TMS 9.3.3.7 - Special Reinforced Masonry Shear walls

[thedaywa1ker]

Does anybody know what the purpose of 9.3.3.7 of the masonry code is?




Special reinforced masonry shear walls are required to have horizontal reinforcement at these spacings (below), and the highlighted portion says reinforcement shall be embedded in grout, implying that it needs to be rebar aka a bond beam...the joint reinforcement above would be in mortar and not meet that requirement. At least, that is why I assume most things I've read say that the horizontal reinforcement of special walls has to be rebar.





Why, then, would 9.3.3.7 allow joint/wire reinforcement in shear walls in SDC D,E, and F, which are automatically special walls? Are we allowed to use joint reinforcement and forgo bond beams at 48" o.c. in special walls?

 

[EBAT75]

Not every Special Wall has to be fully grouted, except in SDC E,F or if prestressed. Whenever a wall is grouted even partially, there is the benefit that goes with embedding the steel in grout. It is not as securely held and is more susceptible to corrosion when placed in mortar in the face shells.

9.3.3.7 - The first “error” is - in E,F partial grouting is not allowed. Leaving that aside, 7.3.2.6 is SDC blind. It is a generic chapter. Chs 8 and 9 are specifics. “...shall be embedded in grout...” in the generic chapter 7 may also mean to include mortar bedding.

Between wire and bars, I think they are now interchangeable because wires also have to be coated now. So I think either can be used.

There is no code requirement for bond beams @ 48 in spacing. The max 48 in limit was an arbitrary choice from having been in the masonry codes for a long time (Commentary 8.3.5.2.1). That said, at roof and floor levels, a bond beam is desired practice as it is more solid and secure from displacement and in some ways acts as a header thus distributing loads more evenly.

 

[EBAT75]

A small addition. Forgot to mention that the minimum shear reinforcement is meant for improved ductility which I think is why embedment in grouted cells would be the preferred method of placement.

The comments are for running bond - most common. Special walls with stack bonds have to be fully grouted anyway.

 

[kevo_55]

Not to take away anything what EBAT said (which is 100% correct), there is some really good information at the Concrete Masonry Association of California & Nevada.

As of right now, their hardcopy books are sold out but it seems you can view them for free online.

http://cmacn.org/bookstore-CMACN.htm
Check it out!!

 

[thedaywa1ker]

Not every Special Wall has to be fully grouted, except in SDC E,F or if prestressed. Whenever a wall is grouted even partially, there is the benefit that goes with embedding the steel in grout. It is not as securely held and is more susceptible to corrosion when placed in mortar in the face shells.

9.3.3.7 - The first “error” is - in E,F partial grouting is not allowed. Leaving that aside, 7.3.2.6 is SDC blind. It is a generic chapter. Chs 8 and 9 are specifics. “...shall be embedded in grout...” in the generic chapter 7 may also mean to include mortar bedding.

Between wire and bars, I think they are now interchangeable because wires also have to be coated now. So I think either can be used.

There is no code requirement for bond beams @ 48 in spacing. The max 48 in limit was an arbitrary choice from having been in the masonry codes for a long time (Commentary 8.3.5.2.1). That said, at roof and floor levels, a bond beam is desired practice as it is more solid and secure from displacement and in some ways acts as a header thus distributing loads more evenly.

Thanks for the responses guys. 7.3.2.6, the section about special reinforced walls, is technically SDC blind, but if you are in SDC D E or F, ordinary and intermediate walls are 'not permitted', therefore 7.3.2.6 is implicitly talking about all shear walls in SDC D E or F.

I think the code is pretty vague about this.

7.3.2.6b - horizontal reinforcement 'shall be embedded in grout'

7.3.2.6d ' Shear reinforcement shall be anchored around vertical reinforcement with a standard hook'

Both seem to imply rebar to me.

And then, 9.3.3.7 is in the Strength Design section of the code. There doesn't seem to be an equivalent wire reinforcement provision anywhere in the ASD section. Does that mean we can use wire/joint reinforcement only when using Strength design methodology...?

I'm flipping through the design guides now Kevo - good stuff! I'll keep an eye out for when its back in stock and pick up a copy.

 

[thedaywa1ker]

It is vague enough that I think you could argue either way in practice...but I'd love to know the actual intent of the provisions for exam purposes.

I could 'maybe' see arguing that the joint reinforcement is 'embedded in grout' in a fully grouted wall, but in a partially grouted wall I don't see it holding up well.

 

[thedaywa1ker]

Sorry for the triple post - 9.3.3.7, in the strength design section, they say wire reinforcement shall be provided at 8" spacing, with (4) minimum 3/16" wires.

There is no such provision in the ASD section that I can find.

So, lets assume that 7.3.2.6 is not requiring rebar, and that we are allowed to use wire reinforcement in special shear walls.

If we are designing per ASD, then what controls our reinforcement spacing? 7.3.2.6b says 48" max spacing, but 7.3.2.3.1, before the special seismic section, says 'horizontal reinforcement shall consist of at least 2 longitudinal wires...not more than 16" on center'. This applies to all reinforced walls.


Why would the special shear wall provisions give a 48" or L/3 or H/3 max spacing, when wire reinforcement is already required at 16" for all reinforced walls? The intent of the special reinforcement provisions appears to be additional reinforcement on top of the wire reinforcement at 16"...meaning what? Use heavier wire at 48" on center, or rebar? Rebar makes more sense when you then go to the special wall provisions and it dictates standard hooks and embedded in grout.

Or, is our ASD design supposed to be dictated by the 8" spacing requirement of wire in the strength design section? Not likely

It still feels like they're wanting us to use rebar aka a bond beam at 48" on center for special walls, but they forgot to update 9.3.3.7, or, for some reason they are okay with wire reinforcement doing all the work in Strength designed walls, but want rebar in ASD.

 

[EBAT75]

I think we are getting caught up between two things.

1. Minimum/nominal horizontal reinforcement for ductility. This is the driving force behind the 8”......48” spacing.
2. Shear reinforcement (our normal tendency is to think of rebar only, wires are also reinforcement) that is required to share the shear demand with the masonry. Remember not until recently, masonry had to meet shear demand without any help from steel. Now, an 8 inch wall may cut it with shear reinforcement instead of a 10 inch wall before the new code.

For 1. Wires can be used but they may not provide the area required for 2. Under 2. rebars may be the better option for placement also. Mu/V*d would drive the amounts and placement in Special Walls. So the options are case specific.

As mentioned earlier, the 48 inch spacing is a relic of the past (this is mentioned somewhere in the code Commentary also). 48 being a multiple of 8, 16 it will automatically be be satisfied and thus make it redundant. There is no requirement for a bond beam every 48 inches. It is a “best practice” thing at roof/floor levels where it can also double up as diaphragm chords.

 

[thedaywa1ker]

Sure, I see where the commentary mentions 48" o.c. being a relic of the past. We still have the spacing requirement of L/3 or H/3 in 7.6.2.3b. For a 10' high wall, this would mean 3.33' spacing of...wire (automatically provided at 16" for all reinforced walls) or bond beams (rebar)

As mentioned earlier, the 48 inch spacing is a relic of the past (this is mentioned somewhere in the code Commentary also). 48 being a multiple of 8, 16 it will automatically be be satisfied and thus make it redundant. There is no requirement for a bond beam every 48 inches. It is a “best practice” thing at roof/floor levels where it can also double up as diaphragm chords.

I guess the whole point of my post is saying that I am confused by the code, because like you say, there is no explicit requirement for bond beams at 48" spacing. However, if we are to only have a bond beam at tops of walls and use wire reinforcement elsewhere, then the code is redundant and contradicts itself, and I don't want to go into the exam without understanding their intent.

7.3.2.6b is completely irrelevant and a waste of page space if they intend for us to use wire reinforcement as shear reinforcement in special walls, because all reinforced walls already have wire reinforcement at 16".

7.3.2.6d also is irrelevant because wire reinforcement doesn't have standard hooks.

That is the crux of my confusion...those provisions shouldn't exist unless we are supposed to have bond beams at H/3, L/3, or 48" on center. But, like you've said, it doesn't look like we are supposed to have bond beams at that spacing.

Did they really mean for almost half of the special reinforced shear wall provisions to be completely irrelevant?

I appreciate your playing along with me here - bond beams at 48" or 3.33' spacing seems like a lot unless you have a very high shear demand wall, but I don't understand these code provisions.

 

[thedaywa1ker]

My boss at my last job would call out bond beams at 48" or H/3 on his special masonry shear walls.

My boss at my current job does not and scoffs when I've suggested it.

The first primarily worked on the west coast, the second has only worked on the east coast. Might be a regional thing.

I see both points of view so this has been bothering me for a while.

 

[EBAT75]

Just 2 things about your immediate concerns.

1. About going into the exam hall with these questions in your mind, as long as you design and detail quoting the sections you are basing your answers on, the graders are not waiting to to penalize you. They are experienced people. There are two separate graders without one knowing the other’s grading, a third one if their scores differ (by more than 1 point I think, but not sure how much). So there need be no concern on that score.

2. Wires - these are not flimsy. They are also easily bent in the field during installation. If wires are lapped one end must be bent into a grouted cell. Also, on a related topic, the end units are always grouted. The detailing of reinforcement in that unit can take care of anchorage in different ways whether bars or wires. Bars are not necessarily the means for meeting the requirements.

Thank you for the posts.

 

[thedaywa1ker]

Thanks for humoring me EBAT. For the exam I'll probably stick with specifying bond beams at 48" if it is on an afternoon question, because you cannot argue that they don't meet all these requirements, whereas the viability of wire reinforcement in its place is debatable.

 

[thedaywa1ker]

As always, eng-tips.com has some good discussions...at least I'm not the first to ask similar questions:

Special Masonry Shearwall - Joint Reinforcement - Structural engineering general discussion - Eng-Tips

Horizontal Joint Reinforcement in SDC D, MSJC 2011, Special Reinf Masonry Walls - Structural engineering general discussion - Eng-Tips

The Masonry Society published an article about it as well, but its $75 that I don't think I feel like spending: TMS Journal Volume 36, No. 1 Published | Masonry Institute of Michigan

The structuremag article linked in the second thread suggest that the 8" spacing required in Strength design provisions for wire (9.3.3.7), isn't applicable to ASD design, and you don't even have to have wire at 8" spacing. Meaning for ASD we use 16" spacing of wire for all walls (per 7.3.2.3.1), with an additional, POINTLESS requirement of 48" o.c., H/3, and L/3, and standard hooks in our wires for Special walls (????? Still confusing....) Meaning zero difference in horizontal reinforcement between special reinforced masonry walls and any other reinforced wall, (besides maybe hooking the ends into the last cell like you said EBAT)...shear wall and seismic region or not.

That just does not sound right, especially knowing all the other 'special' seismic detailing requirements for concrete and steel etc.

In conclusion, I think the answer is a resounding 'maybe'. Clear as mud! But, bond beams at 48" etc are still my safe bet for exam purposes.

If the code makes sense to someone and you think I'm crazy then please tell me so I can move on with my life