Taming the Big Bad Wolf by dfsdf224s


									       Taming the
       Big Bad Wolf
                                                                                 the construction methods used for the walls —
                             An eyewitness account of the                        i.e. brick versus wood. It’s as if the story of the
                             damage tornadoes did                                Three Little Pigs has become the focus of our at-
                             recently in the Vaughan area                        tention. And yet when the Wolf blows, some par-
                                                                                 ticular things happen, but the issue is not how
                             of Ontario, just north of                           the walls are made. In the Woodbridge area, the
                             Toronto, suggests roof nails                        houses were of masonry construction with block
                             are still missing the mark.                         and brick walls. These are strong walls. In the
                                                                                 Maple area, the houses were wood-framed and
Gregory A. Kopp              Multiple tornadoes ripped across Southern On-       clad with either brick or stone facades and they
Professor,                   tario on Aug. 20, 2009, resulting in the tragic     seemed to be weaker.
Canada Research Chair        death of a young boy in Durham and significant        In fact, the method of wall construction has lit-
in Wind Engineering,         damage to hundreds of homes in Vaughan, just        tle to do with the performance and damage ob-
Faculty of Engineering,      north of Toronto. Damage was widespread, hit-       served in these storms. Rather, the main issue has
University of Western
                             ting cars, crops, utility poles and many types of   to do with how the roofs are constructed —
                                                                                                                                       Illustrations by Greg Hargreaves/www.threeinabox.com

                             buildings.                                          most importantly, how the roofs are connected
                               The damage in Vaughan was concentrated in         to the walls.
                             two neighbourhoods, one near Hwy 7 and Mar-           When we joined Environment Canada to exam-
                             tin Grove Rd (in an area called Woodbridge), and    ine the storm sites, we observed the vast major-
                             the other near Jane St and Teston Rd (in an area    ity of the damage was due to roof failures such
                             called Maple).                                      as sheathing (plywood sheets nailed to the
                               Many discussions and opinions about construc-     trusses) and toe-nailed, roof-to-wall connections
                             tion quality have taken place, as always seems to   coming off. Damage caused by debris also played
                             happen after such events. Much of the informa-      a role, as discussed below. The more severe Bar-
                             tion being put forward is incorrect, or at least    rie tornado in 1985 showed that deaths often oc-
                             misleading. In particular, the focus has been on    cur when walls collapse; this occurs when roofs

66 Canadian Underwriter October 2009
are removed. So, while it is perhaps true                                                   these elements will subsequently hit
that double-brick walls offer more pro-                                                     down-wind houses. If debris happens
tection when the roof is off, this was not                                                  to collide with a window or a garage
an issue in the instance of the Vaughan-                                                    door, the window or garage door will
area tornadoes (although there were a                                                       likely fail, allowing wind and rain to
handful of locations where walls failed                                                     enter the building. Although the rain
following roof failure). As a matter of                                                     itself causes a lot of damage, the wind
fact, double-brick walls collapse, too.                                                     entering the building pressurizes it like
We saw a few of these in Woodbridge.                                                        a balloon, adding to the uplift on the
Fortunately, no one was hurt there.                                                         roof and increasing the forces the nails
  When wind blows on a house, or any                                                        must resist. At this point, roofs often fail.
other structure, it causes uplift on the                                                      We saw a lot of evidence of debris im-
roof. To understand the effects of this,                                                    pact in both neighbourhoods. Many
imagine holding the house upside down                                                       windows and garage doors were broken
and shaking it. One can immediately see                                                     by debris. We saw many 2x6 and 2x8
the nail connections become the issue                                                       pieces of wood penetrate roofs and win-
(see ‘All about the nails,’ Canadian Under-                                                 dows like spears. (See Figure 2 on Page
writer, January 2009). The primary role                                                     70.) This is why you are always advised
of the nail connections is to hold the                                                      to stay away from windows in torna-
roof in place. If the roof is not con-        The method of wall construc-                  does and head for the basement.
nected to the walls, only the weight of       tion has little to do with the                  Wind-borne debris is very dangerous,
the roof holds it down. Since our roofs                                                     as shown by the tragic events in
are made of wood, they tend to be light,      performance and damage                        Durham, and should be mitigated as
offering little resistance to the forces      observed in these storms.                     much as possible. A primary way to do
induced by the wind unless they are           The main issue has to do with                 so is to ensure that the roof structure re-
well-fastened.                                                                              mains intact. Reducing the possibility
  The photograph below (See Figure 1)         how the roofs are connected                   of shingle and vinyl siding failures also
shows one example of a failure because        to the walls.                                 helps. In this latter regard, it was good
the roof was not connected to the walls;                                                    that the houses in Vaughan were brick-
rather, in this pitched roof construction,    and yet failed.This was a severe windstorm.   clad, so that vinyl-siding was not an is-
the structural members of the roof were         The other major issue in windstorms,        sue. Also, the brick material meant rela-
placed in slots in the blocks.The red ar-     particularly for tornadoes, is wind-borne     tively little debris penetrated the walls.
row points to where one of the connec-        debris.When upwind structures or parts        We saw many black marks on walls
tions should be, but, in fact, there is       of structures fail, the wind carries these    where shingles had hit the bricks.
nothing holding the lumber to the inte-       elements.They can travel a long way and         Figure 3 on Page 70 shows an example
rior wall. In Vaughan, we also saw many       with high speed. In a suburban neigh-         of the type of roof failure induced by
roofs that had been properly connected,       bourhood, a high probability exists that      wind-borne debris. Here, the second

  Figure 1

68 Canadian Underwriter October 2009
floor windows were broken, leading to       in these F2 tornadoes, is actually rather     existing ones. If the houses in Vaughan
severe internal pressurization, causing     inexpensive — hurricane straps. These         had been built with this technology, we
the subsequent failure of the roof.Thus,    are readily available, easy to install in     believe the overall losses would have
when failures occur in one house, it can    new construction, less prone to errors in     been significantly reduced. Examples of
lead to failures in neighbouring houses     installation and, best of all, inexpensive.   such construction exist in Ontario. Two
because of the wind-borne debris. In        At most, they would add a few hundred         houses have been recently constructed
Maple, debris caused a lot of damage. It    dollars to the cost of a new house. As        following the Institute for Catastrophic
broke many second floor windows and         part of the ‘3 Little Pigs’ project at our    Loss Reduction’s “Designed for Safer
garage doors. The garage door failures      Insurance Research Lab for Better Homes       Living” program. These are in Fort Erie
led to garage roofs coming off in several   (IRLBH) at the University of Western          and Sudbury (with a third in P.E.I.).
instances, while window failures led to     Ontario, we are currently testing such        All were built in partnership with The
roof failures.                              technology in order to find optimal           Cooperators.
  The solution for the problem of roof      solutions that minimize costs and max-          We hope more houses get built this way,
failures for winds as severe as occurred    imize safety for both new homes and           thereby taming the Big, Bad Wolf.

                                                                                          Figure 2

                                                                                          Figure 3

70 Canadian Underwriter October 2009

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