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STEEL MANUAL - TABLE 3-10


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Selecting beams using Table 3-10 requires accounting for the weight of the beam. Cb is 1.0. Simply supported beams, easy enough to take off the moment due to self weight of the beam. But if we are doing a continuous beam, and the maximum moment   includes beam’s self weight how does one deal with the reduction? Usually, the maximum positive moment comes not from all bays loaded with floor dead plus live loads.

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If you add the beams self weight to your applied dead load instead of subtracting it from the capacity that should take care of it, right?  Unless I'm confused, which does happen frequently.  Subtracting the self weight moment from the capacity seems to be a complicated way of doing it. 

You'd have to check unbalanced live load cases like you mention, but the self weight will always be there

Edited by thedaywa1ker
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Verbatim from Manual: The plots include the beam weight, which should be deducted when calculating the maximum uniform load the beam will support. Cb is taken as unity.

TDW, I see what you are saying but what threw me off was I was thinking of Moment. Now I see “maximum uniform load the beam will support“. 

The table is Available Moment vs Unbraced Length. If someone wants to find the maximum uniform load a beam can carry, why would they not use Table 3-6 Maximum Uniform Load instead. There also one has to deduct the beam self weight but it is a deduction from loads.

3-10 is good for moments, to calculate maximum uniform load through the moment route is circuitous.

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Where is that in the manual? I must be going crazy, I'm looking at the paragraph about the charts on page 3-11 and dont see it

The difference between 3-10 and 3-6 is that the 3-10 charts include lateral torsional buckling, and table 3-6 considers the beam fully braced

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Let me explain my understanding.
3-6 has plastic (yielding) up to Lp, then inelastic LT up to Lr. Up to only Lr, fully braced, you can use 3-6 (or 3-2 if selecting by Zx). LT is not confined to 3-10. 3-6 also has the linear reduction.
Up to Lr, unbraced, you can use 3-10 for moments/span. Beyond Lr, one cannot use other than 3-10. Limit is span/depth of 30.

My take was based on floors (concrete, steel decking...) bracing the beam. 3-10 is useful when say, concentrated loads are applied (equipment, cranes, .....) in longer unbraced spans.

Looked at 13th and 14th editions. The paras explaining these tables - 14th, 3-10 has an omission Re the linear reduction between Lp and Lr. 13th has the full explanation.
 

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I'm looking at the 14th - Table 3-6 is based on braced (Lb always < Lp, therefore not considering LTB) beams - I don't see anything about a reduction up to Lr.  I have never used any edition besides the 14th so I don't know if that is different between the two.

Are the moment capacities in the 3-10 charts consistent between the 13th and 14th?  If the 13th has the note about reducing the values in the chart, and the 14th doesn't have that note, that makes me think that they changed the chart so you don't have to do that adjustment.  That makes more sense than them omitting the note but still wanting you to adjust the chart values.

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14th - Table 3-6 is based on braced (Lb always < Lp, therefore not considering LTB) beams -....

If so, why are spans many times Lp listed in that table? For the same material properties, Lp is a function of section property only. Lb can be more or less than Lp. Up to Lp, Mp is constant (compact).  Beyond that M reduces linearly by BF factor up to Mr at Lr (non-compact).

There is no reason for 13th and 14th Values to be different in Section F - Flexure based tables/charts because there is no difference in Specifications between 2005 and 2010 on which 13th and 14th are based. I have compared and confirmed 3-10 and 3-6 values in both editions.

14th, 3-2 para on explanation on non-compact range is the same as in 13th. That language was there in 13th 3-10 also, but not there in 14th 3-10. I think it is an omission. May check with AISC after the exam.

There seems to be a misunderstanding in what I referred to as reduction. What was meant was the linear reduction in capacity (moment/load) from Lp to Lr. It is the term BF in reducing linearly from Mp to Mr.  

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4 hours ago, EnergizerBunnyAt75 said:

If so, why are spans many times Lp listed in that table? For the same material properties, Lp is a function of section property only. Lb can be more or less than Lp. Up to Lp, Mp is constant (compact).  Beyond that M reduces linearly by BF factor up to Mr at Lr (non-compact). 

Because the spans in 3-6 aren't related to Lp - a 40' unsupported span in table 3-6 can still have an Lb of zero if it is constantly braced laterally. The paragraph regarding table 3-6 on page 3-10 says 'Maximum total uniform loads based on braced (Lb<Lp) simple span beams...are given'

If Lb<Lp, then LTB isn't a thing, in that case the linear interpolation you mention would put the strength of the beam higher than Mp, so the reduction isn't applied to table 3-6

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Lb is not zero just because the compression flange is restrained from lateral displacement along the full span. It is the distance between points where both top and bottom flanges are restrained from twisting - the span, say 40 ft in a section with Lp of 10 ft.

....based on...meaning the Mp is the upper limit. BF Reduction factor is applied with Mp as the take off point.

If you use 3-6 and 3-10 for a given section, load, span and do moment/load/span conversions, you will arrive at the same result.

From 14th:

 

Notes on Table 3-2:

For compact W-shapes, when Lb ≤ Lp, the strong-axis available flexural strength, φbMpx or Mpx /Ωb, can be determined using the tabulated strength values. When Lp < Lb ≤ Lr, linearly interpolate between the available strength at Lp and the available strength at Lr
as follows:
 
Notes on Table 3-6:
The uniform load constant, φbWc or Wc /Ωb (kip-ft), divided by the span length, L (ft), provides the maximum total uniform load (kips) for a braced simple-span beam bent about the strong axis. This is based on the available flexural strength as discussed for Table 3-2.

Here is a good source also:
https://www.aisc.org/globalassets/continuing-education/quiz-handouts/steel-design-after-college-handout.pdf

 

 

 

 

 

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4 minutes ago, EnergizerBunnyAt75 said:

Lb is not zero just because the compression flange is restrained from lateral displacement along the full span. It is the distance between points where both top and bottom flanges are restrained from twisting - the span, say 40 ft in a section with Lp of 10 ft.

I am going to disagree with that definition of Lb. From the Spec F2.2 "Lb = length between points that are either braced against lateral displacement of the compression flange or braced against twist of the cross section."

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Lb absolutely is zero if the top flange is continuously braced

The definition of Lb in section F2-3, page 16.1-47 of the 14th edition: 'Lb = length between points that are either braced against lateral displacement of the compression flange OR braced against twist of the cross section'

E720 is quicker to the draw!

Edited by thedaywa1ker
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Over at eng-tips.com they had a monster thread a few months ago about LTB where they argued about lateral bracing of top flanges and a bunch of other stuff, complete with fem simulations and bluebeam sketches...its some great reading.

https://www.eng-tips.com/viewthread.cfm?qid=459248

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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.

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OK dear forum members, thank you for the spirited discussion.

 I will be happy to return to the forum on 23 October, yes 2020 and pick up where I left. 
 

It is not about who is right but about what is the correct thinking behind the Tables. If I fail the exam because of this topic, so be it. 
 

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On 10/12/2020 at 11:24 AM, thedaywa1ker said:

Over at eng-tips.com they had a monster thread a few months ago about LTB where they argued about lateral bracing of top flanges and a bunch of other stuff, complete with fem simulations and bluebeam sketches...its some great reading.

https://www.eng-tips.com/viewthread.cfm?qid=459248

Definitely good read.  I will have to revisit post-SE... bigger fish to fry.  However, I did go about 15% through and it definitely made me feel a little less adequate.

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On 10/17/2020 at 8:48 AM, organix said:

Definitely good read.  I will have to revisit post-SE... bigger fish to fry.  However, I did go about 15% through and it definitely made me feel a little less adequate.

Lol. Yeah those guys are rock stars, somewhere in the thread one of the main contributors reveals that he did his masters thesis on LTB.  My favorite part is how they seamlessly transition from referencing one countries code provisions to another countries, and discuss their differences.  I'm still trying to get fully comfortable with my own countries provisions.

Good luck to everybody this week...

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I am back as I said I would. I took a brief look at the eng-tips thread. My initial impression is that it dwells on too many codes of other countries and not enough on the US codes which is our prime interest. An engineer cannot stand up in a US court and invoke those codes in defense. Also, code making and revising them often has become an industry in itself in the US and code writers are not unfamiliar with at least some of the foreign codes. e.g. after the Kobe earthquake US engineers learnt a lot from the Japanese seismic codes and revised our seismic codes. Australia, New Zealand have very different climatic conditions. Or for that matter India or China. I have seen adoption of some Euro codes. And Canadian codes in wood, masonry, cold formed steel.

Wish to continue where we left off after I have taken some time to dwelt on it.

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