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If structural engineers seem a little withdrawn in social situations (sorry, that's a stereotype unfairly reinforced as...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