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RE: METAL BUILDING DESIGN METHODS

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Juan,

Sorry for no earlier reply... In Florida soaking up the sun last week....

First some of the previous posters were correct in the following areas:
1) Each company does a slightly different frame design approach (actually
all materials) mainly differing in assumptions used.
2) Each company would considered "detailed" design information proprietary.
Info below is about as detailed as I should get.
3) AISC APP. F is not a commonly used section with PEMB companies (note word
commonly... it can be done but usually requires extra effort)
4) The MBMA Low Rise Building Systems Manual is a building code not a
structural design manual. (i.e. similar to ASCE7, BOCA BBC, OBBC, SBC, UBC,
IBC, NC, WI, NY, etc.)

Some missed items:
1) Last I remember the city of Chicago, IL requires all tapered members to
be designed based upon AISC APP. F.  subsection F7. Web-Tapered Members...
this leads a lot of PEMB companies to design buildings in Chicago to have
straight sections.
2) Don't remember if it is Chicago or not but some jurisdiction also
requires "hand" (not computer) calculations to be provided for review.

General design info:
1) All PEMB frames are designed per AISC specs (as far as I am aware of...
However, you could probably get some fly by night place to do otherwise)

2) The stiffness (or displacement) design (load distribution) is done
similar to that of prismatic sections.  "Matrix Analysis for Structural
Engineers" by Willems and Lucas is one reference. (i.e. member stiffness,
structural stiffness, load matrix, etc.). The main difference in stiffness
matrix determination is what basis is used for determining the member
stiffness matrix for tapered members.

3) Generally each member is broken down into smaller "local members."
Stresses are checked at each brace point (Lx, Ly, Lb) and at varying
intervals between brace points (typically using the section properties at
the location being checked... K, Cb, Cm, Depth, Moment, Shear, etc.).

4) Each "local member" is then stress checked per AISC spec.  Main
difference here would be to take some of the simplification back out of the
code as mentioned by Paul (??).  This is required for frames with unequal
flanges.  Both the tension side and compression side of the beams (or
columns) need to be checked.  "Similar" to that used on a pure design check
for a single angle (Sx-top, Sx-bottom, Fbx-top(compression),
Fbx-top(tension), Fbx-bottom(compression), Fbx-bottom(tension), (note single
angles have Sx-top, Sx-bottom, AISC manual shows smaller value of Sx for
simplicity, and behave differently based upon bending force direction).

5) Initial depths are general started with some rules of thumb (similar to
say starting with a W12, etc.)  Final depths are determined based upon
structural (member and/or connection design), serviceability, and clearance
requirements.

6) Depth variation (and each end depth) is typically determined based upon
fabrication capabilities.  A good min/max range might be from 6"+- to 72"+-
with a maximum slope change in the neighborhood of 10 to 15 degrees...
depending on fabrication capabilities and processes.

7) Design analysis does not "typically" include plastic design.

Hopefully this simplified overview of a thorough design check helps guide
you in the correct direction.  The appendices and commentaries in the AISC
manuals also have good additional information (sometimes the older manuals
have more detailed information on where specific code parts came from, esp.
the red (8th) and blue (7th) ASD books). I did not even touch on different
frame types, column shapes (prismatic, tapered, supermarket, reverse
tapered, etc.), fixed column base plates, pinned/fixed column tops, etc. If
you are really into pain then on the next metal building you work with ask
for and go through a set of the calcs verifying the numbers and
assumptions...  Make sure you have a lot of time.  Depending on how thick of
a set of calcs and how detailed you want to review them it could take
awhile. If you wanted to get into cold-formed members (by hand) then plan an
extra month or so.

Hope this helps,
Greg Effland, P.E.
KC, MO USA

-----Original Message-----
From: Juan José Treff De la Mora [mailto:jjtreff(--nospam--at)hotmail.com]
Sent: Tuesday, March 05, 2002 3:28 PM
To: seaint(--nospam--at)seaint.org
Subject: METAL BUILDING DESIGN METHODS


Hello,
    As you probably know, metal building companies use a different method
for the analysis and design of rigid frames with web tapered members than
the one proposed by Dr. George C. Lee and used in AISC Appendix F. I find
AISC's method confusing and not practical. I've tried MBMA 1996 Low Rise
Building Systems Manual without success so I am hoping one of you colleagues

can help me.

    Do any of you know where can I get this "methods used by metal building
companies"?  I know they use a simpler way (initial and final depth
determination, depth variation, design tensile, compressive, flexural,
shear, torsional and combined strengths, K, Cm, Cb, supports, etc.) and the
info provided would only be for learning and continuing education purposes.
Is not like I'm becoming competition of these companies when they share
their information with me. Maybe there's not such info available but an
outline or an example might help me.

Sincerely,

Juan Jose Treff



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