IRC Foundation Wall Reinforcing 3

[SteelPE]

In the 2009 IRC table R404.1.2(1) has requirements for horizontal reinforcing for poured in place concrete foundation walls. The requirements are as follows:

wall height less than or equal to 8'-0" 1-#4 at the top of the wall and at mid height
Wall height greater than 8'-0" 1-#4 at the top of the wall and at the 1/3 points of the wall height.

What is the purpose of this reinforcing? Is it to limit foundation cracking that tends to occur in foundation walls?

 

[KootK]

So far all I've heard is ACI318 is better because its more, and it works in commercial structures.

If that's all that you've heard then your listening skills are marginal, IRC style.

1) I've seen little cracking issues in commercial structures reinforced to 318 and;

2) Lots of cracking in residential structures reinforced to IRC.

It's that simple. Those of us that don't care for the IRC prescriptive wall designs observe that:

a) IRC requirements provide much less reinforcing than 318 requirements (nothing to debate here) AND;

b) In our own, personal experiences, it seems that IRC constructed walls develop more cracking issues than 318 walls (can't really debate someone's personal experiences).

That's it, the end. If your experiences and judgement take you in another direction, so be it. That's why you've got the stamp with your name on it.

Should I throw away any code books with prescriptive requirements that might be less conservative than other code books or engineering calculations.

That's up to you. In your opinion, should I scour time and space for the most permissive wall reinforcing requirement ever put to print and then toss all of my less permissive references? Rumor has it the 1923 Peruvian Residential Code only required one outstretched coat hanger and two hail Marys per wall.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[GWoodPE]

your listening skills are marginal, IRC style

should I scour time and space for the most permissive wall reinforcing requirement ever put to print and then toss all of my less permissive references? Rumor has it the 1923 Peruvian Residential Code only required one outstretched coat hanger and two hail Marys per wall.

Thanks for keeping the discussion constructive and professional.
Last I checked, states weren't adopting or referencing the 1923 Peruvian Residential Code as a standard.

In our own, personal experiences, it seems that IRC constructed walls develop more cracking issues than 318 walls

That's why I'm asking, I'm not saying my experience is the be all end all. I'd rather learn from the mistakes of others than my own. So what are the cracking issues that you are seeing? Can they definitively be solved with more T&S reinforcing? How do you know what reinforcing is in the walls with cracking issues? Are there other issues that could be causing the cracking, high W/C ratio, backfilling without support? You seem to have already acknowledged that shrinkage cracks may not be the main issue

Not necessarily shrinkage cracks but basement wall cracks in general. It seems to me that, when real issues crop up, they tend to be associated with things like frost and hydro-static pressure rather than -- or perhaps in addition to -- shrinkage cracking

So are we talking about different issues? Issues that need to be solved by means other than additional horizontal reinforcing?

Do all, or even most, lightly reinforced basement walls crack up? Of course not. If they did, practices would change. In this context, I think that a "high risk for cracking problems" probably means something on the order of 5% of the homes that get built this way. But, then, that's still too much in my opinion and the opinion of that lucky, one in twenty home owner who has his or her basement go to hell in a hand-basket.

If you are saying that 5% of houses are a potential problem, and most of those are associated with frost and hydro-static pressure, what percentage are we down to if we first ensure there is enough flexural reinforcement, and second make sure proper drainage is installed and maintained. And then maybe third, make sure the contractor is installing the reinforcing properly and using a proper concrete mix. Is there an acceptable percentage?

If they did, practices would change

Isn't that what we are trying to get to the bottom of? Does practice need to change? SteelPE started this thread with the question of what the reinforcing is for and the conclusion he came to is

I suppose I will just continue on not installing it in my foundations and stick with the 2-#5 bars horizontal top and bottom

Seems sad if we can't even convince someone to use the IRC minimum. Or it doesn't make a difference if he uses 2#5 T&B or 3 evenly spaced since anything less than ACI318 is inconsequential?
It SEEMS like that is what you are saying, its ACI318 Table 11.6.1 or bust. I'm trying to suggest that maybe there is some level of conservativeness (?) in ACI318 that isn't maybe necessary in residential construction. This article would seem to acknowledge that.

https://www.toolsofthetrade.net/trucks-equipment/jobsite-equipment/a-new-kind-of-concrete-code_o?o=2

But maybe Baty is just a shill for the concrete contractors, but I'm not sure why the concrete contractors would want to promote the use of inferior concrete walls. Maybe CRSI needs to gain more influence in the 332 committee.

Please don't take my questioning as an attack. There's no need to be defensive. I'm legitimately trying to broaden my knowledge and in turn maybe others as well. Isn't that the purpose of this forum? Like I said, probably 95% of the basement wall cracking issues I see are CMU not concrete, and even the CMU walls with problems are likely 30-40-50+ year old houses. That's not to say we won't be seeing the same issues with concrete walls in 20-30-40 years, so if that is the experience in other parts of the country/world and more horizontal reinforcing will solve it, then we should be working on getting some code changes implemented.

Link Referenced article in case the other link doesn't work.

 

[bob33]

Empirical design often does not meet the requirements of an engineered design. There have been many examples, but one of my favourites is the empirical design of unreinforced masonry walls subject to out-of-plane wind loads. I used to design school gymnasium walls in Canada that were almost 20' tall between lateral supports using 8" block + 2" cavity + 4" brick with 3/16" dia. brick ties (pre-metric days). The code allowed it at that time. Years later I checked one of these walls against the engineered method and was surprised by the amount of overstress in tension in the horizonal mortar joint at mid-height. Still later I learned that the code values for masonry mortar in flexural tension were too high when checked against the American code; the walls were even more overstressed using their engineered method. However, these walls are still standing after 50 years or more. The code has changed so that now the wall could only go 3.8 m (about 12'-6"). Although the masonry mortar values have not changed, the wall (with or without the brick) would still be overstressed if checked using the engineered method.

 

[bones206]

I was talking to my Dad about this basement wall topic last night, and was surprised when he told me he had personally witnessed basement wall failures during house construction projects he was involved in back in the day. On two of them, he said the walls cracked wide open and deflected inward several inches. On one job he said the wall completely broke off at the ends and collapsed. In all cases the walls were backfilled without any bracing elements at the top and had no rebar in them whatsoever. He recalled that in one case a fork truck was driving close to the wall when it failed, and one other case was due to high groundwater. This was probably in the 70's or 80's.

In that context I can see how a couple continuous horizontal bars tying into orthogonal end walls would go a long way towards increasing the resiliency of basements, especially in the construction phase.

 

[Settingsun]

SteelPE, what was the reason for your 2/#5 top and bottom? Seems to be a similar magnitude of steel (minimal) but located differently. Perhaps the underlying reasoning is also similar to the IRC.

I also struggle to see such a small amount of reinforcement contributing greatly to crack control. It's roughly 0.05% so the tensile strength of even weak concrete is around five times greater (order of magnitude). There's more to consider but, with that difference in basic strength, whatever force/action cracked the concrete will also yield the reinforcement and the initial crack would tend to grow large rather than the steel 'catching' the crack and forcing others to form. I don't know why Bones206's wall had several crack at regular spacing but maybe the restraint from the footing prevented the initial crack growing uncontrolled and forced more cracks to form.

I question the ACI comment that their reinforcement rates have been successful over time (either ACI 318 or ACI332 levels). Is concrete cracking not a major reason for upset clients and owners in USA? It is everywhere else. Is cracking more acceptable in residential buildings aka the owner's castle? I think home owners are equally/more concerned but simply have less money to pursue those concerns.

 

[Settingsun]

Bones206, do those walls have a connection to the slab such that they can cantilever vertically? Or are they required to span horizontally until the upper floor in built?

 

[bones206]

So those basements are in my neighborhood and I just happen to walk past them on my evening walks. I don't know how they were designed, but I know they do not have any connection at the footings. No keyways or dowels, nada. But even if there was a connection there, the footings are only 16" wide, so I'm not sure they would resist rotation or sliding much. I assume the designer is depending on the basement slab for lateral support at the bottom. Except the contractor is sometimes backfilling before the slabs are in place.

 

[Settingsun]

I thought that was your site but now I look at the background of the photos can see that they're just relaxed about safety. Fall from height into water with only a red ribbon to keep people out...

 

[KootK]

Please don't take my questioning as an attack. There's no need to be defensive.

I disagree. All meaningful debate is a cyclical dance of attack, defense, and counter attack. I prefer to revel in it rather than deny it.

Thanks for keeping the discussion constructive and professional.

If your feelings were hurt, I apologize for that. I was trying to interject a little humor into a situation where it would probably have been best just to be direct. The direct version would have been this:

1) I find your questioning repetitive and cyclical.

2) My impression is that you are not really absorbing the answers previously given to your questions.

As one of your dance partners in this discussion, I'm frustrated by this.

So what are the cracking issues that you are seeing?

1) When I practiced in WI, cracked basement walls were a thing. Almost a cliche really. If I'd cared to, I could have easily made a living out of just driving around looking at people's cracked basement walls.

2) In Alberta, where I currently practice, cracked basement walls are just as much of a thing as they were in WI.

3) On this forum, I have seen many, many examples of cracked basement walls all over the US and Canada in both concrete an masonry.

Can they definitively be solved with more T&S reinforcing?

In my opinion, yes. I feel that most common concrete cracking issues would be improved by the addition of reinforcement. What particular cases are you thinking of that would not?

How do you know what reinforcing is in the walls with cracking issues?

1) Sometimes I can look in the cracks and observe the absence of reinforcing.

2) Sometimes I can review the construction documents and observe the absence of reinforcing.

3) Sometime wall ares scanned to reveal the absence of reinforcing.

Are there other issues that could be causing the cracking, high W/C ratio, backfilling without support

Of course. I mentioned couple of alternate causes previously, as repeated below. But then, so what? Do I have to prove that ALL cracking issues are related to a lack or reinforcing to validate my opinion that many cracking issues would be ameliorated by additional reinforcing?

- We all know that the wall to diaphragm connections are mostly worthless in residential and, therefore, so is much of the logic vertical spanning walls. It is a dangerous thing to not have some horizontal spanning capability.

- Most real world structural failures have a building envelope failure associated with them. As such, I'd still consider a big, purely vertical crack to be an invitation to future pain.

It seems to me that, when real issues crop up, they tend to be associated with things like frost and hydro-static pressure rather than -- or perhaps in addition to -- shrinkage cracking

So are we talking about different issues? Issues that need to be solved by means other than additional horizontal reinforcing?

It's hard for me to say as I'm not really clear on what your issue is at this point. Can you restate it for clarity such that I might respond to your question? My issue is really just this:

I prefer to see meaningful quantities of reinforcing in basement walls for a variety of reasons. And I do not like it when prescriptive code clauses, combined with market pressures, prevent me from seeing my preference put into practice

If you are saying that 5% of houses are a potential problem, and most of those are associated with frost and hydro-static pressure, what percentage are we down to if we first ensure there is enough flexural reinforcement, and second make sure proper drainage is installed and maintained. And then maybe third, make sure the contractor is installing the reinforcing properly and using a proper concrete mix. Is there an acceptable percentage?

I'm not in possession of the statistical answers to those questions I'm afraid. My 5% number was just spit-balling. Regardless, my feeling is that the possibility of other construction deficiencies, such as inadequate drainage, is just another good reason to have more reinforcing. Stuff happens and nothing's perfect. So providing some strategic contingencies makes sense to me.

Isn't that what we are trying to get to the bottom of? Does practice need to change?

Yes and yes in my opinion.

Or it doesn't make a difference if he uses 2#5 T&B or 3 evenly spaced since anything less than ACI318 is inconsequential?

I, for one, have never said that the small amounts of rebar prescribed by IRC should omitted or that they offer no benefits. I simply feel that more reinforcing would yield more benefits.

It SEEMS like that is what you are saying, its ACI318 Table 11.6.1 or bust.

That is not what I'm saying. Nor is it anything that I have said in this thread. You're putting words in my mouth.

I'm legitimately trying to broaden my knowledge and in turn maybe others as well. Isn't that the purpose of this forum?

Yup. That said, motives here are not always as pure as one would hope. Sometimes arguments are had because folks:

1) simply like to argue or;
2) are not receptive to contrary opinions.

Everyone has an ego after all. The only questions are how large is that ego and how well is it managed.





HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[bones206]

Dusted off my great grandfather’s 1957 CRSI Design Handbook and found an interesting bit on basement walls.

 

[bones206]

Often a pair of bars is added at top and at bottom of a wall to provide longitudinal beam action from footing to footing, pile cap to pile cap, or to bridge across soft and firm subsoil conditions.

 

[KootK]

I do love me those old CRSI manuals. Based on the data there, it seems that an reinforced, 8" basement wall probably stands a pretty good chance of working for realistic loading situations. That, at least for straight up flexure induced by vertical spanning behavior.

As long as we're making a Christmas wish list of things that we'd like to see done differently... If I had my druthers, the code would outlaw using wood diaphragms to resist basement wall lateral thrusts. I simply do not feel that the stiffness, strength, or connections of wood diaphragms are well suited to the task. And, as GWoodPE intimated, issues at stair wells etc only make things worse. I'd like to see all of these systems forced to be either horizontally spanning walls or cantilevered retaining walls. I know... fat chance.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[kissymoose]

I simply do not feel that the stiffness, strength, or connections of wood diaphragms are well suited to the task

I'm going to make a photo album of all the rim boards that overhang leaning basement walls.
Who knew a couple toenails into the sill plate and anchor bolts at 6ft o.c. don't work?

 

[bones206]

I came across this article on a concrete wall trade group website: Link (btw, they state that the purpose of the horizontal bars is for shrinkage cracking control). Snooping around on their website a bit, it looks like this trade group CFA is interested in improving the quality of basement wall construction. Might be a good partner for structural engineers if we really did want to make some code changes. They even have an ACI-style tech note summarizing code requirements: Link

In the presentation of these suggestions for a Building Code, the Committee realizes that perfection has not been attained. In soliciting criticisms it became apparent that changes might be made indefinitely.