Why the Safety Factor?
Let's be practical concerning the article "Strong Rail-Post
Connections for Wooden Decks" (2/05). Looking at it from a
contractor's point of view, I have the following observations:
The bolted connection passed code, so no further refinement is
needed. Perhaps the code already took into account the safety
factor, or the code needs to be changed. A 200-pound load
applied continuously to a single post — as in the test
— is a pretty unlikely scenario.
Readers can certainly choose to overbuild their deck if they
don't mind the added cost, but I don't think they will have any
more peace of mind.
Barta and Sun Construction
Authors Joe Loferski and Frank Woeste,
P.E., respond: Of all structural engineering topics, the
subject of safety factors is the most difficult to explain and
understand. The intent of safety factors is to compensate for
uncertainty — uncertainty in loads and materials,
uncertainty in the models used to analyze the structure, and
uncertainty in field installation or construction. For a
typical application, the goal is to provide a safe structure
for a "lifetime" and achieve a failure rate of no more than
about 1 in 1,000,000. It is theoretically impossible to design
a structure with no chance of failure.
Before going to the deck guardrail post, it may be helpful to
examine a more common case that homeowners personally
experience every day. In the case of a residential floor joist,
the average occupancy live load is 11 psf, based on published
research, yet the design load has been 40 psf for
Assuming an ordinary 2x10 southern pine joist, the allowable
design value would be approximately 1,000 psi. If you were to
test several hundred of these joists in the laboratory to the
point of failure, the test strengths would range from about
1,300 psi to 13,000 psi. The average strength would be about
6,000 psi. What, then, is the safety factor, or SF, for the
joists based on the average test value?
Because the test lasts only a few minutes per joist, the
average test value is reduced by a duration factor of 1.6 as
well as by the safety factor. For the example of the 2x10, the
1,000 = 6,000 / 1.6 x SF
So, in this case, the safety factor (SF) equals 3.75. By
adjusting the average load upward from 11 to 40 psf, and the
bending strength downward from 6,000 to 1,000 psi, the result
is that people do not need to worry about their floor
collapsing throughout their lifetime.
Returning to the guardrail-post case, your point that "the
bolted connection passed code, so no further refinement is
needed" is analogous to using 6,000 psi for the allowable
strength of a 2x10. Without imposing a safety factor, we would
expect failure for about 50 percent of the post connections
that were exposed to a 200-pound load. Safety factors are used
to reduce a 50 percent chance of failure to a failure rate on
the order of 1 in 1,000,000.
We used a safety factor of 2.5 from the current building code
(IBC), and note that it is significantly lower than the 3.75
calculated for an ordinary 2x10 floor joist. As it turns out,
2.5 is the same safety factor required by the ICC standard AC
174 (Acceptance Criteria for Deck Board Span Ratings and
Guardrail Systems), which is used to qualify man-made deck
materials — primarily plastic, wood-plastic
composites, and vinyl. Aside from the fact that we are inclined
to go with the "wisdom of the code," we feel it would be
foolish to use a lower safety factor than that required for new
man-made materials, which should have less variability than
solid-sawn wood products.
Regarding the point about overbuilding, we believe that if
contractors give their customers the choice of going with a
common bolted guardrail-post connection vs. a connection
assembly tested in a lab using a code-prescribed test safety
factor, nearly 100 percent of homeowners will choose the tested
No Labor Shortage
Just a few thoughts as a member of that "other minority" that
most industry publications — and I mean yours
— tend to ignore on most occasions.
Did Mr. Gritmon of Little River, S.C., (Letters, 2/05) include
the unemployed and underemployed African-American males in his
neck of the woods? My quick research shows an unemployment rate
three times higher than that of whites. And did he ever think
about hiring women? If he is willing to pay a "living wage," I
can find him plenty of bodies in my neck of the woods who would
be willing to move there if a job is waiting.
His use of the term "Anglo" could be considered offensive in
many circles. Reading between the lines, it sounds like Mr.
Gritmon bemoans the fact that he can't find any "nice white
boys" to come into the biz. Perhaps if he and those like him
had not locked out and did not continue to keep out people who
look like me, neither of us would be writing about this. Get
Success in School
I have had the pleasure of teaching the construction trade in
middle and high school for over 24 years in the Milwaukee area.
In 1989, I was teaching in a suburban district with an
enrollment of approximately 5,000 students. While many people
felt ours was a model department, the administration decided it
would be better if all students were put on a college-bound
track. Seventy-five percent of the industrial-technology staff
was cut, I was out of a job, and now students sit in study
halls instead of having the opportunity to explore the
In 1993 I was hired by the Mukwonago (Wis.) public schools,
where I teach a 12-week introductory course in construction to
all seventh-grade students — about 400 per year. They
love it! Many students go on to explore one of the trades in
high school. Many of my former students are now owners or
employed in the construction industry.
My daughter is currently enrolled in a four-year degree
program to become a teacher. When she graduates, her beginning
salary will probably be around $30,000. My son, on the other
hand, has just entered a five-year apprenticeship program. His
employer pays for the cost of his education as well as an
hourly wage. When he completes the apprenticeship, his salary
will be twice that of my daughter's.
It's not that we don't have a generation ready to enter the
trades, it's a case of putting too much emphasis on trying to
get everyone into a four-year degree program.
Park View Middle School
Starting a Training Program
I am writing in response to D.W. Murphy's letter "Help Wanted"
in the February issue.
As a contractor, I too have had trouble finding young people
to work in the trades. Shop classes are all but vanishing from
the high school environment. When the shop teacher either
retires or gets pushed out, the shop class is replaced by
another type of class. Fortunately, in my area of Marin County,
Calif., someone did something about the problem.
Ten years ago, the Marin Builders Association began sponsoring
a high school construction class in two local schools. Seventy
students completed the class this past year, and 13
participated in a summer internship program. Each year a number
of scholarships are awarded to those students who excelled in
the class and in the summer internship.
I volunteer to teach at one of the schools. I have a group of
six students who learn the practical aspects of construction
and how to use tools by stick-framing 6-by-10-foot sheds that
are eventually delivered to nonprofit organizations.
I suggest that Mr. Murphy and anyone else with this concern
contact their local builders group and local high schools. If
you need assistance in getting started, please e-mail me at
firstname.lastname@example.org; I am glad to help.
T.H. Eller Construction
The Question Not Asked
The article "Getting Into Insurance Work" (2/05) was good
reading for anyone wanting to get into this business. However,
one question — which poses the most danger to
remodelers — was not asked: "Should I accept the
insurance company's costs for a given job when they declare
that this is the accepted cost for this work in this
The answer is no. A remodeler needs to understand his numbers
well enough to know exactly what he can and cannot do a
particular job for. The remodeler who blindly accepts the
adjuster's cost and scope of work will quickly find himself in
trouble — either financially, from not understanding
his costs, or with the client, from not understanding the
Mark Labourdette, CR
Certified Xactware Restoration Contractor
Cabinet Pull Layout Jig
The cover of the December 2004 issue shows a cabinet jig that
I'm interested in purchasing. Any idea where I can find it?
St. Leominster, Mass.
Try Align-Rite Tool Co. at
www.alignritetool.com or 888/624-1942. The
Door and Drawer Drill Guide is available for
Setting the Record Straight
Your article "Arizona Cracks Down on Unlicensed Contractors"
(In the News, 2/05) contained some misleading
Almost all of the agency directors in Arizona are appointed by
and serve at the pleasure of the governor. In that the new
governor belonged to a different political party than her two
predecessors, she replaced almost all of the agency heads,
including the registrar of contractors, yours truly. The audit
was not completed and released for almost two months after my
Previous to the changing of the guard, I authored legislation
that eventually was passed by the legislature and signed by the
new governor that addressed most of the concerns contained in
the audit, and is enabling the new registrar to go after the
bad guys more effectively.
Johns Manville's Spider spray-applied
insulation (Products, 3/05) is a fiberglass product —
not cellulose, as our story's title implied. According to the
manufacturer, Spider contains about one-fifth the water used in
cellulose spray-in applications, which enables contractors to
put up drywall sooner. Also, because it is fiberglass, it is
mold-resistant without the need for chemical