Truss Top Chord Lateral Support 1

[hoshang]

Hi all,
For a Pratt truss, lateral support is provided by a transverse U-frame consisting of the floor beam and truss verticals. What about a Warren truss? It doesn't have verticals.

 

[hoshang]

Hi BA,
I meant can points A and B in this image be considered as support points?

or these points (my A and B) can be assumed as one point so that there would be 3 points of support: one at each end and one at the middle so it can mimic the analysis model (a two-span truss supported by three points)?

 

[BAretired]

Your points A and B are necessary supports. So there are four supports in total.

The middle truss acts like a doubly cantilevered beam with a truss reaction R at the end of each cantilever. See below.

 

[hoshang]

Hi BA,
My thought is this

analytically maybe represented by this for simplicity (i.e., two hinges and two rollers):

Am I right?
My query is what would mimic this (i.e., two rollers and one hinge):

 

[BAretired]

Hi BA,
My thought is this
View attachment 15328
analytically maybe represented by this for simplicity (i.e., two hinges and two rollers):

Yes, that would be possible.

View attachment 15329
Am I right?
My query is what would mimic this (i.e., two rollers and one hinge):

1. Two rollers and one hinge would be possible, but a pinned support would need a stiff column below it, fixed at the base and pinned at the top. A pin can't exist by itself.
2. The entire structure would deflect with the column in addition to temperature movement.
3. Each truss would span 90' instead of 60', so the trusses would use more steel.
4. Erection would be considerably more difficult without a middle truss held in place while erecting the two exterior trusses.

View attachment 15330

 

[hoshang]

post #38

BAretired states:
If there is sufficient clearance for road widths, I suggest the vertical leg of the orange Tee be 2 panels wide. Alternatively the orange truss should be modified, or a different concept should be considered.

Can you elaborate more on the your red phrase? BTW how would you reference to my posts? I tried it without success.

 

[BAretired]

post #38

Can you elaborate more on the your red phrase?

No, I will not elaborate! The information you must provide are:

• the required vertical clearance under the bridge to allow the passage of vehicles.
• the required width of road.
• the required separation of roads.

I do not understand your last two sentences about referencing your posts. What do you mean?

 

[hoshang]

BAretired said:

• the required vertical clearance under the bridge to allow the passage of vehicles.

20ft

BAretired said:

• the required width of road.

45ft

BAretired said:

• the required separation of roads.

12ft

BAretired said:

I do not understand your last two sentences about referencing your posts. What do you mean?

I meant when you quote my posts:

 

[BAretired]

Okay hoshang,

First, the red text:
Alternatively the orange truss should be modified, or a different concept should be considered.
I meant that if any of my suggested supports interferes with the plan, something must change. Looking at the new information, it now appears that something must change.

Let's see...20' clear height, 45' road width and 12' separation of roads.

The geometry has changed. You do not need 90' spans to cross a 45' wide road. A two span bridge, each span about 50' or 60'with a 10'x12' platform at midpoint could be used instead. A 20' high stair is required at each end.

The dimensions have changed, but the concept is still the same. In theory, one pin and two rollers would mimic the structure we had before. But I prefer a rigid support structure in the middle, particularly with such a high clearance between road and bridge. Deflection parallel to the trusses should be minimized.