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# RE: More questions about rigid plywood Diaphragms

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: More questions about rigid plywood Diaphragms
• From: "Einar Einarsson" <byggtak(--nospam--at)itn.is>
• Date: Fri, 2 Jul 1999 23:21:15 +0100

```It may be late, but I just like to mention that FEMA273 and ASCEE/FEMA273
Prestandard define diaphragms as being rigid, stif or flexible. The
defination for flexible is the same as in UBC1994 and is one that has more
than two times the overage story drift.  The defination for stif is one that
has more than half the overage story drift. The defination for rigid is one
that has less than half the overage story drift. Both stif and flexible
diaphragms shall be calculated asuming flexible diaphragms. The result of
this for wood Structural Panel Sheathed diaphragms and shear walls is that,
if allowable shear is used in full for the nailing of the panels, then it is
very likly that the diaphragms shall be calculated asuming flexible
diaphragms. Most part of the deflection is because of nail slip according to
the formulas 8-2, 8-6 and 8-7.
I would like to see simplified calculations for small houses in the Codes
asuming flexible diaphragms. One methode is to divide the house in three
parts with shear walls in every part. The Danish Building Research Institute
http://www.sbi.dk has a book describing this methode. It is in danish,
sorry, but it is a simple methode.

-----Original Message-----
From: Greg Smith [mailto:strusup(--nospam--at)gte.net]
Sent: 2. júlí 1999 15:20
To: seaint(--nospam--at)seaint.org
Subject: Re: More questions about rigid plywood Diaphragms

I'm sure that there are sections in the code requiring an analysis of
this but wouldn't the code be equally satisfied if you used strongbacks on
the rafters and braced down and across creating an inverted delta?
the lower point of the delta would transfer the shear to a horizontal
diaphram on the ceiling joists which would transfer the force to shear walls
below.
As Nels mentioned, the vertical shearing force would still be at the
ridge but would be decreased in proportion to the height of the delta.

Greg

-----Original Message-----
From: Seaintonln(--nospam--at)aol.com <Seaintonln(--nospam--at)aol.com>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Thursday, July 01, 1999 5:34 PM
Subject: Re: More questions about rigid plywood Diaphragms

>Our discussion some years ago had to do with the problems of calculating
the
>actual forces on Rafter Tied Gabled roofs. The calculated forces for a
rafter
>tied connection often does not work as the resulting forces require a much
>more sufficient connection that what is called for in the Nailing Schedule.
>The discussion concluded that the Rafter tied connection was probably
>determined emperically rather than by rational calculations.
>
>Chuck Greenlaw convinced me that my perverbial butt is in jepordy if I
don't
>abide by the code fabricated standard of practice (since it is not in
keeping
>with the actual practices of the professional community during the reign of
>the '94 UBC) I'm in trouble.
>Nels has suggested that the analysis is pretty straight forward and other
>suggested that we really don't know what will happen until we create a few
>models.
>
>I know this may be a dumb questions, but hasn't the seismology committee
>modeled these system (gabled, scissor truss, vaulted roof systems) in the
>process of creating a deflection criteria?  If they had, why are the
examples
>not provided for us to learn their "Intent".  It's great to be expected to
>comply with the intention of the code, but we have proved on this list that

>the intent never included light framed residential type structures.
>
>Has anyone on this list done the math for these types of structures and if
>so, how about sharing your results with us?
>
>I'm no longer looking to debate, but am more concerned with understanding
who
>to solve different models that are not rectangular or have various pitches.
>
>Specifically, I would like to know how to solve a Donut Shaped Structre
(with
>square sides and an Attrium in the Center, A "U" shaped structure and a "U"
>shaped structure where the diaphragms of the legs are 45 degrees to the
>horizontal rather than perpendicular, A two story where the lower story is
>cut into the hillside for sub-terranean parking (with masonry walls), and
>other types of unusual shaped structures.
>
>Dennis Wish PE
>

```