STEEL MANUAL - TABLE 3-10

[EBAT75]

These are items for consideration in totality:

1. The prime (') and non-compact. Yes, the example section is compact. Why then is the table showing primes at the bottom which tie in with the values in the table for different spans? If anyone is confused, I am too.

2. Lb=0. Yes, < or= Mp. For that to happen, shouldn't Cb then become < 1? The Table is based on Cb=1, the minimum for any condition.

3. Continuously braced, prevented from lateral displacement, twist, etc are terms that need clarification. The Specs simply give formulae for bracing strength requirements per brace point. These are based on the flange compressive force Pf which becomes infinity (as to be expected) when Lb=0. Something to think about.

4. Does not deflection become the controlling factor, not strength as the span increases? How does deflection affect flange force and consequently the required brace strength? 

 

[EBAT75]

Just to clarify 2. above, this is under the F2-2 scenario, whether bracing strength is checked to be adequate for all spans. If as have said before, the bracing interval is < Lp and the stiffness provided is adequate based on that Lb=Lp, then the stiffness requirement would be met regardless of the span. 

 

[thedaywa1ker]

These are items for consideration in totality:

1. The prime (') and non-compact. Yes, the example section is compact. Why then is the table showing primes at the bottom which tie in with the values in the table for different spans? If anyone is confused, I am too.

2. Lb=0. Yes, < or= Mp. For that to happen, shouldn't Cb then become < 1? The Table is based on Cb=1, the minimum for any condition.

3. Continuously braced, prevented from lateral displacement, twist, etc are terms that need clarification. The Specs simply give formulae for bracing strength requirements per brace point. These are based on the flange compressive force Pf which becomes infinity (as to be expected) when Lb=0. Something to think about.

4. Does not deflection become the controlling factor, not strength as the span increases? How does deflection affect flange force and consequently the required brace strength? 

Just to clarify 2. above, this is under the F2-2 scenario, whether bracing strength is checked to be adequate for all spans. If as have said before, the bracing interval is < Lp and the stiffness provided is adequate based on that Lb=Lp, then the stiffness requirement would be met regardless of the span. 

Sorry to beat a dead horse, I was going to just leave it and stop going in circles, but I couldn't help myself.  Brother, I don't claim to be an expert on LTB but your posts are going to confuse the hell out of anyone browsing that is trying to get familiar with steel design.

1.  I'm not sure what you're talking about.  Are you talking about the bottom of 3-6 showing prime values?  The Mp values in the bottom of 3-6 should equal the Mp' values where applicable.  I'm not sure where the confusion is, W16x100 is compact anyways

2. That is not what inequalities mean in the steel manual.  It is giving an upper bound to the values you can use out of the first equation, it is not saying that the equation will always be under the second value no matter what.  This is a common notation in the manual (bolt hole bearing strength is an example off the top of my head), and if that is how you interpret the inequality then you are probably doing other things incorrectly as well.

3. Those definitions are indeed vague.  They leave it up to engineering judgement.  What equation is Pf from in the specification?  It is not in the symbols list of the spec on 16.1-xxvii, don't see it in Appendix 6 either

You have posted a number of things that you say are direct from the manual but are not in fact in the manual.  Can you please be more specific when you are citing the manual

'flange compressive force Pf goes to infinity when Lb=0'  That doesn't sound right, again I don't know what equation you're referencing but it sounds like Pf would at least be limited by yielding.  Can you give a code reference please.

4. Yes deflection controls at longer spans. I dont think that deflection in and of itself would effect LTB at all. I don't know of any way that deflection would come into play with LTB unless it is enough that it starts screwing with the layouts of your bracing

 

[EBAT75]

'flange compressive force Pf goes to infinity when Lb=0'  That doesn't sound right, again I don't know what equation you're referencing but it sounds like Pf would at least be limited by yielding.  Can you give a code reference please.
 

Pardon my reference to flange compressive force Pf......MY SLIP. Meant to say required brace strength. Ref: App 6.3

Let me take up the topic with AISC Steel Solutions. They have questions answered. 
For now, I feel some of these are counter intuitive. 

 

[EBAT75]

https://lsc-pagepro.mydigitalpublication.com/publication/?m=7946&i=676603&view=articleBrowser&article_id=3785160

https://lsc-pagepro.mydigitalpublication.com/publication/?m=7946&i=656537&view=articleBrowser&article_id=3646852

Just after I sent an email to AISC, I saw these. I will post their answers when I get them.

I am sorry if I caused confusion. I am thinking of asking the EB moderator to delete this post if it will help.

 

[EBAT75]

AS PROMISED, HERE ARE THE ANSWERS I JUST RECEIVED. AS I HAVE SAID EARLIER, WHEN THE ANSWERS HAVE BEEN SEEN BY OTHER MEMBERS, I AM FINE WITH IT BEING ARCHIVED OR DELETED IF IT HELPS AVOIDING CONFUSION.

[SIZE=12pt]I have addressed your [/SIZE]questions[SIZE=12pt] below in red:[/SIZE]

1. From Steel Solutions: A direct brace may be provided for a primary member by a properly attached floor system itself ..... What constitutes a properly attached floor system itself .....? I assume you are referring to the Engineering FAQ 4.3.1. The technical answer is “a floor system that is attached such that the strength and stiffness requirements provided in Appendix 6 of the Specification are satisfied can be considered a “properly attached floor system” relative to the statement in the FAQ. It is not common in my experience for explicit checks to be performed relative to the floor system and its attachments. It is common in my experience for engineers to judge such conditions by inspection based on engineering judgment and experience. A concrete floor on a steel decking - is it sufficient? In my experience most engineers would deem “a concrete floor on a steel decking” to be sufficient. It might be possible to attach “a concrete floor on a steel decking” to a beam in a manner that is not sufficient, though it would seem that someone would have to go out of their way to do so, at least relative to conditions commonly found in buildings. It is my understanding that in bridges the deck is sometimes attached in ways that can reduce the ability of the deck to provide bracing to the beams. Does it provide required bracing strength per App 6.3? It can be evaluated using Section 6.3. Is it continuous bracing? In my experience beams are sometimes designed in practice based on the assumption that they are “continuously braced”. I suspect different engineers apply/intend somewhat different subtleties when using the term, but generally it can be taken to mean that Lb<<Lp. I suspect that there are few engineers that would assert that Lb is actually zero, if they stopped to seriously consider what they intend. Generally it probably does not make sense (or at least is not necessary) to treat “a concrete floor on a steel decking” as continuous torsional bracing as addressed in Section 6.3.2b in building construction, though as discussed in the Commentary this can be done.
   

1. Is it correct to say continuous bracing means Lb=0; Mn=Mp regardless of beam’s span? (as another engineer says it is) If a beam is truly continuously braced, then the unbraced length would seem to be zero by definition and since stability would not be a concern Mn=Mp – unless local buckling of the elements governs. However, if the compression element is continuously braced, it is difficult to see how the local buckling strength would not be beneficially impacted as well, though the Specification does not directly address this issue.
   

1. In Table 3-6 Maximum Total Uniform Load, if a beam is continuously braced, and the Mn is equal to Mp regardless of the span (as an engineer on a forum says it is), what is the meaning of values marked with ‘ (prime) at the bottom of the pages? I do not understand the question. Some sections are marked with an ‘f’. This indicates, “Shape does not meet compact limit for flexure with Fy = 50 ksi; tabulated values have been adjusted accordingly” as stated in the footnotes to the table. Note that I have also mentioned this possibility above.
   

1. For compact W sections, are Tables 3-6 and 3-10 equivalent? Tables 3-6 and 3-10 address different conditions and therefore are not (and cannot be) “equivalent”. They should converge where the conditions overlap. In other words they should be consistent or should not conflict.  If we consider a W24x229 (just to pick a section at random) Table 3-6 lists φbMp as 2530 k-ft. Table 3-10 on page 3-97 seems to top off at about this value. This is because stability is not an issue at low (though not necessarily continuous) unbraced lengths. However if we look at a unbraced length of 18 feet in Table 3-10 φbMp is about 2340 k-ft. If we go to Table 3-6 and look at the same W24x229 with a 18 foot span the maximum total uniform load is 1130 kips. This can be converted to w, the uniform load, by dividing by the span to get 1130/18 = 62.77 kips/ft. The moment is wL^2/8 = 62.77(18)^2/8 = 2540, which is roughly φbMp, again because stability is not an issue at low (though not necessarily continuous) unbraced lengths. I am assuming we can simply overlook whatever rounding or minor mathematical error I managed to introduce along the way.
   

 

[EBAT75]

Lest I am considered to have “No Comment”, I give below my last set of comments:

1.        Steel decking in and of itself is not the bracing element unless it is designed and detailed as such. Orientation of the decking i.e. perpendicular or parallel to the girder or beams in a bracing system comes into play. Also is the decking designed as a diaphragm? I am not familiar with the SCM 15 or the 2016 specs. Apparently there is more in them on the topic.

There is no consistency on stiffness, strength requirements in the code(s). Why these checks are generally not done is not because the decked floor is necessarily stiff or strong enough but because the framing layout would be such that beams required at some intervals would be providing that stiffness and strength required. Consider this – a perimeter girder into which beams frame into. Decking spanning across the beams would now be running parallel to the perimeter girder. Is the decking providing the “sufficient” strength and stiffness for this perimeter girder? Wouldn’t there be torsion also on the perimeter girder (loaded only on one side)? Aren’t the beams framing into the girder actually providing torsional stiffness/strength in addition to lateral? Is the Lb=0? Is Lb not the spacing of the framing beams? Now, extend this to the interior girders sans torsion, except the decking being perpendicular to the interior girder would provide more of what was described as “truly continuously braced” condition if the span is such that it can provide the stiffness/strength combination.

If there is no decking with shear studs or welds, is the concrete slab attached to the compression flange as in composite slabs with studs for it to provide the strength and stiffness?

On the one hand, concrete slab (decked or not) is considered sufficient; on the other – use App. 6.3. AISC and SDI or ACI are playing in their own turfs. Does a beam/girder designer also design and specify the bracing elements?

Another item of interest is this. Can one say how much of the reserve strength of the beam/girder is what provides the real, old “Factor of Safety” or as a colleague of mine used to call it “Factor of Ignorance” because a 50 ksi W normally comes with 65 ksi (=/-) yield mill test results. We do not design the 50 ksi yield for 65 ksi yield, but that is the “as built” strength. Can this be a part of the overall bracing, Mn, Mp, Lp, Lp margins in the as-built structure that can mask the bracing issues?

2.       Item 2 in reply – No Comment.

3.       The prime has been mistakenly taken as f.

4.       If Tables 3-6 and 3-10 are not equivalent; should be consistent or should not conflict, the example has shown a relationship. It may not be 1-on-1, direct. Yes, 3-6 is based on Lb<Lp. But the bottom part has the BF listed. It can be used for Mn when Lr>Lb>Lp and triangulation between Max Load, Mp, Lb/Lp is possible. In that sense, there may not be equivalency but there is convertibility.

Ladies and Gentlemen, I apologize for the drawn out discussion on this. I have also been confused at times. As long as we do not get confounded, my hope is that something good also comes out of it.

Please feel free to add your comments too. I do not take these at a personal level. I have no intention to add anymore to this thread unless someone explicitly asks me to. Someone once told me “listening is the better part of the conversation”. I will listen.

 

[organix]

I am confused by your item 3.  The prime isn't shown in the tables.  If you are referring to the screenshot below, that's just a comma to separate the adjacent units.  Otherwise, the prime is shown on page 3-5.

 

[thedaywa1ker]

Thanks for following up EB75

1. I believe most practicing engineers would consider metal decking to be sufficient brace the beam without a second thought, considering it will be attached with welds or TEK screws as a diaphragm.  The perpendicular/parallel flutes question is one I have seen debated before...and theoretically there probably is a difference in bracing capability, even though we don't design the diaphragms as having different capacities in different directions.  But I haven't seen anything compelling enough to make me deviate from what I've been comfortable with for years, barring any special circumstances like excessively large beams in braced frames for example

4. Another example of where 3-6 and 3-10 converge is in my post from 10/12:

In the 3-10 charts, the horizontal portion of each beams line matches the moment given by taking (loads from 3-6)/(L)*L^2/8 - and the 3-10 charts start sloping at Lp because that is when LTB starts to apply...once the line in the 3-10 charts are sloping, the chart and table no longer match.

 

[EBAT75]

Thank you for the screenshot. I mistook it for a prime. As it was in color print I associated them as one. The fonts also did not help me.