Steel Truss-Chord Splicing Location 3

[SamETABS]

Hello All,
I'd be grateful if you share some ideas regarding the best locations to introduce splice for bottom and top chord of a steel truss.

Many hanks,

 

[WillisV]

Where there is the least force.

 

[a2mfk]

For splicing anything that is in compression and tension, I don't worry much about the tension. But try and pick a spot in compression where you have some bracing, or add bracing specifically at braces for buckling out of plane...

 

[MiketheEngineer]

Ditto ---

On a triangular shaped truss - nearer the middle. One a flat truss - nearer the ends - USUALLY... Any moments on these sections??

 

[SamETABS]

Thanks all for your quick response:

@a2mfk:
so you think, I should put them near the end of each panel(say less than a foot)?

@MiketheEngineer-:
yes, the top chord is loaded (unifromly), so there ould be bendng moment on top chord member, so I presume I need to get it closer the area with less moment? what do u think?

 

[MiketheEngineer]

That is generally the way I handle it. But don't forget that differing loading conditions can move your moments around a bit. Somewhere near the 1/4 panel seems to work most of the time.

 

[a2mfk]

If you need bottom chord compression bracing anyway, I'd definitely put it towards your most vulnerable section of the bottom chord- which for you is at the splices. I assume the whole bottom chord is in uniform axial compression so that part does not really matter. You will get in-plane buckling bracing from your webs at each panel point, so to keep things simple you may design for bottom chord bracing at each web, and that way you have the same unbraced length for both axes/planes.

 

[ToadJones]

bottom chord of a truss in compression?

 

[ToadJones]

depending on the span of the truss, availability of material may well dictate the splice location.
I have seen many large mill bldg trusses spliced very near the center. I have designed some trusses 80'-100' with no splices except for full pen welds.

 

[hokie66]

TJ,
That's still a splice, and I like them to be other than at maximum force locations. As for compression in the bottom chord, wind uplift generally controls in areas where cyclonic wind rather than snow is the primary consideration.

 

[ToadJones]

hokie-

touche
touche

I guess one should also not ignore the importance of tension chord bracing.
I believe Fisher wrote a paper on the subject some years ago

 

[ToadJones]

....

 

[WillisV]

ToadJones - touchy subject - we have had lengthy discussions on this in the past...like this agglomeration I started from 5 years ago: thread507-124363

 

[ToadJones]

Willis-
I know it is, but, in my case, all the steel truss I have designed were for mill buildings or similar. Bottom chord bracing is a must anyway.

 

[WillisV]

Look in some of the large stadiums - you will see plenty of fairly slender trusses without a bit of bottom chord bracing.

 

[JAE]

And some with (these are, granted, bottom chord braces applied some way along the verticals - but their purpose was to brace the bottom, tension chords):

 

[ToadJones]

My point was, I needed bottom chord bracing for other reasons.
In mill buildings, bottom chord bracing acts as horizontal truss to distribute crane forces between several bents.

 

[BAretired]

Sometimes, it is best to splice a truss at midspan. A truss with height of h/2 at each end and h at midspan is a case in point. The maximum moment under uniform load is M = WL/8 at midspan. The moment at the quarter points is 3M/4 but the height at the quarter point is 3h/4 so the chord force is the same at both points.

Splicing at midspan means handling only two pieces and for long spans, is usually more economical than quarter or third point splicing.

BA

 

[SteelPE]

A little off topic here... but, this is a good example of why you need to brace the top chord of a truss. This one rolled over due to heavy loading. The truss was installed without any attachment to the diaphragm or with any bridging 20 years ago. It worked for 20 years until seeing a very large loading a few months ago. Now I have a great example of why we have the erectors install bridging at 3'-0" o.c. whenthey install spliced joists (which this was).