Walkway Truss or Walkway Beam?

[CurlyQue]

At what distance is it more cost effective to use a truss design for a walkway bridge versus a beam design with moment frames for the tube all cross braced?

I am told 80'.

Parameters: 60' span; 6'x8' walkway; enclosed on 2 sides with storefront; LL:100psf TL:127; exposed roof deck with insulation above. Columbus OH. Wind: 90 mph, Importance 1, exposure 1, Thermal 1; Seismic: Category C, Sds 0.205, Sd1 0.133, Site: D.

Present design calls for 2 W33x130 main beams with W16x26 cross struts at 6' o.c. with L6x6x1/2 diagonals with HSS8x8 vertical 'columns' with HSS8x6 cross struts over and L4x4x48 diagonals. Deck is 5 1/2" slab including 2", 18ga deck. Support towers are 12" tubes with L6x6 cross-bracing between each floor up to floor 4 at 30'-8"

I'm also told that the drift is 2" and that the thermal expansion is 2". I highly suspect the 2" thermal.

Do engineers ever say 'no' just so they won't have to redesign something knowing a client won't know the difference but will check with an engineering forum or call another structural engineer?

 

[JedClampett]

There's no hard and fast rules on replacing beams with trusses. It's more of a common sense, how does it smell type decision. If I was using W33 beams, I would probably step back and consider it. For a 60 ft span, you could pretty easily set a truss (three or four feet deep) with WT's top and bottom and double angle web members and save quite a bit of weight. And even though the fabrication is detailed, the price is usually bid on weight, so it might be proportionally cheaper.
For a walkway, you have the advantage of using the truss for guardrails.
As far as your second question, it depends. If someone who could fire you, or affect that, is telling you to redesign something, you better do it. Are your just being stubborn? As structural engineers, we very seldom provide the end product. It's for another discipline's use, like an architect or a process engineer. If they're not happy, you're not really providing good service.

 

[ztengguy]

Bill them if they change the design. What do you care as long as your getting paid. Scope should have been provided up front before design really got going.

 

[CurlyQue]

Thanks Jed,
I've been called Jethro.
So a full depth truss probably isn't economical as a solution from what I gather from your post. I like the idea of haveing the top be a guardrail of sorts. Sometimes with structural engineers I feel like a patient who isn't going to question the doctor's advice. I'm always free to get a second opinion.

 

[CurlyQue]

ztengguy,

I agree. You should get paid for a change. The scope was up front: design a bridge. The building budget is 25% less so I'm looking for ways to save cost. The engineer said he'd do one more design but that's it. The thought was a truss might be more economical. The response was, "No, not under 80'." But I agree with Jed, that a truss for the beam might be more economical.