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Re: Slender CMU Wall Design

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> From: rlewistx(--nospam--at)juno.com
> 
> I am doing a preliminary design on a slender CMU wall.  I have several
> 
> 1. The wall is for a pre-engineered metal building.  The wall (eave
> height)is 30 feet tall.  The spacing of the rigid frames are 26.5 feet.
> The architect is bumping out the face of the wall at the frame columns.

> rigid frame.  If the joints are located at this location then it seems to
> me that the wall must span vertically, not horizontally.  Is this a
> correct assumption?

Not necessarily. It's all in the detailing. However, don't expect the
metal building manufacturer to do these details.

> 2. Should I have some type of tie between the wall panel and the column
> wrap that crosses the control joint other than the rubber joint material
> itself?

The devil is in the details. I assume that everything, at some point, is
fastened to the supporting structure ... that is your tie. Differential
deflections will be limited by how everything is fastened to the
supporting structure.


> From: rlewistx(--nospam--at)juno.com

> design section.  I don't have the text in front of me but I know there is
> an AISC manual on low-rise steel construction that would agree with these
> high sidesway values for masonry wall construction with steel framing.
> 
> >From my post several years ago I came away with the conclusion that most
> >engineers design for wind sidesway of h/400 to h/500 using the 10 year
> >wind when masonry walls are present for the majority of the building.  As

The difference is not that pre-eng or low-rise design(er)s follow any
other rules, just that the structure details and wall coverings that
they provide can accomodate larger apparent building deflections. The
important term here is "low-rise".

I know that Butler, for example, has specific internal guidelines for
deflections with supported masonry walls. Expect their quotes to be
higher and be very careful about comparing the quote documentation that
is provided by all manufacturers.

> >I wrote in other post I sent on slender wall design, I have a
> >pre-engineered metal building with a 30 feet eave height.  The exterior
> >walls will be masonry.  H/100 for this condition is a 3.6" sidesway.
> >This seems too much for me.  H/400 would knock this down to 0.9".  I know
> >this will increase the steel cost, but it seems to me that this is more
> >appropriate for this conditions.
> 
> What other opinions are out there regarding sidesway of pre-engineered
> metal building with masonry exterior walls?

Rich,
You want to watch three things:
1) the general frame deflection
2) the deflection of the supporting member at the top of the wall if 
   masonry spanning vertically
3) the endwalls and corners - deflection parallel to the frame lines
You also want to reflect on why you need the deflection limited to
determine what you can accept. (I have a story for this one that a few
beers would pry loose)

Depending on the manufacturer, they may typically use h/100 for the
frames and L/60 for the wall supporting members regardless of the
existence of masonry. The net result in your case may be:
h/100 + L/60 = 3.6 + 5.3 = 8.9" or h/40 (possible stability issue)
at the top, mid-span, relative to the base.
Base crack width <= 1/8" on a bad day
This will be the default condition UNLESS IT IS SPECIFIED OTHERWISE IN
THE RFQ OR PROJECT SPECS.

by contrast:
h/400 + L/360 = 0.9 + 0.88 =1.8" or h/200
at the top, mid-span, relative to the base.
Base crack width <= .04" on a bad day

The pre-eng manufacturer can use whatever deflection criteria is
requested. This is a very competitive industry where buildings have
become commodity items and sales are cost driven more than anything
else. The manufacturers will not unilaterally quote a more expensive
product (e.g. stiffer building), unless it is explicitly requested, so
that all competitors are quoting similarly.

Yes, it will cost more to achieve the deflection limits BUT the pre-eng
manufacturers will not be concerned about losing to a competitor on the
basis of spec variance. Some requirements will push a pre-eng style
building out of the economic options.

The view from the other side (e.g. from the manufacturer's engineers):
They see a wide variety of VERY poorly written specs coming from some
major engineering consultants - IF they get project specs at all. (They
pass around the bad specs for everybody to have a laugh, at your
expense.) They are not in a position to discern your specific
requirements unless you make it very clear in the documentation. The
engineer (if there is one) involved in preparing your project quote will
PROBABLY not be involved in designing the building after the purchase
order is received. After the order is received, it will PROBABLY be
designed according to what was quoted rather than what may be "assumed"
to be common engineering practice (e.g stiffer deflection limits - see
above).

Some manufacturer's design engineers will be courteous enough to call
the client (the party who signed the purchase order, not the project
engineer) to ensure that they really do want a masonry supporting
structure with an h/100 limit. This MIGHT be communicated to the project
engineer for response. If a higher deflection limit is determined to be
required, the manufacturer WILL increase the price commensurately.

This can also be used as the pre-eng equivalent to BIGGIE sizing your
fries or "up-selling." The manufacturers will low-sell until they get
the project "off the street". You must be very careful about comparing
quotes from multiple manufacturers.

These aren't your grandfather's pre-eng metal buildings (or selling
environment), anymore. You MUST be knowledgeable about how to properly
specify the building if you have any requirement that extends beyond the
manufacturer's typical supply of frame, secondaries and steel profile
cladding standards.

-- 
Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ad026(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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