The Witch of November

"With a load of iron ore - 26,000 tons more
Than the Edmund Fitzgerald weighed empty
That good ship and true was a bone to be chewed
When the gales of November came early."
from "The Wreck of the Edmund Fitzgerald", by Gordon Lightfoot

The short term weather forecast and additional warnings and discussions from the Weather Channel and others, indicate a high probability of damaging winds on October 26th and 27th in the Midwest. The storm pattern and intensity is being compared to the infamous 1975 storm that took down the finest inland ship of the day, the Edmund Fitzgerald, and the lives of the 29 crew members on board.

Although being on a boat or ship during such a wind storm would be scarier to me than the best Haunted House you can imagine, these wind storms are nothing to be taken lightly when you're on land either. Problems with strong winds range from trees falling on transmission lines (power outages) to blocked roads limiting access for rescue vehicles or citizens, all the way to collapse of buildings, loss of property inside and even loss of life for the building's occupants.

If structural engineers seem a little withdrawn in social situations (sorry, that's a stereotype unfairly reinforced as... I.... sit alone in my office writing this blog post), it could be because they spend their time thinking about and planning for this type of nasty event on a regular basis when they are designing buildings. While no building can be considered completely safe from natural forces, a properly designed building will have a much better chance of avoiding damage than an improperly designed building. And if a better designed building does receive damage, it will still be better suited to maintain protection for property and occupants located within the building during the event.

Horizontal winds from storms like this (I'm excluding the high uplift pressures generated by a tornado) act upon on a building and can cause 4 basic types of failure. This illustration was developed by APA, The Engineered Wood Association and illustrates the general failure modes. The uplift shown below could result from horizontal winds passing over a roof shape similar to the uplift created on an airplane wing (the Bernoulli principle) or it could result from the cyclonic action of winds in a tornado although, as I've said, forces from tornado winds are generally not calculated or accounted for directly in building design or building code requirements.




In my opinion, there is also another failure type that might not be its own "General Mode", but it is different than the four shown. Internal Pressurization can occur when a large opening exists on one wall without adequate offsetting openings on adjacent or opposite walls. This creates what I loosely call a "Windsock" effect where you could have a strong wind coming into the building through a large opening and the building itself acts like the windsock with pressure building up on all surfaces whether horizontal or vertical. If the skin of the building or windows in the building shell cannot resist those pressures, local or widespread building failure could occur.

In buildings with large door openings, predicted storms like this one are good reminders to keep these doors closed during wind events like this for a couple reasons. For one thing, they will add some stiffness to the building shell to resist racking. The other thing it does is close off the end of the wind sock to prevent pressurization of the building.

Buildings still under construction are especially vulnerable to wind damage as they generally have some significant amounts of material area in place to capture wind forces, but the walls or systems to transfer that load back into the ground may not be completed, resulting in much higher stresses on certain building elements than a similar wind event would ever cause in the building once the construction of the building is complete. Temporary, or construction, bracing is usually left up to the discretion of the builder to make up for the lack of the completed building structure during this phase, but it has been my experience that this construction bracing, when present at all, is often not installed adequately to resist the loads that the completed building will be capable of.


I hope I am wrong, but I will not be surprised if some buildings under construction are damaged in some way by this impending storm. My hope in writing this post and sending it out to friends and colleagues in the building industry is that this damage can be minimized or eliminated over the next two days.

If you have any questions about this, or any other building engineering questions, feel free to call me!

Aaron Halberg, P.E.

Structural Engineering for the Minnesota Fair

I was priveleged to do some structural engineering work for a local company here in Hayward, Northwood Outdoor, on one of the 12' wide "shed" buildings that they donated for use by Ron Schara's "Minnesota Bound" to display and sell merchandise at the Minnesota State Fair. Northwood Outdoor makes some quality small buildings, used by people for a range of purposes as you can see at their website, from storing lawn and garden equipment, to children's playhouses, to bunkhouses or even portable sheds mounted on trailers. Feel free to view their website, give them a call, or stop by the next time you're in the area of Hayward, Wisconsin.

One of the primary structural engineering issues for this relatively simple building that I found very interesting, challenging, and rewarding, was justifying Northwood's method of roof construction. Typically, rafter pairs like they use are either connected to a structural ridge beam or else connected together with a minor connection at the ridge and then tied together close to the ceiling with a "collar" tie. Either method is intended to provide structural rigidity and resistance to deflection for each rafter pair so that no horizontal thrust is imparted to the tops of the load bearing walls. Some illustrations will likely be more helpful than many more words:

Structural Ridge beam provides vertical support at the ridge to prevent rafter rotation and the resulting horizontal thrust to the supporting walls. The ridge beam must be supported by endwalls or columns (not shown).

In the collar tie method of framing rafters, gusset plates or metal connectors are used to hold the rafters together at the ridge while the moment resistance to thrust is provided by the collar tie acting in tension in the lower portion of the rafter.

The stapled plywood moment plates custom-designed by Halberg Engineering for Northwood Industries is shown here as it was used in the Minnesota Bound building at the 2010 Minnesota State Fair.

In Northwood Outdoor's situation, they were looking to use their plywood gussets stapled to each side of the rafter connection at the peak as the only structural rafter connection so they could maximize interior headroom and avoid the expense and support issues of a structural ridge beam. The stapled plywood moment connection developed by Halberg Engineering allowed them to use 2x6 Machine Stress Rated rafters and the plywood gusset stapled in a specific pattern to resist the design snow loads in the Minnesota and Wisconsin areas (up to 42psf).

As you can see from the photos of the fair building, the rafters were left exposed, but other than myself and other construction professionals, I'm sure everyone was looking at the merchandise in this building instead of the rafters. I still get a kick out of the fact that we strive for properly designed structures to be relatively ignored by their occupants while they focus instead on the activities or contents within the building.

Please let me know if I can help you with something similar or if you have any questions.

Thanks!

Aaron Halberg, P.E.

Halberg Engineering

Wood Construction - Looking better all the time!

Wood seems to be the obvious choice for the structural frame work in 1 & 2 family (residential) dwellings because of it's low cost, availability, and the familiarity of the work force with the skills required to work with wood.

So what about Commercial Construction? Many commercial structures are similar to residential, but I can think of three areas that change the most when comparing the needs for these buildings:

1. The Building Code – Although building codes for Residential construction are certainly more complex and complete in their reach than they have been, there aren’t as many trade offs in code requirements for various building materials as there are in the Commercial building codes. The advantage through the code development and lobbying process that goes with it is going towards materials other than wood as the code seems to limit commercial wood construction to smaller building sizes allowed and tighter sprinkler thresholds. Your own knowledge of the building code and easy access to a good Design Professional (ahem!) will allow you to better serve your building owners as you help them successfully navigate the choppy waters of today's building codes.

2. Average Sizes and Spans – the practical limits on span and size of wood structures continue to get pushed larger as we become smarter about how we use wood and combine it with other materials to achieve amazing things. I recently saw that a prominent wood connection manufacturer is currently testing a Seven (7!) story wood building in Japan for earthquake resistance. Also, realize that metal plate connected trusses are being used to clear span 60’, 80’, even 100’+ today by using dimensional lumber with Metal Plate Connectors. Compare these spans to what was possible with traditional rafters and ridge beams or rafters and collar ties that was the norm 2 generations ago and is still used in many residential buildings today. The irony is that the average size of available wood coming out of our forests is shrinking, but producers of engineered products are making stronger, straighter, and more reliable wood building materials out of these raw materials. This progress allows spans in wood construction today at the higher commercial floor and roof load requirements (of the building code, see #1) that would not be conceivable with the wood products available just a decade or two ago. Engineered use of Wood is making a difference!

3. Construction Budget – Typically higher for Commercial vs. Residential, however I think that this generalization has reversed its trend over the past 20 years as many new houses over the last decade have still been built much larger than needed (“McMansions”) while many businesses and multi-family developments are looking to build with the minimum capital requirement because the financial conditions reward the projects with the best cash flow, or return on investment. Advantage in the cost department: Wood!

There is another issue where wood has a clear advantage when comparing building materials and that is overall sustainability. The issue is quite simple but has been made complex in an by various political and special interest groups lately.

More about sustainability soon, but for now keep in mind, wood is good!

• Wood uses a very efficient and natural form of solar energy for its production (Photosynthesis!)... very little energy is required to harvest and process it
• Wood can be naturally grown and replaced faster than it's rate of consumption without sacrificing the environment in the process (now THAT'S sustainable!)
• Wood is easy to recycle or reuse or compost
• Wood buildings, when properly constructed, can last as long as any other material used in construction today.

Plan Review of Lighting for Commercial Buildings in Wisconsin

reposted here from Safety & Buildings website:
Wisconsin Safety and Buildings Announcement
January 29, 2009

Subject: Reemphasis on commercial buildings general lighting plans and about submittal of emergency lighting plans.

General lighting and emergency lighting will receive more emphasis during commercial buildings plan reviews by Safety and Buildings Division staff effective March 2, 2009. The Wisconsin Commercial Building Code has included requirements for illumination and electrical energy efficiency since 1980.
S&B has been flexible over time about requesting submittal and approval of lighting design plans and calculations. At times division staff have asked for more lighting information than at other times. Whether submitted for review or not, lighting code compliance has been necessary. S&B will renew review of general lighting plans because of concerns that compliance with recent energy efficiency code changes has not been demonstrated.
In March of 2008, substantial emergency lighting and lighting energy efficiency changes occurred; some of the energy efficiency factors have become 30 - 40 percent more stringent. With our nation's and state's focus on building lifetime energy usage, the building lighting designs need to be clearly recognized as consequential.The specific aspect of plan review of emergency lighting plans, important to the safety of occupants and first responders, comes from field observations that some emergency lighting installations are not meeting code requirements. The fact that emergency lighting is often installed late in construction, even after partial occupancy of a building, may have led to less-than-necessary coordination of egress paths and emergency lighting design and installation.
Beginning March 2, 2009, submittal of general lighting plans and emergency lighting plans will be required by the Safety and Buildings Division for plan reviews for: - New buildings; - Additions to buildings;- Initial tenant build-outs within buildings for which initial plans were received on or after March 2.
For plans received on or after March 2, 2009, if S&B commercial building plan reviewers find general lighting and emergency lighting plans have not been submitted for review with the initial building plan review, the reviewers will contact the designer with the option of the designer providing lighting plans as part of the immediate review. If the designer chooses to not do so, the general lighting plan and emergency lighting plan can be submitted later, before installation, for separate plan review.
Fees for the plan reviews are not increased:- For general lighting or emergency lighting plans submitted with building or HVAC plans, there will be no additional fee. (Overall fee calculated per code, Comm 2.31.)
For lighting plans submitted separate from building or HVAC plans, the fee will be the current $75 revision plus the $100 submittal fee, a total of $175. (Per Comm 2.31(1)(f).)
For submittal of plans after construction, the standard late submittal fee of $250 will be assessed, as well as the $100 submittal fee, for a total of $350. (Per Comm 2.31(1)(d)6.)
See the S&B WebSite for general lighting and emergency lighting code and submittal information.

Environmentally Friendly Materials and Practices

A great definition of sustainable construction was presented by Professor David Bohnhoff at this year's Wisconsin Frame Builders Association conference during his discussion of "green" practices:
"Sustainable Construction meets present needs without compromising the ability of future generations to meet their own needs."

Prof. Bohnhoff also pointed out that, in almost every measure, when you look at the factors that go into making a building the most affordable it can be, you are also making the building the most environmentally friendly that it can be. This is a direct result of the fact that much of the cost that goes into a project reflects the recouping the costs of the embodied energy, or the energy required to produce, use, and dispose of the building at the end of its life. The less energy required, the lower the impact on the environment, and less energy also translates to lower cost. Post frame construction is a Win-Win-Win in this regard.

In another point of the discussion, minimizing the life cycle energy costs to condition our occupied spaces leads us to make our spaces as energy efficient as possible to start using less energy per unit of occupied space as soon as possible. This is basically a reason to perform construction sooner than later at any time as improvement in energy efficiency serves as a positive return on investment with constant energy costs, but that return rate would improve in a scenario of increasing energy prices.

So, if you're going to build a structure that uses less energy to condition, you should also look at minimizing the total material usage (embodied energy) that goes into the building. Some types of structures are not particularly efficient, including Timber-frame construction and even full log construction if the logs are harvested, debarked and shpaed on a site away from the buildnig site. The amount of material used can be minimized in our building shapes by looking at regular shaped buildings with good aspect ratios (length to width), not too square (inefficient for natural lighting and structural framing) yet not too narrow (inefficient for energy loss because of high exterior wall area per unit of floor area).

Post Frame construction is just one of many options for energy efficient and environmentally friendly construction although it is obvious that wood material is a replenishable resource (trees, switchgrass, corn: all are "biomass"). But even concrete can be made in more environmentally friendly today when the heat used to produce the concrete is recovered waste heat from other emission systems. This drastically lowers the embodied energy in concrete.

Professor Bohnhoff made some great points about why we should be very wary of mandated green building standards as we've seen over and over again that standards tend to stifle creativity and innovation, which is really what leads us to improvements in all areas of our society. The man-made impact on Global warming should be held in suspicion as many of the promoters of that theory are using more fear and less facts which seems to move towards mandated standards to combat such a large problem, regardless of the cost of implementing those standards.

In so many ways, there are reasons to be optimistic about the future of the built environment as the quality of materials improve, innovative use of energy continues, and intelligent choice of building design continues to get better.

Is it easy being Green?

According to Kermit the Frog, "It's not easy being green." (I believe he also said "Times fun when you're having flies.") I wonder if Kermit's song lamenting the color green was a result of not knowing about the environmental advantages of Post Frame Construction? Probably not.

Yet it seems to me that as people learn about environmentally friendly building systems, or "Green Buildings", they will discover the advantages of building with the easily renewable, reusable, and highly resilient resource: Wood! I believe we have been given this renewable resource to use intelligently with proper technical knowledge, and also to harvest it in an efficient manner.

There will undoubtedly be more and more emphasis on this topic as time goes on since people have become very politically and emotionally engaged in the move to cause no damage to our environment and to reverse the effects of damage done in the past. While I don't agree that we had the power to change the global climate that many are giving us credit for, I do believe we should do what we can to be good stewards of this world in all of its natural wonders.

One of the green building rating systems is supported by the Green Building Initiative headquartered in Portland, Oregon. The GBI uses a Green Globes rating system. What exactly is Green Globes? According to their web site at http://www.thegbi.org/:

Green Globes is an easy-to-use online assessment tool that reduces your risk and speeds ROI by providing continuous feedback on project ratings as you move through the certification process. Green Globes was adapted from a green certification tool in use in Canada for over 10 years.

Mark Rossolo from the GBI presented an interesting talk at the NFBA Expo last week in Columbus, Ohio, and as I understand it, the Green Globes rating system is approved as an ANSI standard and will result in green projects being rated on a Green Globes scale of 1 Globe to 4 Globes with 4 Globes being the highest rating.

GBI offers a free 30 day trial to the Green Globes online assessment tool for one project which I will be trying myself over the upcoming weeks, in addition to attending a free online webinar on Feb. 27th at noon Central time. You can register for this webinar yourself at the GBI website here.

Over the next weeks and months, I will be investigating the Green Building ratings for wood frame structures, specifically Post Frame structures which, because of their efficient use of wood framing members and ability to use a minimum of steel and concrete, should be among the highest rated green buildings available today. I will report back here soon to keep you up to date on what I am learning.

Have a great day!

Aaron

SIP Construction and APA Siding for Post Frame

A new APA Publications Update features a new Structural Insulated Panels publication and a variety of new and updated Technical Topics.

Featured Publication
APA and the Structural Insulated Panel Association (SIPA) have jointly published a new 20-page guide on Structural Insulated Panels (SIPs). The full color publication covers SIP advantages, applications, design and construction considerations, assembly and related topics. Numerous connection details also are included. This brochure is available for purchase or as a free downloadable PDF. Form H650, 20 pages, $2.

Related structural insulated panel CAD Details are available at http://www.apacad.org/.

Technical Topic of interest to Post Frame:

Technical Topics provide recommendations and explanations for a variety of design and construction details.

New Technical Topic: APA Rated Siding Applied to Post Frame Structures. Available only as a free downloadable PDF. Form TT-023, 2 pages.

The APA e-newsletter is sent to those who request it when registering to download publications. Many of the publications are available as free downloadable PDFs with registration on the APA website or can be purchased in printed form.

Wisconsin to Adopt IBC 2006

The State of Wisconsin, in addition to being the home of the Green Bay Packers and Halberg Engineering, is also the home of the oldest building code in the Union. The state-specific code changed for the first time in July of 2002 when Wisconsin adopted the 2000 edition of the International Building Code (IBC). The adoption of the 2006 is currently underway and is in the hands of the State Legislature.

I learned today that the anticipated date of enforcement for the new building code in Wisconsin is March 1, 2008.

Building plan submittals that are received at a State Safety & Buildings office before this date will be reviewed under the current code (IBC 2000 with ammendments) and building plan submittals received at a plan review office on or after March 1, 2008 (or the final date of enforcement) will be reviewed to the IBC 2006-based code.

Just one example of changes in the 2006 code, A-2 Assembly occupancies (Restaurants, Taverns, Banquet Halls) currently have a sprinkler requirement for occupant loads of 300 people, but the 2006 IBC code requires sprinkler systems at an occupant load of 100 people or more. Of course the code is a relatively complex set of documents and standards, so I would always encourage owners and general contractors to work with their Design Professional as early in the design process as possible to ensure that any advantage available may be obtained by performing the plan submittal before or after the March 1, 2008 implementation date.

Post Frame Ring Shank Nails in NDS!

During a revision of ASTM standard F1667 a few years back, ring shank nails were removed from the standard and were subsequently dropped from recent editions of the National Design Specification (NDS) for Wood Construction. Now, thanks to the work of the NFBA T&R Committee under the lead of Patrick M. McGuire, P.E. and with special effort on his part, the ring shank nails are acknowledged by the American Wood Council of the American Forest & Paper Association, keepers of the NDS.

In a just released update to the NDS in the form of NDS Design Aid No. 4, ring shank nails are making their return to the NDS under the new name of "Post Frame Ring Shank Nails". The design aid contains design values for single shear connections to supplement Table 11N (wood to wood) and Table 11P (steel plate to wood). The design aid can be found at http://www.awc.org/pdf/DA4-RingShank.pdf

The NFBA T&R committee has been prioritizing this issue pretty highly in recent years to not only reinstate the nails as part of the standard but also to re-evaluate the design values that were allowed for these nails, believing that specific testing and research will result in more accurate and higher allowable loads for both shear and withdrawal values for use in design.

So, although this release of Design Aid No. 4 is a significant step, the design values it contains are limited to shear values only and these are not significantly higher than a similarly sized common nail. Testing currently being performed for the NFBA should provide the basis for justifying higher allowable shear loads and also the withdrawal values in future editions of the NDS.

Horizontal Siding for Post Frame

Horizontal siding is often fastened over vertical studs in conventional wood frame buildings, so horizontal 2x4 or 2x6 girts used instead of studs on post frame buildings presents a bit of a dilemma... or is it an opportunity? Recommendations for new construction include the creation of a drainage plane behind the siding by using a housewrap over the structure and vertical air channels under the siding (picture at left).

To create the vertical air channels, both conventional wood frame and post frame buildings require vertical members behind the siding. This moisture drainage plane assists with more than just exterior moisture as the membrane is designed to allow moisture out of the wall to atmosphere.

Vertical nailing members will be required at a spacing no greater than the maximum fastener spacing required for the siding. For horizontal vinyl siding, installation specifications recommend 16" o/c fastener spacing and for Fiber Cement siding (such as HardiPlank), 24" o/c fastener spacing is recommended. By attaching vertical members to horizontal girt members, Post Frame wall will have less energy loss through conductivity than an equivalently insulated conventional wood frame wall (since, as shown in the picture, vertical studs would be lined up with vertical strips).

Keep in mind that when horizontal lap siding is used instead of light gauge metal wall panels, a separate method for resisting lateral loads will still be required, such as a layer of structural sheathing or adequately designed diagonal bracing. Also, Post Frame vertical strips spanning from girt to girt should be larger than the 1/2" x 2" strips shown above.

If this topic is of interest to you, or if you have comments to share with others, please share your comments using the link below.

AWC online calculators

The American Wood Council has made a couple handy calculators available for free online. The span calculator will tell you what size and species will satisfy a given load and span conditions or you can have it determine the the maximum span a given size, species, and load condition can achieve. Their most recent calculator is a connection load calculator that can specify the allowable capacity for bolts, screws, nails, and lags. This calculator is based on the latest data available in the 2005 edition of the NDS, which is adopted in the 2006 IBC.

Again, these are handy tools which I encourage you to explore and use. If you have questions about the factors that the provided help cannot answer, contact your favorite design professional for assistance!