Bar Splices in CMU for IBC are too much? 2

[seattlemike]

Hello fellow structural engineers,

Several years ago, I think I went to seminar on the UBC transition to IBC, and it mentioned this problem with Masonry, and I am just now running into it:

IN the WSD design method:

IBC has an equation for lap splices that overrides ACI 530, and for #5 bars at the centerline of CMU, the values for splices from equation 21-2 (page 419) are similar to the UBC's 48 db even for low strength CMU. However, when you have (2)#5 each cell, eq 21-2 gives 67*db for 1500 psi CMU and 45*db for 3500 psi CMU.

For #6 bars, one of the factors goes from 1.0 to 1.4, and things get really ridiculous....

For 1-#6 per cell, 70*db for 1500 psi CMU, and 46*db for 3500 psi CMU.

For 2-#6 (1 E.F.), it's 112*db for 1500 psi CMU, and 74*db for 3500 psi CMU.

In the seminar, I remember they said this was a big problem, and that steps were being taken to address it, otherwise you'd just have to do one continuous bar top-to-bottom of your wall.

Anyone know if this has been resolved? How much should I use for lap splices of #6 bar? #5 bar each face? #6 bar each face?

Thank you,
Seattle Mike

 

[jike]

I wrote a spreadsheet to determine the laps since the laps vary by wall thickness and whether the bars are centered or on the face of the wall. As you pointed out, the lap lengths for #6's and larger are ridiculous. We have stopped using bars larger than #5's. Larger bars may require the use of couplers!

 

[seattlemike]

I agree, seems like #6's are on the out unless you go to couplers... which probably isn't so bad.

 

[Boiler8019]

What version of IBC are you using? I thought this problem had already been corrected in the newer code, or possibly it was just my local jurisdiction that trumped the over conservative requirement. ICC basically admitted the problem.

 

[seattlemike]

We are using IBC 2003 and ACI 530-02. Hmmm.. yes, I will see if the ACI 530-05 is available. Thanks Boiler8019!

 

[henri2]

For WSD/ASD, IBC-06 Section 2107.5 modifies MSJC-5 Code Section 2.1.10.7.1.1 so that the equation for lap length by ASD is similar to the UBC WSD equation.

For SD, IBC-06 Section 2108.2 modifies the ld equation (3-15) in MSJC-05 Section 3.3.3.3....and it states amongst other things that the ld need not be greater than 72db.

For those who are familiar with or have used IBC-06 in conjunction with MSJC-05, what are your views on lap lengths? Do they appear reasonable?

 

[WillisV]

They are more reasonable - the IBC '06 working stress lap length equation provides a lap length for bars that are stressed to less than 80% of Fs and requires the lap lengths for bars stressed to greater than 80% be increased by 50%.

So either you design the bars to be only 80% stressed and use shorter laps, or you go with the more conservative lap lengths, or you simply ask the masons to put strain gauges on all the bars and cut them as required.

The equations for typical cases (center of 8" wall - 1500 f'm) work out to be:

Bars <80% Stressed:

#3 15" = 40db (TYPICAL)
#4 20"
#5 25"
#6 30"
#7 35"
#8 40"
#9 46"

Bars >80% Stressed:

#3 27" = 72db (TYPICAL)
#4 36"
#5 45"
#6 54"
#7 63"
#8 72"
#9 82"

 

[seattlemike]

Yep, looks like I will have to get my hands on IBC 2006 as soon as possible to solve all my cases. I have a lot of edge bars and 3500 psi masonry... with both 80% bars and 100% bars...

Is the 1/3 increase for Masonry still in IBC 2006 and MSJC-5, aka ACI 530-05/ASCE 5-05/TMS 402-05?

Thank you henri2 & WillisV. Great comments.

SeattleMike

 

[WillisV]

1/3 Increase: NO - hasn't been allowed since the IBC 2000 except for alternate load combos. The next MSJC is also probably going to pull it out all-together...it already is pretty clear it is only allowed if the governing building code allows it.

 

[seattlemike]

WillisV:
Hmm... I thought that the IBC 2003 alternate load cases defered to the material standard for 1/3 increase. AISC 341-05 (steel) strickly forbids the 1/3 increase, but NDS (wood) allows Cd = 1.6, and the MSJC-02 section 2.1.2.3 allows the 1/3 increase.

...except in MSJC-02 2.1.2 it's not clear whether the "E" is working stress level or strength level... I had previously assumed it was working stress level E.... like Section 3-2 of Amrhein, but perhaps that is wrong?

 

[WillisV]

Seattlemike... yup thats why I said "except for the alternate load combos" - then the 1/3 increase is allowed.

The MSJC has always allowed this 1/3 stress increase, but for the standard (non-alternate) load combos, the IBC has overriden the MSJC since 2000 and disallowed it.

The NDS material increases are a completely seperate matter and are allowed in the regular IBC combos.