BIG BEAM BABIES

By Michael F. Malinowski, AIA

Massive beams and columns are the norm in Serene Lakes, where it takes nearly twice
the timber to build a home that would be used in more benign climates.  Those of you recovering
from the shock of your first lumber bill, might appreciate a professional perspective on what's 
over designed and over engineered and what's just plain necessary to keep houses standing up.

As a licensed architect and structural designer experienced in the snow country, I'm
amazed at some of the ramshackle, old buildings along the Old Donner Pass Road that have
stood up over time.  Of course, you don't see the buildings that have collapsed, so don't use a
sagging, leaning building remnant as an indication that new homes are over designed and over
engineered.  Even with modern code requirements, a structure will occasionally fail, since snow
engineering is not an exact science.  Yet older buildings that don't come close to meeting current
codes may weather storm after storm.  Wood structures are, by their nature, very redundant.  That
means that if one part fails, often the whole structure won't collapse.  Longer spans and bigger
open spaces reduce redundancy, making a failure more catastrophic.  Wood performs according
to a probability curve.  That is, most members meet the strength criteria, but out on the edge of
the curve, a small percentage will not pass muster.  Thus, a beam sized to carry a certain load
will do the job 99.9% of the time.  That still leaves 1/10 of 1% of circumstances for failure, 
even when engineering meets code.

The weight of snow is another big variable.  The code itself devotes an entire chapter to
snow design, and has evolved over the years to consider such things as ice dams, snow drifting
and even impact loads from falling snow.  Houses however, may not get the careful scrutiny that
a commercial building might, especially in regards to more particular requirements such as impact
loads.  There is also nothing in the code that specifically prohibits you from placing a stair in a
location where it will be destroyed year after year by snow unloading off your steep metal roof. 
The Serene Lakes area has a design snow depth of 14 1/2 feet and a ground snow load of 430
lbs. per square foot.  To put this into perspective, residential floors are designed to carry about
40 lbs. per square foot, which will comfortably support even pianos and waterbeds under most
circumstances.  

A parking garage is designed for only 100 lbs. per square foot!  

For the parts of a house where the eave height (edge of the roof) is less than 14©1/2 feet, the entire 430 lb. load
must be used.  This makes sense because the snow has nowhere to slide off to.  In addition, the
code requires a lateral clearance from the eave equal to the height of the roof.  This also makes
sense when you think about all that snow having a place to go when it does slide off.  Metal
roofs are rewarded in the code which recognizes their slippery nature by reductions in loading
permitted when slopes get steep.  At a 9 & 12 roof pitch (9 inch rise for a 12 inch run), the load
drops to 390 lbs. per square foot.  At 16 & 12, it's 180 lbs. per square foot.  At 36 & 12, i.e.,
rising 3 feet for every 1 foot of depth, the load drops all the way to zero.  Of course, that steep
of a roof is impractical on a real building.  Some engineers are even more conservative than code
provisions require, although there has been generally good performance with current code design. 
Remember,  however, that meeting code is no guarantee that your structure can't collapse!

My garage at 4200 Lake Drive has a truss roof that's designed for 430 lbs. per square
foot.  Since water weighs 64 lbs. a cubic foot and snow density is roughly 50%, that means my
roof can support about 12 feet of snow before it is at its design load capacity.  That doesn't mean,
however, that the trusses couldn't fail with less load.  If you have trusses that have been damaged
by something as simple as a minor split from a nail, it might dramatically reduce the capacity
of the structure.  On the other hand, factors of safety in wood engineering and redundancy noted
previously might create enough reserve strength to carry up to five times the design load whopping 
2,000 lbs. per square foot) twenty times the design load for a parking garage!

My next article will explore why you don't see more deep overhangs in spite of their 
design benefits, and the mystery of ice splitters!

Questions or comments?  mfm@appliedarchitecture.net

BIG BEAM BABIES, PART 2

By Michael F. Malinowski, AIA

There's an obvious reason why the homes in Serene Lakes are "Big Beam Babies":  SNOW,
SNOW, SNOW!  We have one of the highest snow loads in the country, resulting in the fact
that a well designed deck in Serene Lakes is over four times stronger than the floor in a high
rise parking garage!

As an Architect and Structural Designer working in snow country, one of the things I have to
consider is the effect that a house's geometry has on the structural loads from snow.  Some of
these effects are specifically addressed in the Uniform Building Code, and they help explain
some of the characteristics that our houses have.  Have you ever wondered why most houses
in Serene Lakes have little or no overhangs?  The main reason is that the structural load on
an overhang must be doubled from that of the rest of the roof.  This can result in design loads
of nearly 900 lbs. per square foot.  To put that in perspective, that is about the weight you'd
have from fully grown elephants sitting on the edge of your roof, cheek to cheek!  Needless
to say, it takes some challenging structural engineering to accommodate those loads.

Roof overhangs have huge benefits, however, in directing snow that slides off your roof to
where you want it, or keeping snow from piling as high against the front of your garage.  The
3 1/2 foot overhangs that I have on my own home at 4200 Lake Drive are about the limit of
what conventional framing can produce, however.  Anything deeper requires the braced strut
approach you see in some chalet style designs.  Why are the loads so high?  It is because of
possible ice dams.  These are blocks of ice that form from melting snow.  Especially in older,
less insulated homes, heat loss would allow for snow melt in the middle part of the roof. 
When that water hits the outside edge where there is no heat, it can freeze.  Over time,
massive blocks of ice would build up that would not only weigh huge amounts, but also trap
water and snow.  Modern insulation--at least R38--and ventilation of a metal roof to keep it
uniformly cold can reduce ice dams, but the structural design must provide for them anyway. 
When you see huge icicles on your overhang, you'll know why! 

Another issue dealt with in the codes are ice splitters.  The code recognizes that an
obstruction such as a chimney will act to dam up snow and ice, leading to potentially huge
horizontal pressures.  That is why chimneys are often placed near the high point of the roof. 
The forces of a chimney low on the roof can become incredibly big, on the order of 3 tons or
more!  When good interior design demands that the chimney be on the outside wall, as it is
on my house, a complex system of drag struts and straps, coupled with a steeply pitched
"cricket" is required to create an "ice splitter."  My ice splitter is designed for over six
thousand pounds of pushing snow and ice!

Further complexities of structural design at Serene Lakes include large wind forces and even
the analysis for an earthquake with a partial load of snow on the roof!

Questions or comments?  Drop me a line:  mfm@appliedarchitecture.net 

or 

2420 K Street
Sacramento, CA 95816
(916) 442-6955.

Copyright 1998 Michael F. Malinowski, AIA