Reinforced CMU Jamb / Column for Lateral and Gravity.

[StrEng007]

Part 1:
I often see CMU jambs, located between large window and door openings, being constructed with (1)#5 bar centered in each cell for (2) or (3) cells.

Let's say you have (2) filled cells side by side acting as your wind jamb, w/(1)#5 bar centered in each cell of an 8" unit. Treating this shape (8" x 16") as a flexural element, this provides ρ = As/bd = (2)0.31in²/[16in x 3.8125in] = 0.0101

TMS 602-16 / ACI 530.1 limits flexural elements to ρmax = 0.286 f'm/fy.

For f'm=1500 psi and fy=60,000 psi, ρmax = 0.00715, which is less than 0.0101.

So what gives here? Is this jamb "over reinforced"? This error came up in Enercalc while trying analyze a wall for a 16" strip (ie, 2 filled cells)

Part 2:
I've seen this debated before and have not found a consensus. In a situation as described above, utilizing the jamb for combined lateral and gravity, does the jamb constitute a column or some other structural element (pilaster). How do you handle this design?

 

[DaveAtkins]

You could assume the area of steel is the maximum area of steel permitted. But that technically violates the intent of rho max, which is to ensure a ductile failure.

DaveAtkins

 

[StrEng007]

Upgrade the f'm?

2600 psi would get you there, but would generate a lot of questions for the intent of a couple filled cells.

 

[Roukkia]

Are you sure you’re not supposed to be using 32,000psi instead of 60,000psi?

 

[jerseyshore]

What you're designing is masonry column, not a wall. If you had #5's @ 8" o/c in a wall yes it would be over-reinforced.

The max As for a column is 0.04 * An so that will give you much more typical results.

 

[CConrad]

There are a couple things to consider here.

Section 9.3.3.2.4 provides a condition that would remove the upper limit on the flexural steel, based on the M/(Vd) ratio.

If that section does not apply, the max of 0.00715 is based on the assumption that the axial load in the column is zero. If the column has axial compression for the load combination of D + 0.75L + 0.525QE, then the upper limit on the flexural steel is actually higher than 0.00715 by the ratio of (Axial Compression)/60000.

Director of Engineering
ENERCALC, LLC
Web:

http://www.enercalc.com

 

[EZBuilding]

Last time I looked at this - the ASD provisions do not limit maximum reinforcement.

 

[StrEng007]

What you're designing is masonry column, not a wall. If you had #5's @ 8" o/c in a wall yes it would be over-reinforced.

The max As for a column is 0.04 * An so that will give you much more typical results.

The steel does not contribute to the axial capacity since it is a wall element and the longitudinal steel is not tied. It's a single bar in the center of a grout filled cell.

Section 9.3.3.2.4 provides a condition that would remove the upper limit on the flexural steel, based on the M/(Vd) ratio.

I'll look into that.

If that section does not apply, the max of 0.00715 is based on the assumption that the axial load in the column is zero. If the column has axial compression for the load combination of D + 0.75L + 0.525QE, then the upper limit on the flexural steel is actually higher than 0.00715 by the ratio of (Axial Compression)/60000.

Same for wind? I'm not familiar with this section. As mentioned above, I'm not looking to analyze this as a column per say, since the the longitudinal reinforcing is not tied.

Last time I looked at this - the ASD provisions do not limit maximum reinforcement.

I'm limited to using the Strength Design methods as my wall designs classify as slender, are subjected to combined axial and gravity, and utilize a closed form solution for determining the P-Delta effect of said loading.

 

[CConrad]

@StrEng007 What you're looking for is Section 9.3.3.2.1 of TMS 402-16.

The bit about considering the effect of the axial compressive load is in item (d).

Director of Engineering
ENERCALC, LLC
Web:

http://www.enercalc.com

 

[JLNJ]

I wouldn't spec anything less than 2000 psi for f'm which puts you close enough to the #5 per cell max.

If you looked into it, you might find that the committee used f'm=2000 and d=4" to create the .286f'm/fy limit just to include the #5 in each cell case.

 

[masonrygeek]

Just a reminder that the minimum f'm for masonry is now (TMS 602-2016) 2,000 psi, and in fact could be higher than that based on typical block strenghts.

 

[StrEng007]

@CConrad
Is there a way to make Enercalc reflect the provisions of the sections you mentioned?

My design doesn't have anything to do with seismic forces. Doesn't that make the consideration of Qe and R irrelevant?

 

[CConrad]

@StrEng007 Feel free to send in a tech support email on the question of Section 9.3.3.2.1 of TMS 402-16. It's something I would have to do a little research on.

Director of Engineering
ENERCALC, LLC
Web:

http://www.enercalc.com