Wednesday, December 30, 2015

US Building construction practices, revisited

In 1999, after the 03 May tornado outbreak of that year, I wrote a web essay on home construction practices and how that affects wind damage.  The recent December tornado events have re-awakened this topic, and it seems appropriate to offer some additional remarks after 16 years have passed with virtually no comprehensive change in construction practices.

The damage surveys I (and others) have done since then have continued to reveal not only the inadequacy of existing building codes, but also just how widespread violations of existing building codes are.  The existing codes remain, for the most part, pegged to a 90 mph standard for resistance to structural damage from winds.  The operational EF-scale puts 90 mph winds (a 3-second gust) at the low end of EF-1 tornadic winds (86-110 mph).  According to this standard, an EF-1 tornado (or anything stronger) is considered to be capable of initiating structural damage.  This isn't a very good standard for most of the United States east of the continental divide.  If a home is poorly anchored to its foundation (which is, unfortunately, all too often what is observed in American frame homes, despite such practices being below code standards), an 80 mph wind might well be able to slide it off the foundation, resulting in total loss of the home.

The reason for the widespread occurrence of code violations (in all buildings, including schools, not just homes) is simple.  Although structural enhancements can be added to new construction for about $1000 or so, the real issue is the time it takes to add those clips and strapping (see Fig. 1) to the frame and roof.  The added cost to the homeowner (passed on by the builder), amortized over a 3-year mortgage, is trivial.  For homebuilders and contractors, the key to profit is speed of construction.  In far too many cases, this means "shortcuts" are taken by the builders.  For instance, the code standard for attaching the wall plate to the foundation is the use of "J-bolts" embedded in the concrete, with washers and nuts tightened onto the threaded end of the J-bolt (see Fig. 1).

Figure 1.  An example of strapping used to attach a wall stud to the wall plate and the use of a washer and nut to attach the wall's bottom plate to the concrete foundation via a J-bolt.  Toe-nailing the stud to the bottom plate is poor practice (but acceptable by most current building codes) because it offers little resistance to forces acting to lift the wall.  The strapping uses nails (or screws) that are at right angles to lift forces, thereby creating much more resistance to those forces. 
Image courtesy of Tim Marshall. 

Surveys after tornadoes reveal too many examples where builders have installed the J-bolts, but failed to attach the washer and nut to the end!  That makes the J-bolt completely useless but of course it saves time!  Another common practice is for builders to be granted an "exemption" (by local governments) from codes requiring the use of J-bolts, allowing builders to use powder-driven cut nails for attaching the wall bottom plate to the concrete foundation.  This results in an extremely weak attachment (Fig. 2) when lift forces are applied to that attachment, as they are in tornadoes.

Figure 2.  A powder-driven cut nail left behind in the foundation after the wall bottom plate was torn away.  Note the damage to the concrete caused by the process of driving the nail through the board into the concrete.  Sometimes this shatters enough of the concrete to utterly negate the attachment of the nail but it would not be visible to the builder.  Image courtesy of Tim Marshall.

In my surveys, I've seen that shoddy, sometimes appallingly weak construction practices are widespread.  Buying an expensive home is no guarantee of high quality construction.  We see about as many code violations in expensive homes after tornadoes hit as we see in low-cost tract homes.  We also have seen that in many (not all) cases, the homes rebuilt after tornadoes are no better constructed than the destroyed homes they replaced.  The lessons learned from previous tornado event evidently are not being used widely to change construction practices.

If the standard for wind resistance to structural failure were raised in the tornado-prone parts of the US (everything east of the continental divide) to 120 mph (in the middle of EF-2 tornado winds - 111-135 mph), this would result in a substantial reduction in tornado damage.  Structural damage would primarily be associated with EF-3+ tornado events.  Flying debris in tornadoes from structural failures initiates considerable additional damage and increases the casualty risk to anyone caught in the tornado (Fig. 3).  Enhanced construction codes would thereby reduce tornado damage considerably and also lower the casualty risks.

Figure 3.  Damage from the 03 May 1999 tornado in Oklahoma City/Moore, OK.  Note the prevalence of broken 2X4 timbers within the debris - these are structural frame elements that may have been carried considerable distance by the winds.  FEMA image taken by C. Doswell.

Note also that even in the strongest tornadoes (EF-5), only a tiny fraction of the damage path experiences the strongest winds (at most only a few percent of the total damage area).  And EF-3+ tornadoes represent only about 10% (or less) of all tornadoes.  Limiting damage to parts of a tornado track with EF-3+ winds would represent a significant reduction in the amount of structural damage, and reduce casualties, as well.

At the very least, there's a need for more rigorous enforcement of building codes.  It would be an important advance if all buildings actually were built according to the existing building codes, to say nothing of additional benefits from enhanced code requirements.  We as a nation need to be benefiting  from our experiences with tornadoes, not ignoring the lessons they've provided.