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RE: Residential Design Spreadsheet

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Dennis sent this to me:

Mark,
Possibly you can pass this on to the list as I am not a subscriber and can
not post to it.

The Lateral Design Spreadsheet - flexible analysis provides a wall
deflection analysis that assumes the wall to be cantilevered. The method
follows, very closely, the methods presented in the 1998 Wood Design Seminar
AND the ICBO Volume II of the Seismic Design Manual. If the wall occurs in a
location other than above a wall below, the designer is responsible for
further analysis to justify the design of the member below which is used to
resist the reactions on the wall (such as a beam below).

The spreadsheet follows UBC method for lateral force distribution in a
multi-story building to arrive at a story shear for each level. The
distribution of shear to resisting walls in each level follows a more
complicated proportional distribution based on the following design
assumptions of a series of simply supported beams. The process becomes a bit
more complicated when walls do not stack. The following method was created
to distribute shear to walls which do not align from level to level:

1. Every line of shear from the first floor to the top must be accounted for
at each level. If you consider the building to be transparent from roof to
foundation and place a clear Mylar over the top - each wall that resists
shear - regardless of the level it occurs on - is represented by a grid
line. The total grid layout occurs at each level whether a wall occurs at
that level or not.

2. The tributary areas occurring between grid lines is calculated and the
shear from the upper level is distributed by proportion into the walls
below. It becomes a bit more complicated to explain:
First, the shear at the level under consideration is distributed by
tributary area into all walls. If a wall occurs above that is between two
walls in the level below, the force from the wall is distributed to the two
nearest walls below by proportional distribution the same as you do with a
simple span beam with a concentrated load placed anywhere on the beam. I
follow the formula Rleft = Pwall * (L2/(L1+L2)) where L2 is the distance
from the wall above to the wall reaction at the right of the load.

3. As the loads are cumulated in lower levels, the reactions from the walls
above are distributed in the same fashion as stated in number 2 above. This
allows the user to omit and add walls as the program will redistribute the
loads accordingly.


Mark, one thing that disturbs me is the ease in which the spreadsheet is
simply discounted by some of the respondents. A great deal of effort went
into the creation of the spreadsheet by myself and others. We placed it on
the SEAOC website for others to use freely. We expected feedback and reports
of errors and inaccuracies - which we fully intended to address. However,
many users downloaded the software, found a bug or two and simply discarded
the tool as useless.

The time and energy spent to create this tool were done by dedicated
engineers who believed they would be contributing a useful tool to their
peers without expectation of compensation. Over one hundred and fifty names
were accumulated when the software was first released. Only three or four
responses were received.

The software was then donated to the public domain and left unprotected on
the SEAOC server where anyone with spreadsheet skills could take the
software, improve upon it and offer it back to the community as a
contribution - none have.

Personally, I do use the spreadsheet and found the results consistent with
hand analysis that I perform. There are issues that remain unanswered by the
code writers and until that happens, neither our spreadsheet or the two
commercial products on the market will meet the needs of everyone in the
profession.

I am amazed at how often it is easier to criticize than it is to contribute
something useful. For those of you who would like to invest a bit of time in
learning to use the spreadsheet, I remain available to discuss your concerns
rather than receive them second hand.

Regards,
Dennis S. Wish, PE
The Structuralist Administrator for:
http://www.structuralist.net
AEC-Residential Listservice
admin(--nospam--at)structuralist.net
(208) 361-5447 E-Fax


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