Anyone who lived in Miami-Dade County in 1992 experienced first-hand what a Category 5 hurricane looks like. Although where I lived was not particularly hard-hit, even though there was a great deal of damage to the trees and landscaping, the next day I was walking around the devastated areas of the south side of the county.
Architects, engineers, and the building departments of the area learned much from the experience. As a whole almost no gas stations or screen-enclosures were left standing. In most homes which had block walls on the ground floors and wood framing on the second floors, at least parts of the second floors were gone. I saw one house where the front façade on the second floor was left, but as I walked around the yard to the back, all the other three walls on the second floor were gone with the clothes and furniture littered all over the yard. In some areas the majority of house lost all the windows and doors, in addition to the roof finish. In others, even the roof sheathing and sometimes even some of the wood trusses were gone.
For about 6 months it looked as though South Miami-Dade had been bombarded continuously and we were in the middle of a war zone. In actuality the houses of the county had gone to war with Hurricane Andrew and lost.
Six months after Hurricane Andrew a report came out citing the reasons why houses failed during the hurricane. These were simple enough:
- Unbraced gabled ends of the roof framing trusses failed
- Corners of the house lacked the required steel reinforcing which were supposed go from the foundation to the tie-beam
- The roofing paper and tiles or shingle blew away and it rained in the house
- The wood roofing sheathing was not attached corrected and it blew away
- The roof trusses were not attached correctly to the time beam and were displaced
- The connections between steel members or wood members were inadequate and the structure failed
- The windows and doors were not properly attached to the surrounding concrete block walls and blew away
- The windows did not have shutters and the glass was broken, the high winds would get into the house and do great damage
- The second floor wood framing was not properly attached to the tie-beam at the top of the ground floor and was destroyed
So what did we learn from the Andrew experience?
I think the most valuable lesson was the importance of the attachments or connections from one material to a different material, for example, from wood to concrete or metal to wood, etc. The weakest link can expose a whole house to disaster.
Now all the straps that connect from the wood trusses to the concrete beam are long and strong enough to sustain hurricane wind forces. The favorite construction has become reinforced masonry where the concrete block cells are grouted every 4′-0″ or so with a steel reinforcing bar which goes from the footings to the tie-beam. In addition, all window or door openings in the exterior walls must have the same reinforced cells with the steel bars at each side of the apertures.
The favorite roof configuration is the hip roof. Because of the way it’s built, it is naturally self-bracing. All gable ends are braced with cross-bracing, at a minimum at each of the last two trusses at each gable. Many times what kept a gable end from collapsing during Hurricane Andrew was one 2 x 4 bracing. Imagine such a small piece of wood made all the difference.
The code now requires that all new windows, even in just an addition, must either have shutters or be impact-resistant. Impact resistant windows are a great invention. The glass is laminated in layers, similar to the way plywood is made out of layers. Impact-resistant windows are so strong that you can take a hammer to them and not break them. However, unlike regular windows, if a piece of glass should break, the glass alone cannot be replaced. The window must be replaced in its entirety.
The way the roofing membrane and the wood sheathing are attached to the wood trusses below has been improved as well.
Where wood to wood connections in a trellis or pergola prior to the hurricane were installed intuitively, the code now requires real structural calculations. So are calculations required for the connections between each truss and the concrete tie-beam below. Nothing is left to the imagination. The signed and sealed calculations are presented along with full structural plans at the time of plan review for permitting.
Although great strides have taken place over the last 16 years, there are still construction techniques which could be improved. For me there is no better construction system than a concrete slab roof. Because they are difficult to build on a slope, and therefore expensive, we continue to building roofs with wood trusses, sheathing, roofing paper, and shingle of tile finish. This system is still very susceptible to damage in high velocity wind conditions. Hopefully, someday someone will come up with a better, practical system which will not succumb to hurricanes that easily.