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.
