Work Access Walkway

[lsj103011]

I work at a fabrication facility and need to design a gangway for workers to safely access a project. The span must be 120LF which if I follow the building construction code of L/360 I get 4.0in max deflection (delta). Given my load conditions the required Ix=15,600in^4 which is very large (W40x211!). Deflection is my controlling case.

Am I being too conservative on my deflection? Does anyone know what the general rule of thumb would be for TS&CD that would satisfy any inspection and give workers good access?

FYI: I am a young engineer so keep that in mind before you say "i can't believe you engineer these and dont know!" I do have bosses that review these....

 

[Once20036]

120 feet is a huge span and regardless of any other criteria, you're going to get huge sections.
Are you looking at deflection under live load? dead load? combined? What live load are you using? Have you reduced the live load at all? You probably won't be loading all 120' at the same time.

I don't have a code book in front of me, but I believe that L/360 is the standard when you have sensitive finishes (plaster, glass, etc) that could be damaged by the deflection. I'd consider L/180 to be the starting point in an industrial facility.

 

[dhengr]

A walkway of some sort, spanning 120', with people and some equipment as loading. It probably needs some handrails too. Why not make these handrails into two trusses with floor framing btwn. the bottom chords, or at some height above the bottom chords. You will also have to study the bracing of the top chords, but they potentially cantilever up from the floor framing, on the vert. and diag. truss members. You also must watch baluster spacing, kick or toe plates, handrail loadings, etc. I think OSHA has something to say about this type system.

 

[SlideRuleEra]

I agree that a truss design is like best for a 120' span. You could consider a manufactured span, such as:

http://www.excelbridge.com/for-owners/bridge-types

(I am not affilated with this company).

Even is you decide not to go this way this will give you some ideas of cost and design issues for elevated walkways.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[bridgebuster]

Does the 4" include dead load deflection? You can camber the beam for DL; then you just need to deal with LL. If your walkway isn't supporting a ceiling, utilities, etc, you can make a judgment call on the allowable LL deflection.

A truss might be the economical way to go. 120' is pushing it for a rolled beam, in terms of economy. We recently did a bridge with a 145' span using rolled beams - clearance issue - and it required a very tight beam spacing.

Take a look at AASHTO's guide spec for pedestrian bridges. It gets into LL reduction and vibration criteria, which is important for a ped bridge.

 

[BAretired]

I agree with dhengr. Use a truss on each side of the walkway extending to top of handrail. A total truss depth of about 8 to 10 feet should improve the deflection quite a bit.

Use a horizontal truss at floor level to carry lateral forces over the 120' span.

Consider the natural frequency of the structure and avoid resonance problems.



BA

 

[hokie66]

A truss solution would be best, I agree. If height permits, make the bridge deep enough so that the top chord can be braced by a horizontal truss as well. Similar to a conveyor gallery.

 

[SAIL3]

as dhengr mentioned the truss option seems to be the optimum solutuion...this always brings up the problem of bracing the top member of the truss in the horiz direction....I have, in the past, extended a horiz bm @ the walking surface level out 1 1/2 to 2' beyound the truss and added a vert brace @ 60 to 70 degrees up to top memb and count it as bracing the top memb in that position...in other cases of bracing requirements, providing a brace to resist 5% of the compression in the memb is sufficient...in this case, however, having a compression memb braced at repeated intervals by a bracing system that depends on flexural rigidity of the bracing, for some reason, leaves doubt in my mind, as to the overal adequacy of the stability of the compression member.....I have not been able to put it to rest, one way or another....

 

[bridgebuster]

The AASHTO guide spec for ped bridges provides an example for designing a pony truss. It's also covered in "Guide to Stability Design Criteria for Metal Structures" - Galambos (editor).