Lateral Bracing of Garage Wing Walls 1

[RG88]

I am looking for a double check on my reasoning in applying the provisions of the International Residential Building Code. The section in particular is 602.10.1 of the 2000 IRC.

This section requires wall sections that are less than 2'8" long to be braced by special design as the code does not address this point. This comes into play for the little wing walls on either side of a garage opening. I have been working in both the commercial and residential areas of structural engineering for quite some time. I must agree with the code that these wing walls must be braced properly, but I have found myself in disagreement with 'other' engineers in my area.

As one would analyze a lateral load resisting wall as a shear wall, I am analyzing the opening of the garage as a portal frame that must resist induced moments from wind loading on the side wall. My analysis is very similiar to how one would analyze a moment resisting builiding bent for a commercial structure.

In this lies my issue: I am factoring the moment resistance all the way down to the footing and I am coming up with larger than average footing sizes and that the reinforcement must extend into the footing. This not unlike the analysis and detailing one would expect to find on a set of commercial building plans. Most of the 'other' engineers in my area are not carrying the reinforcement down to the footing, merely to the masonry foundation wall below with J-bolts...but unreinforced masonry cannot resist axial tension. Alot of my clients have voiced concerned over the difficulty in constructing this type of braced frame system but they do agree with my thinking. Unfortunately several people want to use the 'other' engineer's methods because they are easier to construct. I would like to reference any respondants to this thread to the Simpson Strong Tie 'Strong Wall'...basically this is the same thing that I am doing, but like Simpson I am connecting the wall through the masonry foundation wall to the footing.

The way I look at it, if the code doesn't cover it, one must exercise sound engineering judgement in the analysis. My judgement is to not cut corners and do what I feel is right, but the local building jurisdictions in NC are accepting the, in my opinion, lackluster designs of others. Any other interpretations on this codes section? Esecially from those working in Hurricance country. I have the ethical issue well in hand, but I am starting to think about the client retention issue.

To elaborate: The typical garage opeing is 10' tall by 18' wide and assume that the header is the top of the bent. The wing walls are 2' wide braced panels that rest on a 8" wide unreinforced masonry wall of the same length, these are the side verticals of the bent. The lateral force applied to the top corner of the bent is around 10,000#. The way I calculate it, the moment at the bottom of the braced panel must be transmitted to the footing, through the foundation wall with reinforcing bars, not just J-bolts in the masonry alone.

Thanks for any help or insight you may offer.

 

[rccon]

I fully agree that your anchor rods would have to go down to the footing, but don't see how you can resist 10 kips of lateral force with two 2' wide shear walls. This is 2500 plf of unit shear which is about twice what you can resist with a two-sided shear wall. The Simpson 2' x 10' shear wall has an allowable shear of only 1590 lbs, while you have 5000 lb. per wall. Also the hold-down force would be too great for any available anchors for a wood wall. It seems you need a steel bent, and the footing beneath the opening designed for the resulting moments, with adequate weight to resist overturning. 10 kips is a much greater force than I would ever encounter in this situation, but maybe that's because I'm in Seattle, WA where we use 80 mph wind speed.

 

[RG88]

Thanks rccon. I am glad that someone agrees that anchorage should extend to the footing.

By the way, you are right, I somehow typed the wrong number in there. Every house is different. Each wing panel will take any where from 1000# to 2000# each. The typical value being closer to 1000#. The wings vary in size as well, ranging from 18" wide on smaller garages to 2'-6" wide on larger houses.

For a 2'8" wide wing, the code governs and requires the panel be connected to only the foundation wall with a connector rated for 1800# of uplift (in the code writers mind how does unreinforced masonry carry tension????).

Also, our wind speeds are 100 MPH in central NC and 120 MPH on the coast.

 

[boo1]

Foundation:
Dowel with standard hook the blocks at corner and at opening, top with bond beam.
Hold Down Connector:
Mud sill anchors or through bolts are easier to install then HD2A's.
Wall System for short wood shear segment
Consider solid studs and plywood on both sides.

 

[boo1]

See link:

http://www.buildingscience.com/resources/walls/shear_panel_test_results.pdf

 

[rlflower]

I encounter this type of problem quite often. Your choice of a masonry stem wall might have been dictated by cost. I would think deputy inspection will be required for the masonry to allow a design to full strength. In any case, I agree that the anchorage should extend down to the footing. I do not agree to the bond beam solution proposed by Boo1; we need a to pick up the weight of the stem wall and the footing to resist overturning. Detail threaded rods as substitutes for the manufacturer specified anchor bolts, with a plate washer fastened with a double nut at the bottom end of the threaded rod. Embed the rod to full development.

In cases where the overturning proves to be greater than 90% of the resisting dead load moment, design a grade beam continuous for the full length of the garage below the two stem walls. Embed the rods into the grade beam, thereby picking up the grade beam and the stem walls.

I do this all the time, and it proves to be a clean detail. The contractors I work with don't mind it a bit.

 

[boo1]

The block stem wall in residential construction in my area is layed on the footing. Concrete footings are minimum 8" x 16" with two steel 5/8" reinforcing rods, 10" x 20" with three 5/8" for two story. the stem wall is comprised osf standard 8x8x16 CMU blocks.

What we use is a bond beam on the top row, that is tied to the footer with rebar hook (#5) and grouted at the corner and garage opening. I thought this was following, SSTD 10-99. You indicate these details are not adaquate, can you describe why?

See Link:

http://www.sbcci.org/North Carolina/ncirc/ch-4.pdf

 

[RG88]

rlflower

It sounds like my detail and analysis is very similiar to yours. I have developed a set of details and instructions that allow the contractor to provide the anchorage pre or post footing pour. They don't mind the intensity of this detail, they want something simpler (really though, the construction community is upset about the new building code). It will take a little time for them to come around. Every time the code is updated everyone is upset because they don't like change.

boo1

I believe the method you describe will work as well. Unfortunately, I have to contend with residential masons who are not familiar with bond beams. The method you describe is very similiar to an approved alternate method in the code. I considered the bond beams and associated anchorage. Around here we use similiar footings to what you describe, but everyone use 4" brick-4" block foundation walls (8" multi wythe), so there is no way to develop lap length of foundation anchor bolts to veritical dowels in the footing. So I have called out for a continuous A307 threaded dowel embededed into a thickened footing and then continuous through the masonry wall to the framed shear panel above.

Most of the stem masonry walls at the garage front are less than 2' high (typical residential construction in central NC).

I am happy to see that others agree with me on the need to treat this subject properly and to utilize anchorage tied to the footing. I appreciate your comments. FYI, I have approached several of the local building jurisdictions and have found that they agree with me on the need for continuous anchorage as well.

 

[boo1]

Typically one story garage opening designs (with small aspect ratio shear segments) are not overturn but shear driven. To reduce the loading also consider adding plywood to the inside garage wall too.

SSTD 10-99 requires exterior foundation walls to be min 8" CMU. See fig 303C for the details to attaching foundation tie-down methods.

 

[rlflower]

boo1:

As an engineer in Southern California, which is entirely a Zone 4 seismic catagory, I stand with my comments made earlier. The issue I see that you are overlooking is that of what is commonly called "deadman". A funny term for such a serious subject, but it is descriptive. You have to determine that you have enough weight provided by structural materials (i. e., "deadman&quot to resist the uplift of the anchor. Your idea of anchoring to a bond element on the top of the masonry wall does not at all address this concern. You and I can easily imagine how this anchor can pull up without much resistance if it is terminated in the bond element.

RG88 and I agree - along with a good share of the engineering community - that we need greater depth in the anchorage to pick up the "deadman".

I hope this clears things up for you.

 

[boo1]

rfflower,
What elements does tying the bond beam to the footer with rebar miss?

 

[rlflower]

boo1:

Our basic disagreement is this: is there or is there not overturning of the walls on either side of the garage door? You contend there is no overturning in these walls. I hope you can explain to me how this could be possible.

Perhaps what you are assuming is that we have a moment-frame condition with "pinned" supports; such a condition would in fact absorb all the moment into the frame and zero moment at the supports. However, even in this case, there is still overturning of the system due to siesmic/ wind loads - and the question still remains: is there enough dead load to resist this overturning? If not, add the deadman to resist the uplift due to overturning.

 

[boo1]

I do agree there is overturn in the wall.
The vertical steel ties the bond beam to the foundation. The detail is lacking? please help me learn!

 

[rlflower]

boo1:

Ok, we're on the same page. The problem we are now faced with is that of detailing.

If I have not been clear enough on this previously, my appologies. Just step back from the problem for a moment and get the big picture perspective. Any element placed on top of the masonry wall - bond beam or whatever - is not sufficent to resist uplift simply because it is on top of the masonry wall. The elements we need to rely upon for uplift resistance are the masonry wall and the foundation beneath this wall. We need to properly engage these elements to "pick them up" - thus resisting the uplift force.

The detailing of this engagement is therefore critical. How can we ensure that the vertical anchor rod used to transfer the uplift force to the masonry wall/ foundation system will actually transfer that load? To answer that, take it piece by piece: 1.) is there enough embedment of the rod in the material it is engaging?, 2.) Can we develop the classic "shear cone" from the end of the rod to determine the shear capacity of the material the rod engages? This shear cone starts at the plate washer I have recommended previously and enlarges at 1:1 ratio to the face of the material the rod engages (i.e., the bond beam, or the masonry wall, or the foundation). 3.) Does the material the rod is engaging have enough bulk to reist the uplift? Keep in mind that elements such as concrete and masonry do not have tensile capacity, so the bulk we are referring to is that which is adjacent to and above the rod.

And here is my point: if you cannot satisfy all the above concerns by simply engaging the bond beam (thus not "picking up" the masonry wall), then engage the masonry wall; if the masonry wall does not offer enough material to staisfy the above concerns, then engage the foundation; if the foundation does not satisfy all the concerns addressed above, then enlarge the foundation until it does.

Ok, I think I have been as clear as I can possibly be; I hope you now understand what I am trying to say.

 

[boo1]

Yes I understand, your second too last para. was very well written. But back too our earlier posts, how is the detail I indiated wrong?

Seldom do residential garages have overturn issues. The designs are usually shear and flexable diaphragm design driven.

 

[boo1]

I do understand the foundation should be designed so that the overturning moment of the structure due to the wind load does not exceed 2/3 of the dead load stabilizing moment of the structure.