Single angle strut 1

[dlew]

I am investigating an existing building built around 1965. The building has long steel trussed girts made of WT's chords and single angle diagonals. One leg of the angle diagonal is fillet-welded to the WT's stem, what makes the connection to be eccentric. The diagonals seem to be overstressed when using the LRFD method. I followed the example shown on page 2-49 of the AISC's LRFD manual.

On the example, the ratios of the compression load, and the moments about the W and Z axes are added to get the unity check. But it looks to me that in the example, they are adding the stresses (or ratios) without regard to where they occur. Per example, the moment about the Z-axis produces compressive stresses on the vertex of the angle and tensile stresses at the two tips. The moment about the W-axis produces maximum stresses at the tips (one tip in compression and the other in tension), and zero stress at the vertex.

My question is, can I check the angle for the actual stresses at each point (tip and vertex) instead of lumping all together?
If I do that, I would be adding the following stresses (using the unity check formula, of course):
At the tip: those due to the axial compression, plus those due to moment W, MINUS those due to moment Z.
At the vertex: those due to axial compression, plus those due to moment Z.

Thanks in advance

AEF

 

[jheidt2543]

dlew,

This comment may be a little off the topic, but not too much. When checking out an existing structural system, such as you describe almost 40 years old, should it be checked under today's code or the code it was built under?

I was in a seminar a few weeks ago where a participant (not a presenter) made the statement that ..."not only should the structure be analysed under the old code, but using the old methods"..., that is, even a slide ruler if need be! My opinion is that is foolishness, but what do others think?

I sometimes check out old industrial building structures and roof trusses, once even a wrought iron truss bridge (140 years old). The toughest part is making a realistic estimate of the existing condition and stresses using original allowable stresses. If found deficient, then the structure has to be reinforced with today's materials so, you end up with a mixture of allowable stresses, and code requirements. I wouldn't say you throw a dart at the board, but lets call it "engineering judgement", has to play a big part.

Back to your problem, realistically, how much eccentricity is there, an inch or two at the most? I think I would look at the system first ignoring the eccentricity. Then look at the effect on the individual member. My guess is that the amount of bending the eccentricity causes in the member is really quite small compared to the axial loading. Just my thought.

 

[LittleWheels]

Hello jheidt2543,

In Britain, there is both a new design code and an assessment code for old highway bridges. If an old structure fails the current code, it can be assessed under a (generally) slightly easier set of criteria and conditions.

 

[DaveAtkins]

I analyze single angles using the ASD method, but it appears to me that you are checking the angle correctly.

So now you must use your engineering judgment. In this case (trying to justify an existing structure), I think it is reasonable to combine stresses the way you suggest, even though this does not satisfy the AISC. I "pull out all the stops" when I attempt to justify an existing structure. For example, are your wind loads too conservative? Can the WT's take some torsion, thereby reducing the moment in the angle? Is your interaction equation at least close to 1.0 (say 1.10 or 1.15)? You must decide what you can justify, and still "sleep at night".

 

[amokhta]

dlew:

I assume that you are investigating this existing structure to renovate it. The question of whether the renovated building shall conform to te requirements of the code for new construction is far from clear. The local building official should be contacted to establish the degree of compliance with the current code. The UBC allows the local building official to authorize renovations without full conformance to current code, if the building does not become more hazardous than before.

You may want to consult UCBC (uniform Code for Building Conservation) as well as UBC Appendix Chapter 34 where it deals with existing structures.


Regards