Posted on October 30, 2020 by Curt Gear
This methodology, commonly referred to as ‘Hardened Construction’, allows the engineer to pick a wind speed that is lower than the 250 mph and use a standard ASCE 7 design criteria. With 98.8% of all wind events being that of and EF-3 or less we choose to work this ideology with a wind speed of 167mph (top end of an EF-3). This allows us to use much more standard precast elements when creating these large volume spaces. Most importantly we are able to design the roof with a double tee element without using a post applied integral topping. The flange thickness on a double tee is increased from the standard 2” to 4” and there is an increase in the flange connections form DT to DT to develop the necessary amount of shear resistance. With the lower wind speeds the uplift is in a more manageable zone where it can be accounted for with the self-weight of the double tee and increased connections to the wall panels. There is also increased prestressing in the upper portion of the double tee stems. This helps with resisting the upward camber of the roof elements during a high wind event. This type of construction is significantly less expensive than achieving the 250 mph resistant structure that is required in ICC 500 design. Generally speaking you are only looking at a $4-$6/sf increase on the roof costs only. The wall panels in their normal design state are already capable of handling the loads created from a 167 mph wind event.
In summary, precast concrete components are the most durable and resistant materials in their natural state to large wind events and with a little thought upfront by the design team a school district may be able to afford the increase costs of a hardened construction gymnasium where an ICC 500 structure may not be in the budget. There are currently several of these EF-3 Hardened Construction gymnasiums standing the State of Iowa and more are being considered and designed every year. I hope this information provides some food for thought as you move forward with future K-12 projects and please feel free to contact me directly with questions/comments about this post.
Curt G. Gear
Director of Business Development