Steel Bowstring Truss Design 1

[jammermatt1]

Greetings,

I'm getting ready to design a couple of different steel bowstring trusses. Length to depth ratios will be about 6 to 1. I will be using HSS sections. Configuration will be radiused top chord, and suspended bottom chord (top chord bearing), sort of in the Vulcraft frame of thinking. Max length will be 77-feet. Full-contact (no gussets) welded joints.

These are the biggest I've ever designed, so naturally I'm a little wary. I'll be using Risa-3D, and IBC 2003.

Oh, and over a pool, so corrosion issues are present.

Any hints, tips, or known pitfalls I should be on the lookout for? Or, can anyone recommend any good publications?

Thanks x 10^6

 

[JAE]

Bowstring trusses should have careful consideration regarding lateral stability - in other words, be sure to have good bridging - probably X bridging - detailed or required as they are less stable than flat trusses.

 

[jammermatt1]

JAE,

Thanks for the response. Good point. On that subject, I'm wondering if we need to specify some "TLC" when transporting, storing and erecting these fairly flimsly elements.

Thanks!

 

[JAE]

You could add a note sort of raising the "red flag" that the contractor should exercise caution in handling these trusses as they are inherently unstable.

Then be sure to add that means, methods, and safety are the contractor's responsibility.

 

[RARWOOD]

I would be interested in a good book on truss design also. I only design timber trusses, but a lot of what is good design in steel also apply to wood.

One book I have, is "Simplified Truss Design" by Michele Melaragno copywrite 1981. I wouldn't buy it, but if you can find it in a library it might be worth looking at. However, it is on a pretty basic level. You might also look into some old wood design books since bowstring timber trusses were common in the past.

I don't think you will have a problem with a span to depth ratio of 6 and a total span of 70'. Forty years ago it was not uncommon to see timber bowstring trusses span 150' or more.

If I was designing a steel bowstring truss, the part I would be unsure of is how to design the welds.

 

[jammermatt1]

Yeah, the welds are a whole other story

 

[Lutfi]

JAE's input is right on target. Bracing is critical so is lifting and handling.

You may want to consider cambering the truss. I would require camber that would neutralize the dead load deflection to start with.

I would use 2-D modeling. I do not see much value added for doing 3-D modeling.

The jitter bugs will go away. This is normal. Good luck.

 

[jammermatt1]

Lutfi,

Thanks. I was wondering about the camber issue - I didn't know whether truss manufacturers did it for bowstrings. My preliminary swipe at it resulted in about 1/4 to 1/2" DL deflection, and I was wondering what that might look like. I'm thinking that cambering might be in order. Even a tad of overcamber would help to mitigate the eye's natural tendency to see deflection in level objects. Also, a tad of visual camber gives you that sort of psychological warm-fuzzy that deflection doesn't.

Thanks

 

[structuresguy]

Wow, only 1/4 to 1/2" of DL deflection. That is nothing. I would never consider cambering for that minor amount of deflection. Based on my experiences, I would have expected something like 2"+ of DL deflection over that span, though I have never designed a bowstring truss.

I have however, designed 200 ft span trusses for a hangar, where the depth/span ratio was about 12. I had, if I remember correctly, about 6"-7" of camber called out for these trusses, maybe a little more.

As for weld design, if you don't already have it, I would get a copy of AISC HSS Design Manual. It is a very good reference on design of welded tube connections. And also a book called "Handbook of Structural Steel Connection Design and Details". Also a great reference for connection design. Here is the link to the latter book at amazon:

http://www.amazon.com/exec/obidos/t...f=sr_1_1/104-9404755-3091153?v=glance&s=books

There is another book which you might find helpful, called "Design of Building Trusses" by Ambrose. It is a bit simplified but might have some tips or lessons learned for bowstring trusses. It is generally a good reference book for ideas, but does not go into great detail on calculations.

Hope this helps.

 

[BFPartners]

You might spec that the truss installation requires an engineer or inspector to be on site during installation/ removal from the truck. You should perform an inspection before the trusses are raised into the air to ensure no damage to the trusses. I know of many cases where the trusses are damaged and only years later the damage is found.

 

[Gordy2]

I have a school that is just being completed where I designed 100' long, 17' deep bowstring trusses (depth was an esthetic issue, not structural). Construction bridging was by the fabricator and erector, and final bridging was two lines of angles on top of the bottom chords. The connections were fully welded and were by far the most challenging part of the design. I agree with structuresguy, no camber is required (or practical) for that amount of deflection. I also like his recommendations for connection design. You might also consider "HSS Connex", it's a free program from the Steel Tube Institute

http://www.steeltubeinstitute.org.

 

[jammermatt1]

Gordy2,

Wow, that's cool. Thanks a ton! We'll be doing fully-welded also. To complicate life, the architect has decided he wants full-width skylights in every other bay. So, now I have very little roof diaphragm, and I will be desigining horizontal X-bracing in the skylight bays to resist diaphragm forces and out-of plane wall loads (tilt-up with brick veneer).

Good times ahead!