Pedestrian truss "pony truss" 10

[Gus14]

My boss wants a pedestrian truss like shown in the attached picture to support 6 meters long floor beams {W 5*16). He wants the pedestrian bridge because it can serve as a hand rail and not decrease floor clear height. I did the analysis and found that the maximum axial load in the top chord 100 kn in comp and in the bottom chord 100 kn in tension. I chose section HSS (2*2*3/16) for top, bottom chords, diagonal and verticals FY IS 50 ksi. After revising the AASHTO GUIDES for pedestrian bridge i found that I need to weld another ( HSS 2*2*3/16 ) to prevent top chord buckling and be conservative.

1) in the ASSHTO guides stats the following : the connection to the floor beam to the vertical shall not include the HSS chord member, i.e. the vertical and the floor beam shall not be connected to different sides of the HSS chord.
SO WHERE SHOULD I PUT THE BOTTOM CHORD IS IT POSSIBLE TO PUT IT ON TOP OF THE FLOOR BEAMS AND HOW FROM THE INSIDE OR OUTSIDE or in between verticals ?

2) I really have no idea how to design the gusset plate connection any simple reference would be very appreciated. And any conservative answer is also welcomed.

 

[BAretired]

The design live load has not been mentioned to date, unless I have missed it. What kind of vehicles will be crossing this bridge? Are there wheel loads or other concentrated loads which need to be taken into account? It seems extremely wide if it is solely for pedestrian traffic.

Why is the client so fixed on limiting the beam depth to 140mm? Does he realize that he will save money, not to mention improve performance by using deeper beams? If not, he should be told.



BA

 

[Gus14]

Its not a bridge. its a truss that will support the ends of a 6 meter spaning W5×16 floor beams in the transverse direction.
A welder gave him an offer that is half what a good design costs and that uses only 14 cm deep beams.

 

[r13]

You have a 6 m long w5x16 floor beam spaced @0.4 m on center, simply supported, and deflects 70 mm. If I didn't make mistake in calculation, the beam load is about 790 plf, and the floor load is about 600 psf. I don't think this bridge is for traffic, where is the load came from?

 

[Gus14]

The softwate section database I used was edited and it gave me a very low moment of inertia. I went back to my AISC Manual and deflection is 30 mm. Well that will be embarrassing but still its too much.

 

[r13]

IBC table 1607.1 specified uniform live loads
1. 250 psf for vehicular driveways,
2. 60 psf for walkways and elevated platforms,
3. 100 psf for yards and terraces, pedestrian.

After your finding, you still have a design load of 800*(30/70) = 343, say 350 psf. Is this the true/correct load?

 

[dhengr]

Gus14:
This whole design scheme seems all messed up and ill thought out, as mentioned above, in a number of ways. I believe you said your boss would handle it from here, and I would gladly let him deal with the dumb/difficult client. Better yet, let the welder design and stamp the drawings, since he’s going to build it his way and at his price. Then, when things go wrong, you won’t be stuck in the middle. When a client hires you and then won’t allow you to do a proper design; won’t listen to your advice, but rather takes someone else’s poor advice and asks you to bless it and make it work, that’s a good time to look for another/better client.

 

[Gus14]

not really the total serice loads are 8.4 kn/m2 (175 psf). More deflection could be predicted because they will probably store wood (They said they wont but I don't really believe that).

 

[STrctPono]

Gus, what is this structure? You say it's not a bridge and yet it is only meant to support some 6 meter spanning W5 x16 floor beams. Can you provide a better sketch? More than just the elevation that you showed in your first post? Can you describe it in more detail?

 

[BAretired]

Right on, dhengr!

BA

 

[Gus14]

Yeah, I'm done with it. I was assigned to another project already. I think they will even use a british ( IPE 140 )section which is half the moment of inertia of my section (w5×16) to save money They will solve deflections somehow.

 

[r13]

If you can use w6x16, then Ix = 32.1, which is 50% greater than Ix = 21.4 for w5x16. The resulting deflection is now 30*(21.4/32.1) =20 mm (L/300). You can further go to w8x15, the deflection would then be 13.4 mm (L/450).

 

[TrussBridgeboy]

One other thought since you all brought up K-factors - We generally use a K factor of 2.0 for the truss vertical in the out of plane direction unless you have a good reason to use something less like 1.5. You can use a K=0.8 or 0.85 for the in plane K-factor for the truss verticals.

 

[hoshang]

You have a 6 m long w5x16 floor beam spaced @0.4 m on center, simply supported, and deflects 70 mm. If I didn't make mistake in calculation, the beam load is about 790 plf

Please see the attached:

https://www.screencast.com/t/olTB1Twbt0nn

or:

https://www.screencast.com/t/jOQWXWfw

 

[r13]

hoshang,

Yes, I made mistake in my calculation. The total beam load that causing the deflection should be 474 plf. My results differ from yours is due to the parameters used, L=20', δ=2.75". Also, as we are back check the applied load, there is no need to add beam weight again. Thanks for checking though.