What are the limits of prescriptive design? 9

[DTS419]

The IRC, as we are all familiar, provides prescriptive design standards meant to cover common construction of “one and two family dwellings and townhouses up to three stories.” Examples of this prescriptive design include connections such as from wall to foundation, headers over openings, etc.

But what are the limits of these prescriptive provisions, and whose responsibility is it to identify them?

Let’s take a closer look at connections to foundations, for example. It’s not uncommon for large custom homes that fall within the IRC’s scope to have finished basements with tall ceilings resulting in deep foundation walls with significant unbalanced soil load. There can also be significant uplift loads that must also be transmitted to ground depending on the proportions of the superstructure. These forces can easily exceed the capacities of the prescriptive provisions, that, if I had to guess, were developed long ago with simpler construction in mind.

It’s also not uncommon for many home builders to skip architects and engineers and simply follow the IRC. I’ve seen too many projects where this happens, and the result is connections that are over capacity, lateral systems without adequate diaphragm and shear wall detailing, etc. This often doesn’t result in total failure, but rather a final product that doesn’t meet current standards of practice, making it hard to call the builder’s attention to flaws with “the way we’ve always done it” that might be code compliant but not necessarily sufficient. And of course, failures can and do happen in the worst cases.

So what mechanisms, if any, are in place to ensure that simply following prescriptive codes are adequate for every situation, and whose job is it to identify when an engineered design is required? And, who is responsible if a code compliant prescriptive design ultimately proves to be inadequate for the situation?

 

[lexpatrie]

We've been talking about limiting the span of masonry foundation walls, turns out it's already in there.

(Courtesy of another thread).

Keep in mind, generally, the floors/joists are "considered" the point of support, i.e. wall is presumed to span vertically.

As a side note, the masonry wall reinforcement tables and the concrete wall reinforcement tables are revised a LOT (or they were when I was paying more attention in 2006 or so), and I think many of the tweaks in the tables come from the WABO folks.

Oh, as a Side note, I may have found the anchor bolt spacing that DTS419 was discussing previously, look for Ehrlich, page 87 of the document below...

2009 IRC Resource, Approved changes

As a side note, RB116-07/08 proposal by Steve Skalko, of the Portland Cement Association, for foundation wall reinforcement, that suggests to me these are based on calculations.

 

[Mike Mike]

lex,

Great find on the repeal of the 2006 wall anchorage requirements. Here's a screenshot of the 2006 code:

5" OC!!! quite a difference from 6', no wonder the powers that represent the contractors flipped, here's the movement to squash it:

I have no issue with their conclusions. If they have collected 200,000 data points from unbiased samples and found no issues resulting from lack of connection at top of basement walls, then maybe IRC is right. Maybe it's the IBC that needs to change. Let's dig in and call the IBC and NDS numbers into scrutiny.

That being said, I am skeptical only 41 problems were reported. Let's dig into the Concrete Foundations Association's "members" and their affiliations and validate the data. Certainly seems to be at odds with Kissymoose's observations.

as to NAHB's comments, I suspect Gary threw together this list without consulting a structural engineer (or a spelling/grammar checker). I could poke holes in every one of these items, but I'll focus only on "no resistance to lateral pressures provided by the floor framing and floor diaphragm is taken into account" uhhhh in order for the load to get from the wall to the diaphragm doesn't it have to go thru the anchors? not sure what subject Gary got his PE in but it certainly didn't require tracing loads.

one thing that doesn't make sense to me about all of this push back from contractor's associations, if the price of homes has to go up because of mud sill to foundation wall anchorage, that price increase will be passed onto consumers. perhaps NAHB members are scared because basic economics says an increase in price will cause a decrease in demand, and therefore a decrease in company revenues and share prices. however, in this case, shelter is virtually universal, and most consumers do not see forgoing shelter as an option.

moving on to your other items lex, 1992 CABO Figure R-404.9 appears to only allow balloon framed masonry walls, which takes care of the OP's original concern. this is similar to my 9 Jun 24 09:17 post on 1995 CABO.

a couple issues with your code masonry lateral support requirements:
1. concrete wall chapter 608 contains no equivalent provisions
2. length is not limited where walls are supported by floors, highlights below

concrete walls 608 does not appear to pertain to walls resisting soil pressure, but I did find the following enlightening, and may give the OP some ammo for future conversations to make his engineered designs a little less embarrassing. these tables appear to require 1/2" bolts at 24" or tighter, depending on wind loads.

 

[lexpatrie]

Found this by accident.

Code Prescribed Limitations of Conventional Construction and How to Mix them with Engineered Design, Osterberger, Lockyear, Brandt, Wood Design Focus, Fall 2003.

There's a graphic!