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RE: HSS Connections - Not Conventional

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Thomas,
 
Thanks for the input.
 
As for trying different truss configurations, I really exhausted that option.  My problems, unfortunately, weren't coming via the web members.  It's basically getting members good enough for the top and bottom chords.  And, according to the contractor, making the truss deeper isn't an option because that would change his estimate for roof sheeting.  *shakes head*  I am beginning to regret taking this task on.  And nothing pisses me off more than a contractor that gives me the money sob story when he went ahead and bid the project before consulting with an engineer.
 
Thing is, his idea is pretty good.  He uses conventional wood framed construction, but instead of using wood products, he uses all steel and welded connections.  Really not a bad idea.  But there are some things, like a 50' simply supported mono-sloped roof truss, that seem to fall outside his realms of comfort.  Hence the phone call to me.  And now I have to call him and tell him he has to spend more money on materials.
 
It kinda reminds me of an episode of Hell's Kitchen.  Chef Ramsey was asking for food to be done in a matter of 4 minutes and one of the apprentice chefs said, "It's impossible to cook anything in 4 minutes.  It defies the laws of physics."  That's how I feel when I talk to this guy and he keeps asking, if I can do more to get it to work.  No, I can't.  It defies the laws of physics.  I cannot twist and bend the rules of logic in my profession so that you can stay within your budget and still get an adequate structure.  AHHHHH!!!!  TGIF!!!!
 
Thanks again.
Dave Maynard, PE
Gillette, Wyoming
-----Original Message-----
From: THunt(--nospam--at)absconsulting.com [mailto:THunt(--nospam--at)absconsulting.com]
Sent: Friday, August 19, 2005 1:50 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: HSS Connections - Not Conventional


Dave,

As long as your analysis is sound and connections well detailed you are probably OK.  One thing I would consider is that if your sandwiched connections start getting rather large with lots of welds your member ends may be more fixed than pinned.  You may want to run your analysis with both pinned and fixed ends.  There may be significant moments at the ends of your members, depending on your configuration, which may need to be considered in your connection design.

If the members have not yet been cut, you may also want to look at other types of truss configurations.  Changing the strut direction could possibly put the critical members in tension rather than compression.  If the cords are the problem, making the truss deeper might help.

Thomas Hunt, S.E.
ABS Consulting




"David Maynard" <davemaynard(--nospam--at)ceincorp.com>

08/19/2005 12:40 PM

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HSS Connections - Not Conventional





Sorry for bugging everyone with these problems, but if there is an area of the AISC design manual that isn't extremely clear, I believe the HSS one to be it.  Then again, it could be the lack of clarity results from a VERY long week.  Never the less....
 
My contractor made the brilliant move of estimating a project that he is doing based on, what he thought were, members sizes and geometry that would suit the purpose of a 50 ft mono-slope roof truss.  He's using rectangular tube shapes, 2x4x14 ga., ASTM A588 (got the mill specs on this one to verify).  However, after running several analysis on varied geometries, I can't get a single 2x4 to work for his application, with respect to the top and bottome chords.
 
His next option was to double up the members on the top and bottom, so that two members would share the load.  His connection option was to have the web members "sandwiched" between the chord members, much like a typical steel bar joists (bar ends are connected between the vertical legs of the angles of the chords).
 
I have never done this type of connection and I was hoping to get a little assistance on this.  If I can develope enough weld to make the connection (which may be a feet in and of itself, seeing as how the walls are so thin), is this connection a viable option???  And if so, what sort of local stresses should I be checking for this condition???  It isn't very conventional, but this is something that the contractor is looking into in, what he believes to be, a cost saving option.  (I tend to not argue with contractors about "cost savings" when they come to me with this type of post-proposed problem and ask me to "make it work")  Any assistance would be greatly appreciated.
 
Sincerely,
David Maynard, PE
Gillette, Wyoming