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Mixing Solid Sawn and Engineered Wood Products


hausdok
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All engineered wood components are manufactured in a relatively "dry" state. The moisture content of engineered wood products at the time of manufacture ranges from approximately 4 to 12%. During the manufacturing process, the wood-based resource must be dried to these levels to insure that a good glue-bond is developed. A range of values is given because some adhesive systems used in some products have different moisture requirements.

It is also important to realize that these are not average moisture contents as traditionally measured. If a certain adhesive system requires a maximum 6% moisture content to develop an adequate glue bond, then every piece must meet that maximum during fabrication. A traditional average where 50% are above the maximum and 50% are below just doesn't work. Only those pieces that were at or below the maximum will ever get to the market place.

Traditional "dry" lumber on the other hand is dried to a much higher moisture content, typically 19%, although some lumber is dried to 16%. Because of natural variability, the range of moisture content of the lumber pieces in a given bundle may vary widely. A given lumber element may even have moisture gradients along the length or across the width.

In service, however, such as in a residential structure, after 4 to 8 months of drying, all wood elements will reach an equilibrium moisture content of from 6-10%, depending on the season and location of the structure. As the engineered wood products are very close to this normal equilibrium moisture content as manufactured, and because they are typically shipped in a waterproof protective wrapping, they take on little or no additional moisture and their dimensions vary imperceptibly during this period. The sawn lumber, however, during this period dries down through a relatively large range of moisture content. Along with drying comes an equally significant shrinkage. A 14" deep sawn lumber element can shrink as much as ¾" in its depth as it cycles from the as-dried to in-service equilibrium moisture content. This difference in behavior between solid sawn lumber and engineered wood can lead to structural failure if the designer is not careful.

APA EWS I-joists and APA EWS Rim Board products are manufactured in 9-1/2", 11-7/8", 14", and 16" depths. It is no accident that these sizes are not compatible with, and are larger than, traditional lumber net depths for 2x10's, 2x12's, 2x14's, and 2x16's. There are many applications in roofing systems and especially residential floors, where other elements are used in conjunction with the I-joists for the express purpose of transferring load through the floor system without overloading the floor joists. Some examples of these other elements are blocking panels over an interior bearing wall and rim or starter joists. In these cases, the vertical load from the structure above the plane of the floor is transferred through the floor into the structure/foundation below by way of direct bearing on the blocking panels, rim or starter joist.

Because the load is transferred in direct bearing, it is essential that the blocking panels, rim or starter joist be the same height as the floor joist. Solid sawn lumber cannot be used in applications like these because of the very likely potential for shrinkage. Shrinkage by as little as 1/8 of an inch (3 mm) can be enough to transfer the vertical loads from the walls above directly to the floor joists, thus inducing possible bearing or reaction overload conditions at these locations. The solution to the problem is to use engineered wood products for these applications. They are manufactured in the correct depths and have the same dimensional stability properties.

While the previous discussion concerns vertical loads, the same is true of lateral loads such as those caused by wind and seismic events. The small gap between the floor sheathing above and the sawn lumber rim joist or blocking panel below resulting from shrinkage of the lumber members can have a small but negative impact on the performance of the structure during the design event. Even greater however, will be the impact on the deformation of the structure caused by the potential slip at this location under design lateral loads. While not necessarily life threatening, these greater deformations can result in increased damage to the non-structural components of the building, such as drywall, windows and doors, cabinets and interior and exterior finishes. If the deformations are excessive, they can cause the structure to be irreparable.

Every application where solid sawn lumber is used in conjunction with engineered wood must be looked at very carefully with respect to the different moisture states of materials at the time of construction. The safest alternative is to not mix engineered wood with solid sawn lumber in any situation where load sharing might be an issue.

For more information, visit:

http://www.APAwood.org

http://www.SouthernPine.com

http://www.SIPS.org

TIJ wishes to thank Mr. Ray Clark, Product Support Specialist, APA Help Desk, for a very enlightening and useful contribution. Mr. Clark can be reached by phone at: 253-620-7400; via fax at: 253-565-7265 or via e-mail at: ray.clark@apawood.org.

For a printer-friendly, MS Word version of this document, click the link below.[/b]

Download Attachment: icon_word.gif Mixing Engineered and Dimensional Lumber.doc

Disclaimer

APA, the Southern Pine Council, the Structural Insulated Panel Association nor their members make any warranty, expressed or implied, or assume any legal liability or responsibility for the use, application of, and/or reference to opinions, findings, conclusions, or recommendations included in this communication. Consult your local jurisdiction or design professional to assure compliance with code, construction, and performance requirements. Because APA and the Southern Pine Council have no control over quality of workmanship or the conditions under which wood products are used, they cannot accept responsibility of product performance or designs as actually constructed.

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  • 3 months later...

Hi All,

Thought I'd tell the brethren about this and ask for some assistance. The issue below was what inspired me to run the series of articles by J.D. Grewell and the article above back in July. Since then, things have escalated a little bit.

Here's the situation. One year old home is one of more than 75 in a development that are all done the same way in Washington State. I was hired to do a warranty inspection for the homeowner, prior to expiration of the one-year warranty provided by the builder. This is the warranty that builders tell homeowners about here - what they don't tell them about is the mandatory 6-year "habitability" warranty and the 10-year structural warranty on new homes here.

The builder bought a custom "framing package" from a lumber company that included specs and stamped drawings from an engineer specifying sawn rims to be used with engineered I-joists.

Initially, based upon what one of the builder's employees told me, I'd reported that these were 9-1/4 inch high I-joists, but I was wrong on that point, as I later learned from the builder via a letter after the client asked for correction and was turned down. The joists turned out to be a custom-sized joist made to a depth of 9-1/4 inch, so they'd match the height of the sawn rims, unlike most manufacturers who make them 1/4 inch high on purpose so these can't be matched.

There are no web stiffeners or squash blocks under any of the typical point loads. Instead or using I-joists at these locations, the builder doubled up 2 by 10's. Blocking panels are inconsistent. Some are engineered lumber while others are blocks cut from 2 by 10's. Everything is on 19.2 inch centers and walking through the home is like walking on a drum head, everything shakes and rattles. Some unevenness in the floors where some bumpouts have been cantilevered. Again, instead of anchoring the inner end of the cantilever to engineered lumber, I-joists were replaced with 2 by 10's and the transition can be felt when walking across the floor. Where this was done with engineered lumber one doesn't sense a transition. These are two-story homes and the same system has apparently been used between floors.

The builder maintains that it's fine, because the lumber company's engineer signed off on it and the AHJ approved it. The engineer says it's fine because the sawn lumber rims will sustain more load than a doubled-up I-joist or APA-rated rim board will. The lumber company says he's been selling these packages like this since about 1990 or 1991 and has never had a problem.

APA-Engineered Wood Association folks say, "No way," the sawn rims will shrink - some more than others - and shrinkage of even 1/8-inch is going to place an unacceptable load on the I-joists at the perimeter, therefore the perimeter is essentially, all point loads and everything should have been reinforced with squash blocks to prevent damage to the trusses if/when the rims shrink.

The code being used by the locals specifies "used in accordance with the manufacturer's instructions."

The sales folks for the I-joist manufacturer say that they've know about this lumber company selling these packages for nearly 20 years but that they haven't the power to dictate how their product can be used by private companies. They say they know that there have been a number of battles over it with some local code officials and it usually manages to get approved, although they are aware of one job that went bad because of it.

The chief engineer for the manufacturer says it's wrong, that differential shrinkage could cause some issues, but, like the sales folks, he has no say over the vendor or the end user's use. He defers to his company's installation manuals and the Wood I-Joist Manufacturer's Association's guidance. He points out that they have pre-fabricated squash blocks and a computer program that would have shown the builder exactly where to place all reinforcement to make this work right. He also points out that the lifetime warranty on the joists will be null and void if this system fails because they failed to follow their (manufacturer's) instructions.

Their own documentation doesn't even discuss what one would need to do to mix these two together in this situation, and only specifies what types of engineered APA-rated rim board can be used. WIJMA's tech library specifically addresses it in a paper and says it's a no-no.

Back to the lumber company - the vendor maintains that APA and WIJMA and the manufacturer only want to sell APA-rated products, so naturally they'll say only engineered rim board material may be used.

Back to the engineer who designed the package - he says shrinkage isn't an issue and maintains that APA and WIJMA and the manufacturer only want to sell APA-rated products, so naturally they'll say only engineered rim board material may be used.

Paul Fisette, frequent contributor to JLC and head of the Wood Technology program at UMass Amherst, thinks they're playing with fire.

J.D. Grewell, thinks it's a nightmare waiting to happen, John Bouldin agrees.

To me, it's non-standard construction and at the least, an experiment, and I felt that the homeowners should have at least been informed about it before they'd purchased the home, so I made sure to explain the potential issues in my report and recommended they have it looked at by another engineer, who is intimately familiar with how I-joists are used in modern-day construction, just to ensure that the vendor's engineer hasn't made a serious mistake.

Homeowners went back to the builder to demand an explanation and are looking for a warm and fuzzy - either through correction or verification by another engineer. The builder refuses to do either.

Since the first one, I've done four more inspections up there and found them all the same. Three were in the same development and one was in a separate development by the same builder several miles away. In the past, I've seen it in other homes in that general region, but didn't realize that it was a standard lumber package sold by a single lumber company - I'd thought it was individual builders screwing up and had reported it thusly. Obviously, this will be the same in every home in that development and this builder has developments all over the county.

Now, I see I-joist systems done by other buiilders all the time. In fact, I did a one-year warranty inspection on a home yesterday morning that had 9-1/2 inch I-joists. Engineered rims, web stiffeners and blocking panels of APA-rated materials where needed and sawn-lumber squash blocks where necessary. Floor system was solid, and quiet with absolutely no vibration. Like night and day when compared to these one's.

If APA, WIJMA's and the manufacturer's fears come true, and a system fails, one would expect that it's ultimately going to come back on the engineer. One, maybe even two homes, might not be a huge issue, but what if dozens, maybe even hundreds start to fail, the engineer goes bottom-up and the manufacturer refuses to honor the warranty, because of this non-standard "experiment?" Scary stuff - especially for the homeowners, many of whom are Navy servicemen and women on limited, fixed-income budgets.

What are your various thoughts? No problem because there's an engineer's stamp? Shrinkage issues are over-rated? APA and WIJMA are just covering their butts and there's nothing to be concerned about here? Have any of you ever seen this successfully done and have plenty of evidence to show that it's not an issue? Or, are the vendor, engineer and builder out of their gourds?

I'm being pegged by the builder as the bad guy here, for knowing that it's an atypical installation, making my clients aware of it and recommending that they do a little more homework to ensure it won't be an issue, but, if it looks like it will be, to demand whatever corrections are recommended to them by other experts. For doing that, I've received a cease and desist order demanding that I send letters to my clients retracting what I'd reported and, if I don't, threatening legal action, including seeking an injunction to preclude my dissemination of further "erroneous" information, pending the outcome of a trial. I've obtained counsel and it's being handled appropriately.

A future article in TIJ will probably deal with the issue of whether or not a builder has the legal right to try and muzzle home inspectors through legal action vs. simply examining the issues brought before them and dealing with them straightforwardly and honestly in a cooperative manner for the benefit of their buyers.

If any of you have encountered a similar issue with an I-joist installation, have had the builder challenge you, because the AHJ approved it and an engineer's stamp was on it, and then had the manufacturer back you up in the same way, I'd like to talk to you about it. Please e-mail me off-line at hausdok@msn.com.

ONE TEAM - ONE FIGHT!!!

Mike

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  • 2 weeks later...

Hi Mike,

I've read though the discussion of this over at the JLC forum and for what it is worth, I think you are doing a great service for your clients in the way you are approaching this problem. Unfortunately, you are running up against a builder who is trying to strong-arm you into keeping your mouth shut. As an aside, I know that many HIs, builders, and tradespeople knock the municipal building inspectors by saying that the building inspectors cannot be sued. The situation you find yourself in is the primary reason why the building inspectors are protected. They are protected in order to keep the threat of lawsuits from compromising their judgment. It is not to protect them from incompetence. Any building inspector (or engineer in public service -- I was one for over 21 years) can be fired by their employer for screwing up or for abusing their authority.

The situation for these homeowners as you have rightly pointed out is as follows (and you'll recognize some but not all of this in a post from another engineer at the JLC forum):

The code says either build to the code and follow the manufacturers instructions, or do the engineering.

The builder chose to not follow the manufacturers instructions and hired an engineer.

The engineer says that what the builder plans to do will be ok.

The AHJ, who typically is not a PE, relies on the engineer's PE that all is ok, and gives the ok to the builder.

The manufacturer usually voids their warranty if their product is not installed per their instructions.

If the builder follows the engineer's design, then the adequacy of this departure from the manufacturers instructions falls squarely on the shoulders of the engineer who gave the ok.

======================

I certainly hope there was more engineering in that PE's decision than "the manufacturer doesn't want you to mix engineered and sawn lumber because they want to sell more engineered lumber". Perhaps there is, and he hasn't disclosed it yet. There should be a really good reason for the builder wanting to do this, because I can't see the cost savings in the rim joist (by using sawn lumber) justifying the potential cost of correction, should the engineer be wrong.

As an engineer, the logic behind the APA stance makes sense to me -- engineered and sawn lumber absorb and release moisture in different amounts and at different rates, resulting in differential movement between the two types of materials. I feel like I'm in good company since there seem to be several experienced HIs, engineers, builders, tradesmen and manufacturers that feel the same way.

Further support comes from the "JLC Field Guide to Residential Construction - A Manual of Best Practice" which says in the section on wood I-joists:

Acceptable rim joist materials include: the same size I-joists as those used in the flooring system; one or two layers of plywood or OSBs; or an engineered lumber or metal product. Don't mix wood I-joists with solid lumber on rim joists.(bold is theirs) Not only are the two materials difficult to match in height, but also solid lumber will shrink after installation, leaving too much weight on the I-joists."

RS Means "Residential & Light Commercial Construction Standards" 2nd edition, 2002, also advises against using sawn lumber at the rim in I-joist construction.

If I was a homeowner and I knew about this situation before I purchased the home, it would be a deal breaker for me. Based upon what I know about mixing sawn and engineered lumber within the load path, I wouldn't want to be the guinea pig for the builder's experiment on the structure of my home. If the experiment fails the fix could be costly.

The homeowners who have already taken title to the home are in a difficult situation if the builder digs in his heels. It's going to take an independent engineer, one who has no dog in the fight between builder and manufacturer, and who is more qualified than the original engineer, to examine the as-built homes and the original engineer's work and render an opinion if this is ok or not ok. That opinion could come about as either speculation based upon theory of what might happen over time, or through failure analysis if symptoms of differential movement have already become apparent.

If I was in your shoes and the builder started trying to muscle me, I'd begin contacting my former clients in that development, one at a time, explain the situation to them, ask them if it was ok if I disclosed their name and address to my other clients in the development, whisper the words "class action" in their ear, and suggest they start talking to their neighbors.

BTW, if I ever start acting like one of the engineers that is posting in that thread at JLC, I want you and Chad to take turns smacking me upside the head until I come to my senses.

Brandon

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