16”- 4 ½” = 14’-5 15/16” you will find that if you plug in that number it will be a few sixteenths off.
You will need to measure your top chord member from the center of the reference to the peak tip, which adds a few sixteenths and would lower your top chord by the same amount. 
 
 


So our true “truss height” needs to be 14’-5 ¾” to fit.

That’s all fine and dandy. You now have a truss that works pretty well after jumping through all the hoops.

But, let’s just say the engineer comes back and says…oh we need that top chord to be a 2X6….and we have to change 7 types of trusses with variable heights and peak offsets….yikes! (this is where I lean way over towards unpinning and aligning the top chords.


Autodesk can we please get an easy button for truss height calculations? Please?

  •  Attachment disorder for custom truss profiles


God forbid you get pissed off at all the craziness and just try to attach this truss to your roof deck now.
Revit will give you the big middle finger and mash your truss to pieces. I am sure this works fine with basics trusses with no overhangs or customizations; in fact there are many YouTube videos to support that. But here is a real world example
 

 


You get something very similar if you try to “edit profile on the truss. I did try creating the overhang as a separate truss but that presented a whole slew of problems with plan representation/grouping in and of itself. Another option, more problems.

 
Fixing Plan symbology

Fixing the stick symbol break back at the bearing point can be achieved by unpinning the two bottom chord members and creating a bit of an overlap. (if your symbol location is set to bottom chord)



Truss distribution
I think most people are grouping and arraying trusses. This works well but I have seen it where once you try to edit the family and load it back in members start to disappear in fine view (bug?)
Recently I tried creating a truss as a structural framing family which worked well for distribution as I could use it in a beam system, the drawback with that is that you cannot do a BOM or material takeoff of all of the members of your trusses. So if you are just going for a pretty truss with opportunities for spatial coordination then this may be a better option. It does make placing the “truss” easy as you can set the bottom as the bearing.
Adaptive components work well if you are just looking for a general layout of the truss profile. You can’t use typical structural member profiles in adaptive components and you can’t generate a BOM if you need to later either. I love AC’s but I don’t think I will be able to use them for this.

Conclusion
I will probably be using the OOTB truss families if we win the project we are pursuing, but I am going to have to decide on best methods and lay down the law in a document for all consultants to follow.

Below are a couple of trusses I created as structural framing families. The corridor piece and peak needed to be adjustable so this worked best in this case. The overhangs change like crazy along the footprint of the building so I need a ton of flexibility in all views. 




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