Beam chart Difference Between Black and Green Steel Books 1

[EngineerGeneral]

Hi there,

Did ya'll noticed that the moments given by the black book's beam charts are generally larger than that by the green book's?

For instance on the black book, for W14X43 with 20ft unbraced length, the available moment is 109.5 kip-ft;

On the green book, the available moment is 81.5kip-ft.

Anyone gotta an explaination for this, please?

 

[RobertEIT]

Black book is based on LRFD, so the listed moment is refer to factored load (combination with load factor).
green book is based on ASD, basically load combination without load factor except sometimes with a 0.75 factor in some combination.

 

[nutte]

Robert, that is not correct. The black book is both ASD and LRFD, and the 109.5 value represents the ASD capacity.

I don't know why they're so different, without digging into the equations. The lateral torsional buckling equations changed between the two versions, but I wouldn't have expected the values to differ so drastically.

 

[PEinVA]

We just noticed this the other day in comparing an W8x15. I think it was somewhere near 20% increase in capacity @ 9'-0" span, if I remember correctly. If AISC is going to force you to learn a new code by removing an old code, shouldn't they offer some type of free manual? I mean I'm only 2 years out (office is ASD) and I've forgotten all LRFD stuff, and now I've gotta learn the new ASD method.

I feel for all the old-timers out there, who've fought as long as they could. (including my boss)

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[SteelPE]

I agree with nutte. The difference is within the revised LTB equations. If you calculate the numbers by hand you will get the same numbers that are in the charts (I wrote a spreadsheet for this). As for the large difference, I would guess that it is based off more research that has been performed since the 89 spec…. and one of the reasons why to go to a combined spec.

 

[lrhg]

As I understand it the AISC 13th ASD numbers are the LRFD values divided by 1.5

 

[csd72]

The black book is for black steel and the green book is for green steel.

 

[EngineerGeneral]

Irhg, no, it's not true. The factor of 1.5 was already in the formula on the chart, i.e., Mn/Omiga. So you really can not apply that 1.5 one more time to the values.

 

[structuresguy]

Also, the equations were recalibrated for the latest code to bring the results from either ASD or LRFD closer together, so there is not an advantage to using one over the other. In the previous codes, depending on your DL/LL ratio, either ASD or LRFD would result in a more economic design. I can't remember how it went, but at a low ratio, one was more economical, but at higher ratios, the other was more economical. The PEMB industry took full advantage of this by always using whichever version resulted in economic design at low LL/DL ratios typically seen in PEMB.

If you plotted a curve of the design results versus LL/DL ratio for ASD and LRFD, there was a point where the two curves crossed.

It is my understanding that the new black book has the two curves basically in line with each other now. So choosing between ASD and LRFD is now more of a matter of convenience than economic advantage.

 

[EngineerGeneral]

However, Structuresguy,the ASD beam tables for fully braced beams for black and green books are very similar.

 

[frv]

structuresguy-

That's not the case. The equations are calibrated at a LL/DD ratio of about three. Lower than this, LRFD is more economical.

Think about this. The capacity is static. There is no way to tweak capacity based on LL/DL ratio.

 

[jt12]

Yielding limit state is now based on Z rather than S. Gives sometimes up to 10-20% kick.

 

[nutte]

But it already was based on Z, with the 0.66 Fb term (as opposed to 0.6 Fb). Z/S for wide flanges is on average 13%.

 

[Lion06]

I remember hearing something in my ASD steel class in school that the LTB equations were based on the LARGER resistance based on lateral or torsional resistance to buckling (even though both resist buckling). Is it possible that both are figured into the equation for LRFD?

 

[ash060]

It is funny that they call it ASD in the 13th edition, but it kind of isn't. The equations are all LRFD you just don't factor the loads if you are using ASD. None of the equations require you to calculate stress like the green book. It is now Pa/Pn Ma/Mn, not fa/Fa or fb/Fb.

They are very sneaky.

 

[JRGse]

ash060 - The sneaky part of the name game is ASD no longer stands for Allowable Stress Design, it means Allowable Strength Design!
AISC calls is stress design in the preface but then calls it strength deign in Chapter B.

Yes it is very sneaky.

 

[structuresguy]

Well, I am just going by what an AISC presenter told a large group of engineers at an AISC presentation for the new black book at UCF last fall when the new black book came out. He explained the differences from the old to new codes regarding LL/DL ratios. And also explained that the new ASD and LRFD equations were set to result in similar design by either method. So you can believe me or not, that is what the AISC engineer told us.

 

[JRGse]

Getting back to the original question, the reason why the two books are differant is that the green book does not take advantage of the modidied rt value that is allowed in the commentary for chapter F. If you use the modified rt value (rt'=2.63) you get an allowable moment of 103.2 ft-kips