Where you run into the kind of problem you describe is when a strong but
brittle material is substituted for a weaker but more ductile material.
The ductile material will bend before it breaks, the brittle material will
As for something "designed and built correctly" showing "excessive
deflections before failure", certainly one can design things that way but
that doesn't mean that it's the only correct way. Concrete for example
doesn't flex noticeably before it breaks so by your reasoning concrete
should never be used as a building material.
When dealing with wooden beams, making the beam stronger than called for is
not going to result in sudden failure with no warning unless the original
design would also fail suddenly with no warning at a lighter load.
I would like you to quote the statute which makes it a criminal offense to
build something stronger than is required. Or provide reference to a civil
case where someone was successfully sued for building something stronger
than was specified.
*NOT* necessarily true.
Engineering for systems under stress, particularly dynamic stresses, is a
_complex_ and _complicated_ subject.
*ALL* the components have to be considered, =both= singly, and in combination.
'Over-building' _one_ component can result in excessive transfer of stress
to _other_ components, Leading to failure of _that_ component under conditions
that are _less_ severe -- as measured for the overall system -- than the
original design was spec'ed to handle.
There are numerous real-world instances of this *exact* thing happening.
One of the easiest places to find them is in the world of home-built, plans-
built, aircraft. Firstly, in general, the 'safety margin' on _any_ aircraft
design is extremely small. "1.6" is typical for commercial construction.
Homebuilts usually are designed with higher margins, because there is more
variability in the quality of construction. However, there are =many= cases
on record, including after-the-fact engineering analyses, where a home-builder
has modified a design -- to =strengthen= some part of it -- where said mods
have led to _premature_failure_ of other areas of the design. Higher "point
stresses" occurred in the modified design, as a result of the modification,
than the original design was designed to handle.
Please provide a case in which replacing a weak ductile material with a
strong equally ductile material results in the kind of failure you
describe. Please include the analysis.
Yes, necessarily true. Please provide an example of a case in which a
wooden beam was strengthened and there was subsequently a failure with no
warning while an identical structure subjected to the identical loading
gave warning. Please prove that this was the case, I don't want someone's
Buildings of the kind where an individual would be installing or removing a
beam are not typically under "dynamic stresses" to any significant extent
unless you want to count wind loading. If you want to talk skyscrapers
it's another story, but they typically have little or no wood in the
How can it "result in excessive transfer of stress" if the loading is the
same? Please demonstrate the mechanics of this. Show me an analysis of a
case where under identical loading increasing the strength of one member
increases the stress in other members.
Again, show me an analysis that demonstrates that this happens.
I was not aware that the OP was talking about an aircraft. Houses,
workshops, and other buildings typically have much higher margins than
Now why would "higher point stresses" occur under the same loading? That's
a matter of forces and geometry. While I probably could design a structure
in which stiffening one member increased the static stress somewhere else,
I'd have to work at it.
What usually happens in such incidents is that the stiffness of a structural
member was changed, resulting in an altered natural frequency, which put it
into a range to resonate with shed vortices and there by causing a flutter
problem. But putting a heavier beam than required in a house is not going
to cause a problem such as this.
Look, the bottom line on this is that you seem determined to overengineer a
simple problem like spanning a doorway.
Show us how to make a house fall down by making the headers too strong and
maybe someone will listen. In the meantime you're just crying gloom and
doom to no purpose.
You've made two different statements here.
If something is stronger, the fact that it's ductile doesn't necessarily make
it ok. You can change the load regime without entering the point where ductile
failure of the replaced element comes into play.
I've done dynamic analyses of some pretty small structures. They don't have
to be skyscrapers to have dynamic loading problems.
The problem is that the loading isn't necessarily the same. Just because the
design load is the same, doesn't mean that the load in use is the same.
If someone overloads a properly designed building element, they will see
precursors of failure. If the element is overdesigned, those precursors
(e.g. excess deflection) don't show up. Proper design means that you get
a warning if you have overloaded the structure.
The poster said that overdesign is never a problem. No mention of headers.
So in what mode does the replaced element fail? Or are you saying that the
other elements which one supposes to be properly designed will not give
this warning that you describe, that the ONLY element which will give this
warning is the beam that was replaced?
Seems to me then that you need to do something about those other elements.
How small is "pretty small"?
So let's see, it's all right to overload the structure and have it show
"precursors of failure" but it's not OK for it to just sit there holding
I see. So you're basically saying that your properly designed structure
will give warning if the _beam_ is overloaded but not if the _posts_ are
overloaded? Do tell. Sounds like sloppy design to _me_.
Read the title of the thread. We're talking about something in the ballpark
of two two-by-tens, not about the effing Space Shuttle.
GET SOME BLOODY PERSPECTIVE.
You are assuming that the only design criteria is strength. Serviceability
and stability are also limits. An element can be more than strong enough
is serviceability is the governing criteria. As another poster said, you have
to consider the structure as a whole.
No, I'm taking you at your word that a "properly designed" structure will
"give warning". If the beam doesn't "give warning" then it's not near
failure. So the failure has to be somewhere else, and the only other place
that can be would be in the vertical members. So one would expect, with
this "proper design" of yours, that _they_ would "give warning". If they
do then there's no problem, if they don't then by your own standards the
structure was not "properly designed".
You don't seem to be able to follow the ramifications of your own argument.
Try that sentence again. It makes no sense as written.
Yes, you do. So what? So you're saying that using a 2x12 header instead of
a 2x10 is going to make the house fall down? If not then what are you
Damn! there is going to be a lot of pissed housing contractors around
here if they have to stop using doubled 2x12 lintels in the 3ft wide
doorways and windows under 6-8ft wide ... way oversized! - especially on
the non-bearing walls! Hey, Joe! saw me up a 1/2doz of 5"inch
cripples will ya? :-)
This is a key distinction. I would guess that issues of dynamic stress
drop to insignificance in a structure like a house, though. (But I am
not an engineer, nor do I play one on tv, so take that with a grain of
I was thinking of exactly the same example, but it seems different in
that dynamic stress is a very big factor here.
So far, I've done this only as a thought experiment, but I'm tempted:
Go to a hobby shop and buy some balsa and build 4 "structures": 2 24"
beams consisting of a 1/4" square piece of balsa, and two more such
beams where 20" on one end is stiffened by gluing another 1/4" square
above and below the primary one. Clamp all 4 structures to your bench
so that 2" is held rigidly, and the remaining 22" is without support.
Now, from the ends of one of each type of beam, hang increasing
weights until they break. My guess is that the breaking point will be
pretty close to the same. (you might have to adjust for the decreased
arm over which the force is applied in the case of the bending beam if
that gets significant.)
Part two is to take 1/2 that weight, attached by string to the end of
the beams, and drop it from various heights. My guess is that here the
stiffened beam will break much sooner, as the more flexible beam
absorbs the dynamic stress with the springiness over its length.
If reality matches my thought experiment, that would say that
increasing the strength of a header in a house is probably a harmless
waste of materials, while increasing the strength of a wing may well
Another dynamic example is automobile suspensions: if you are going to
significantly strengthen (stiffen) the springs, you should make sure
that the structure to which the suspension is attached has the
strength to accept the increased dynamic load.
Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.
Sorry everyone. I don't take offense by your replies. My post was
severely lacking in details. I'll try to give them here.
The 2x8 beams are for a deck, about 30" above the elevation.
I don't think there is much risk to life and limb, but beyond that, I'm
over-engineering the design. The footer spacings are 6', the table said
I can use 8'. The joist spacings are 16", the max was 24". The max
joist span is 6', I'm using 2x8 PT (I could use 2x6 PT according to the
All of the wood is PT for obvious reasons (an outdoor deck). I'm being
VERY conservative on materials and design.
The source that told me to use carraige bolts was the Stanley book,
"Building Decks" from Home Depot. That's the same source that said to
seperate the beams with PT plywood spacers. It's NOT an old reference-
it's Copyright 2002. Making a BEAM is on page 44, where they describe
the plywood spacers and the carriage bolts.
Given all of that complexity, I'm considering maybe just "sandwiching"
the 4x4 post with the 2 beams. It looks simpler is perhaps even
Anyhow- my original question was "Do I really need the plywood spacers"
if I choose to build 4x8x20' beams from 2x8's? Should I just sandwhich
and forget bamking beams?
I am glad to see you were able to hang in and reclaim control of your
I am far from familiar with Florida state regulations and suggest it be
worth a browse on the state web or look in a local library to ensure
conformance to the state requirements at least. Your municipality won't
necessarily apply the same rules but unlikely to be far off but I would
keep pressing them to at least acknowledge conformance so you don't end
up with a liability issue at some later date.
I think most of the group gave the indication that using parameters
above minimum code requirements is not going to get you into trouble for
The spacer issue might be a code requirement for built-up beams in your
rather moist environment and likely intended to prevent (or at least
minimize) potential for rot. When shim spacered I would be inclined to
use carriage bolts through the shims to ensure the integrity of the beam.
You don't specify that you plan to lay the joists across the top of the
support beam or hang them on hangers on the beam. This will influence
your "sandwich" vs "built-up" choice considering a potential for
deflection on the hang joists on a sandwich beam. And in this over top
model you don't indicate the amount of cantilever beyond the beam. Our
local reg's indicate a maximum of 24" but in context of the supported
span to a max 1/4 or 1/3 ... can't recall.
From a Canuck view, you seem to have a reasonably sturdy plan that will
serve your needs and should meet codes with the provision that you need
to be sure about required insect and moisture protection.
I did a quick DAG on Florida building codes and found some useful hits
to look through.
Concrete is a perfect example of the problem and one where overdesign is
a problem. Too much steel reinforcement in a small beam compared to less
steel in a deeper beam - the lightly reinforced beam will fail slowly with
the ductile steel failing in tension. The overbuilt beam with too much
steel will fail suddenly and in a brittle manner by failure of the concrete
The lighter beam would bend considerably before failure. The heavy beam
can carry a significant overload and can cause it's supports to fail without
warning. You can't look at a building by considering its components
You have to look at the entire structure as a system.
If an engineer or architect is responsible for the design of a building,
they are required to ensure that it does not fail in a manner that does not
give warning (i.e it must fail in a ductile manner). If the design of one
component results in an unexpected failure, whether from over- or underdesign,
this results in professional liability. Maybe not the Code of Hammurabi, but
there are still legal consequences - such as criminal negligence causing death.
This is the classic case, but it is NOT a case of overdesign. It is a case
of WRONG design. Overdesign would be sizing the beam twice as deep as it
needs to be, not providing a faulty design.
But, we are considering overdesign. In this case we must assume that the
design with the lighter beam is SUFFICIENT to carry the load. It is not
expected to fail. Thus a heavier beam would be sufficient as well.
Wouldn't the supports be overdesigned as well? You seem to be considering
loads that cause buildings to fail. In my mind these are things like
earthquake, wind and perhaps snow. With an earthquake if the supports are
going to fail under the load it doesn't matter if the beam is oversized or
not. You are crushed. Likewise with wind. Snow is a different story as it
accumulates slowly, and there you might have a point. But even then I
contend that while an engineering system tries to be balanced, in practice
there is enough variation in materials and fabrication that it is impossible
to be 100% certain how it will fail.
Sure, and what if the overdesigner does this and overbuilds everything?
Perhaps you can quote the regulation that states this?
Also, give us a case or two where overdesign has resulted in criminal
negligence causing death.
If they exist you should be able to cite one or two examples.
Over design version "just throw some more rebar in there - it's
never bad to overdesign".
For the design load - yes. For the actual load - no. See my other post.
The issue is whether you get warning of impending failure. Overdesign
can result in brittle failure without warning.
The case from which this derived is one where someone is considering
a single element, not designing the whole building. If someone in
a forum like this who is not an engineer or architect gets hold of
a ridiculous claim like "overdesign is never bad" all sorts of
unexpected evil can result.
You made the assertion, it's up to you to support it. If you don't want to
support it them don't make the assertion. Now, do you have case law in
which making a part of a structure stronger than required resulted in a
conviction for criminal negligence in Canada or are you just a Chicken
I didn't say it has happened, I said it could happen. If the circumstances
arise, all it takes is a zealous crown prosecutor to make the case. Since
the law has been applied in other cases, that's not much of a stretch.
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