CMU - Bond beam reinforcement minimums? 1

[lexpatrie]

Looking at a proposed detail from a contractor, it's a below grade basement wall 12" with a bond beam more-or-less at the top, they have (2) #4 rebar. My typical detail shows (2) #5. Is there a minimum reinforcement ratio that's based on or is it just "we feel like it" engineering detail?

Is anyone aware of a percentage requirement here? This reinforcement doesn't seem to be based all that explicitly on a design scenario..... (Non seismic, Seismic requirements are fully deleted in the locale). Wall supported at top by a concrete slab and at the bottom by a garage slab. About 8' with 7-6" of 45 pcf soil is what I'm looking at as a design scenario.

 

[driftLimiter]

When I use those bars explicitly it is usually for a diaphragm chord.

 

[BSVBD]

lexpatrie...

Although we (both) SHOULD know what the ratio of reinforcement should be for CMU, due to me only having an ancient "1999 Masonry Standards..." rather than a "Building Code Requirements" for Masonry, referring to my ACI 318, and applying interpolations relative to the differences between concrete masonry and poured concrete design compressive stresses, one could EASILY justify using (2) #4 continuous longitudinal rebar in a 12" bond beam.

HOWEVE... I have been specifying (2) #5 bar, in a 12" bond beam, NOT because "I feel like it", BUT, similarly and, admittedly, with no more credibility, "just because" that is what I've been advised to do over 30 years ago.

IN ADDITION... Unless I'm misunderstanding...
The area of horizontal longitudinal rebar in a concrete masonry bond beam at top of a "basement" wall (per description), has absolutely NOTHING to do with the described 8' unsupported vertical length of a restrained (basement) CMU (quasi-retaining) wall. If ANYTHING, which, generally, is not reasonably or empirically required, for an 8' tall (or short) wall, vertical rebar WOULD be the ONLY consideration. For commercial structures, I DO specify vertical bar in an 8' CMU (or concrete) basement / restrained / retaining wall.

Bond beams at top of walls, are "typically" only "necessarily" (structurally) used to provide a solid bearing surface for trusses or joists, or grouted member for anchoring fasteners to support ledgers or angles or other.

Despite a popular belief that a bond beam provides lateral support, that lateral support can ONLY be negligible! ESPECIALLY in a basement or restrained/retaining wall as you have described above.

More than you really wanted to ... hear!?

 

[BAretired]

2-#5 continuous top bars are the minimum I have always used. 2-#4 just feels wrong, but there is no engineering reason that I know why it should not be used.

 

[phamENG]

Other than as diaphragm chord or to serve as a lintel over openings, I'm unaware of a standard to check them to. There are seismic requirements for minimum horizontal reinforcement, so I usually go by that even though I'm in a low seismic area. And the joint reinforcement covers that.

We can fall back on our local favorite, the IRC, which requires a single #4.

In 8" walls, I do one #5 or 2 #4s. I've seen them try to do 2 #5s...there's no room for grout at corners and intersections. For a 12", 2 #5s is fine, but 2 #4s probably is, too.

 

[bIockhead]

I've typically only known the "best practice" bond beams at the tops of walls to be mainly for the grout accompanying the steel, filling all voids, and providing a solid bearing for anything being built in or connected to the top of the wall, but the steel is still important to tie it all together. If it were for seismic then I'd expect there to be horizontal steel along the height.

I doubt the contractor really cares all that much if there is a bond beam there regardless. That being said, I don't know why a smaller bar would be a problem (or only a single bar on that note)

 

[BSVBD]

bIockhead... I agree with everything you stated EXCEPT.. that expression, "to tie it all together" is what I have heard too often!

While I can agree that the horizontal bond beam rebar "is nice", it can ONLY provide a MINIMAL, diaphragm-type lateral support action... "kinda"... for only a FEW of the adjacent courses, at top of wall only (with a few exceptions).

For an interior (commercial) wall (of limited height), with no exterior wind applied, this if acceptable.

For a basement wall, WITH resisting lateral support (horizontal floor structure - IN PLACE / APPLIED), only 8' tall, this can ALSO be (empirically) acceptable.

HOWEVER... When you have a basement wall exceeding 10' (for example), the horizontal rebar in top course bond beam only, or even MULTIPLE horizontal bond beams throughout the vertical height, will NOT be acceptable and will NOT "tie it all together" without vertical rebar.

I only elaborate on this because I do NOT want anyone to be mislead into thinking that horizontal bond beams and their rebar will provide ANY resistance to significant lateral forces that will be present in any type of retaining wall or wind wall situation, in walls of any significant height. Our job is responsible for safety... Just be aware!

On a slight tangent, in 2016, I started a post entitled, "How Do Residential Basement Walls Stand?" (Sorry - I don't know how to create links)

Since I deal exclusively with commercial and industrial buildings, the basement, restrained and retaining walls that I design are generally taller than 8'. Therefore, sparing the details, the title, "How Do Residential Basement Walls Stand" was intended to be rhetorical and humorous!

 

[dold]

BSVBD, I don't think anyone in here is talking about replacing vertical bars with a bond beam at the top of the wall. Nor do I think anyone is specifying any sort of walls without vertical bars. Maybe some of the residential guys...?

I think bIockhead's comment about "tying it all together" is saying that the bond beam at the top sort of captures all of the variable reactions (in and out of plane) at the top of the wall so your cells don't "separate", if you catch my drift.

But like others have said, the top bond beam is where you'll get some collector action to distribute in-plane loads to the rest of the wall piers if you have a bunch of openings and such. And chord action.

That said, I have designed tall walls of exterior stair towers to span horizontally to the adjacent perpendicular walls though, by utilizing the steel in bond beams every 48". They still had vertical bars too though.

 

[BSVBD]

dold... Thank you... I agree... I understand... I "catch your drift"... I also appreciate you bringing up the tall wall stairs and use of horizontal spanning to perp walls, not ONLY in stairs, but, multiple similar scenarios! Good (related) point!

lexpatrie's (OP) ending statement mentioned an 8' vertical basement wall being (only) top and bottom supported with no mention of vertical elements, during the "design scenario" inquiry.

Granted, an 8' vertical wall does not (necessarily) necessitate v-bar, but, I have FREQUENTLY dealt with contractors, supervisors, foremen, estimators, architect's / "designers", expeditors, etc, who have believed and expressed / suggested that a bond beam, in various situations should "take care of it" or "tie it all together" rather than installing the v-bar that I specified just because "I/we feel or felt like it".

bIockhead... I FULLY agree (and DID agree) with everything you stated, but, I refer to the previous paragraph and my previous reply where I JUST wanted those who may not, to "be aware"... therre are always some, of which, I have been among, and have appreciated the insight.

Thank you all! Good day?

 

[bIockhead]

BSVBD, thanks for the reply and for clarifying. Wouldn't want someone extrapolating the wrong idea. All I'm talking about is providing a solid area so that eccentric loading isn't dependent on a single face shell or web, as well as connecting a unit to adjacent units so that I'm not counting on the oop shear bond of the mortar interface around the unit to keep it in place. Purely talking about the local response, and not the lateral oop flexural response of the wall below.

As dold noted, I think we are all on the same page here.

For what it's worth, I don't think the area of steel matters if it's "best practice". For walls of that thickness, 2 bars is typical but the main headache for the builder is putting the bond beam there at all. Once they have the right units there and have shoved enough garbage in the wall (assuming partial grouting), whether it's 1 or 2 bars, and what size, isn't a big deal. I'd stick to what you have there now and maybe revisit if someone is getting into very granular embodied carbon accounting since extra steel is low hanging fruit for embodied carbon savings.

 

[zackngineer]

Standard in my area in Florida is (2) #5 bars... However, I've seen a few engineers actually go higher. I've been seeing some plans with #6s and #7s. I haven't seen a project that was built in the last 30 years with anything less than #5s.

 

[lexpatrie]

Thanks for the responses. Appreciated.

I suppose this will sound like a setup or a troll question, but I did excavate my copy of Narendra Taly, Design of Reinforced Masonry Structures, 2001. It's not Amrhein.....

As I mentioned (somewhat), this is a set of hand-drawn proposed details from a masonry contractor for repair of a basement wall, i.e. partial replacement. Below grade residential, tuck under garage wall retaining soil. Non seismic. Above is a 5" concrete slab (a bit atypical for a kitchen, but there it is). Bottom of wall has/will have at least the 3.5" slab for a basement that is considered a restraint in the IRC. Design is being checked for 45 pcf effective fluid pressure which is the middle of the road in the IRC tables, and drawings will note. (30 pcf and 60 pcf being the other options), and like I said, the contractor has a wall reinforcing I'm checking which is a bit enthusiastic, which is fine. Once I am more familiar with their work and quality working on less reinforcing is a possible conversation down the road, if they install what I have on the drawings....

Anyway, the whole thing looks like it could be done under the prescriptive code, with a little help from the Lintel Design Guide by NCMA, but the contractor really wants stamped drawings, and the State I work in just pitched a bit of a fit regarding redlines (for an architect), and I'm intending to redraw what all needs to be redrawn, and, of course, do the necessary calculations.

So regarding the bond beam, here's what I found in Taly.

In retaining walls, bond beams can be used as an alternative to providing horizontal reinforcement to distribute stresses that occur because of expansion and contraction of the wall. Typically, a bond beam is provided at the to of the wall and at 16 in. on center below. For 8-in.-thick retaining walls, two #4 bars usually are provided in all bond beam courses. For 12-in.-thick retaining walls, two #5 bars are provided in the bond beam course at the top, and two #4 bars in the courses below. Retaining walls are discussed in chapter 8.

Admittedly, this is not a retaining wall, per se, as it's not free at the top, pinned at the bottom, but it's at least a source for the 2 #5 continuous I tend to show in masonry details. Taly likes to treat these separately as subterranean or basement masonry walls (i.e. partial height soil load, pinned at top, pinned at bottom)...

Anyway, thanks for coming to my TED talk.

 

[zackngineer]

Anyway, thanks for coming to my TED talk.

It was great. God bless

 

[XR250]

I typically use #4's unless #5 is required by design.
Easier to bend and handle #4's so I might as well give the guys a break when I can.

 

[lexpatrie]

But, XR250, what load is "by design" for when you specify the #5¿? That was my question. What load are you resisting thats needing the #5?

 

[XR250]

If the bond beam is acting like a girt. Typical example is a basement stair running up along the exterior wall which interrupts the floor system.

 

[masonrygeek]

Hey everyone, let's try not to overdesign masonry here. Find out what the bond beam is supposed to do and design for that. As many have stated it may come down to providing movement (shrinkage) control of the CMU. If that is the case then design according to NCMA (now CMHA) recommendations for movement:

https://www.masonryandhardscapes.org/resource/cmu-tec-009-23/.

Settlement and foundation movement is always a concern and a bond beam can help tie the wall together and act as a panel. In other cases what you really need is a grouted course in which to embed anchor bolts, but that doesn't necessarily require horizontal steel. As noted in the NCMA TEK on control joints, No. 4 bars may prove adequate for reasons of movement control, especially when you are trying to design a wall without control joints.

 

[lexpatrie]

That's like the world's longest technical article that doesn't seem to have any code references.

Is there a code requirement for reinforcement ratio, explicitly in the code, that I can't find, regarding bond beams.

 

[masonrygeek]

Lex, typically there aren't any code requirements for movement joints since there are multiple ways to handle movement in a masonry wall (the code just says you must address and design for movement). The code isn't prescriptive so as to allow designers more freedom to design movement joints and/or horizontal steel. In this case there aren't any minimum "code requirements" for steel in bond beams. In fact, you can design a CMU wall with just joint reinforcement to meet the requirements for movement control. Look at Section 4.0 where it talks about an "engineered method" and specifically at Section 4.2 and Table 5. For movement control purposes, you could use typical 9 gage joint reinforcement every 16" o.c. and meet the requirements without the need for a bond beam. Or if you want to use a bond beam for other reasons, it could be spaced quite far apart.

And minimum and maximum reinforcement for vertical reinforcing is different, but that's discussion in another thread.

 

[lexpatrie]

If you had those vertical minimum and maximums at hand I wouldn't mind, for the sake of completeness having it here. We pretty happily divert from the main thrust in these forums but it's all interrelated.