Fully Welded Truss 1

[slickdeals]

Folks,
Can a truss with CJP welds between the diagonals and the chords still be analyzed with diagonals being pin-ended members?

For example, the chords are W14 (web-horizontal) and the diagonals are also W14 members with their flanges connected by full penetration welds.

1. Can the truss be analyzed and designed using moment releases at their ends?
2. If the moments at the ends are not released, is it okay to use an effective length factor of 0.65 in their design for compression (assuming a column with fixed ends).

 

[Lion06]

If you do it both ways I think you'll find there is little difference between fixing the diagonals or pinning them.

 

[jike]

I agree, analyze both ways.

I would not recommend using a K=0.65 since the stiffness of chord may allow too much rotation of the web members. Use K=1.0 for that condition.

 

[dik]

You might consider increasing the member sizes to something that will yield about 70%-80% of the member capacity and get away from CJP welding... generally less expensive. I try to avoid CJP's like the plague!

Dik

 

[dhengr]

Slick:
1. What on earth are you designing here, a new football stadium roof truss, or what? Does architectural appearance make any difference? Or, are you analyzing an existing truss?
2. You should not treat any of the joints as pinned; given the member sizes, joint geometry, and welding you are suggesting, they certainly are not pinned, nor will these heavy members act that way. We certainly have the computing power to treat this truss like it will really act, and you will need the moments and shears at every joint to design the joint properly. These are all highly restrained, complex joints, and should be designed that way. The pinned joint truss has always been a calculation (design) convenience (assumption) which was nice when we were using slide rules or hand held calculators, but you still had to be careful that the truss approximated those assumptions, in action. These trusses just don’t fit that bill.
3. The top and bot. chords are continuous members in compression and tension with applied moments from the diags. And the diags. are fixed end members and the magnitude and direction of their end moments leads to the effective length, don’t they. All of these truss members are beam-columns members of one kind or another.
4. Complete penetration welds are expensive in any case, and doubly so on these skewed joints, thus my question about arch. appearance. Get your friendly fabricator and welding consultant involved in this early, they will have some ideas about how to make this as practical as possible, without full pen. welds at every web and flange in the joint, many times they are not required, and just lead to problems and extra cost.
5. Then, what you are talking about probably can’t be shipped in one piece either, so you might end up with some big bolted connections in the t&b chords.

 

[slickdeals]

Thanks for your responses.
The reason for the question was that I was looking upwards (as I usually do) and in the Miami International Airport, I saw these humongous W14 truss members that were all fully welded and I started wondering how one would have analyzed this truss (using true-pin theory or actual end conditions). Then I started wondering that a truss diagonal with ends fixed might act as a fixed-fixed column warranting the use of a lower K value than the traditional K value of 1.0 for truss members.

 

[Ron]

My condolences on being in Miami International Airport!

I analyze "rigid" trusses both ways, but usually stay with the way they are built as to how they will react. Even multiple bolt connections don't act as pinned, so those have to be analyzed both ways. This one won't act as a pinned truss as others have said. I agree with jike...use k=1.0

When you analyze it both ways, the ultimate capacity is close to the same, but individual localized conditions can sometimes vary a lot.

 

[slickdeals]

Offer me more condolences for living and working in Miami

 

[Ron]

My former employer has an office in Miami, so I've spent plenty of time there. Was doing a structural inspection once on an elementary school in Liberty City...on the outside of the fence they found a body in the trunk of a car and one was dumped over the fence. My assistant that day was a young, black Nigerian engineer. He was scared "s(p)itless". I pointed out to him that the little kids in the school didn't think anything was abnormal....that didn't give him any comfort.

 

[seny]

i do not know scale of your project, but CJP W14 to W14 sounds like budget of your client is unlimited, may be try (2)C-channels w/equal to W parameters (or many other possibilities) w/ gussets and fillet welding.
classical approach for trusses calculation during good old times was pinned connections, according your explanation about what you have there are no pins just use MC.

 

[paddingtongreen]

What dhengr said. These are "highly restrained" connections that you describe. These days, ductility at the connection is the thing, especially in LRFD work. See what AISC has to say about highly restrained connections.

Timing has a lot to do with the outcome of a rain dance.

 

[dik]

Ron: Reminds me of the skit by a comedian several years ago. He loves to hang around school yards. He loves to see the children shouting and screaming and running to and fro... They don't know that he's only using blanks!

Dik