Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: SI Metric

[Subject Prev][Subject Next][Thread Prev][Thread Next]
• To: seaint(--nospam--at)seaint.org
• Subject: Re: SI Metric
• From: ad026(--nospam--at)hwcn.org (Paul Ransom)
• Date: Sun, 1 Jan 2006 12:17:47 -0500

```> From: Christopher Wright <chrisw(--nospam--at)skypoint.com>

> On Dec 31, 2005, at 6:11 PM, Paul Ransom wrote:

> > Complicating this are the "other" measurement conventions that
> > use kg as force, simply because of common use carry-over.
> Which is my point--there's nothing about either system that's purer
> scientifically. Both have the same built-in misunderstandings

No. The force-mass unit confusion does not exist in the "SI" measure.
Newton (force) is derived from the basic properties, mass (kg) and
gravity. The standard mass is not determined using either spring or
balance scales.

> > In conversion tables, a pound is ALWAYS converted as mass units but a
> > kip is ALWAYS converted as force units.

> Pounds are more commonly used for force measurements--pressure in
> lb/in^2; reactions in lb; gravity force in lb; engine thrust in lb;
> moments in ft-lb etc. Anyone fool enough to do engineering analysis
> using the same name for the unit of mass and the unit of force
> distinguishes the two with the terms lbf and lbm. The reason kips are
> spoken of as forces is because they _are_ forces, just like meters and

For engineering applications, I agree on all points. Yet, we see gages
marked lb/in^2 with kg/m^2 (or similar) because somebody found a
conversion table. My tires and the gas station air pumps used to be
marked this way (now kPa). Why? Because the average person is not an
engineer and doesn't know a kPa but they believe that they "know" a
pound and a kg.

> kilometers are both lengths. Pounds are always force. When you say that
> something weighs 1 lb, you're saying that the the something experiences
> a gravity force of 1 lb. If you're talking about a mass of 1 pound
> you're saying you have an object whose mass is such that the object
> experiences a gravity force of 1 lb.

My point is that not everybody agrees with this interpretation of pound
or "weight". As a result, we have slugs (ptuuiii) and commercial
measurement standards by goverment decree that define pounds as mass -
not lb-force associated with a mass, etc.

So, steel and concrete are sold on a mass basis (pcf and plf) but are
applied on a force basis (pcf and plf), which have the same numeric
values (gravity acceleration only) but very different meanings. It sure
messes with units in MathCad sreadsheets.

IBC2003: pound ~ N (force) as defined in the footnote to many tables.

ASCE7: pound as force - converted values in text/tables only (no
confusion). No conversion table.

AISC: pound-force and pound-mass explicitly (ASD 9th Manual, conversion
table). Section properties provided as plf which is commonly applied as
force/length but is probably mass/length (see ASTM A6/A6M to confirm),
despite the good practice in the conversion table.

ACI: can't make up their mind(s)
pound ~ N (force)
lb/ft^3 ~ kg/m^3 (mass density)
http://www.concrete.org/COMMITTEES/pdf/051024-Conversion%20Rules%20for%2
0ACI%20318.xls
For static load applications, lb/ft^3 is commonly applied as a force
"density".
It is not clear if there is a conversion from lbf to lbm for ACI's
applications or if they rely on engineers to make that little brain
twiddle to accomodate.

NIST: http://ts.nist.gov/ts/htdocs/200/202/mpo_home.htm
"Mass (weight)" - pound ~ kg
Force - "pound-force" ~ Newton (kg*m/sec^2)
http://ts.nist.gov/ts/htdocs/200/202/fs376-b.pdf
http://ts.nist.gov/ts/htdocs/200/202/forum/2005%20ForumReport.pdf

Engineers REALLY want to see this:
http://physics.nist.gov/Document/SIFedReg.pdf
http://www.gsa.gov/gsa/cm_attachments/GSA_DOCUMENT/Metric_Design_Guide_R
2E-c-oW_0Z5RDZ-i34K-pR.pdf
This document also discusses rebar sizes.

It is interesting to see that it will soon be permissible, in the US, to
label consumer products in SI, only.
http://ts.nist.gov/ts/htdocs/200/202/forum/2005%20ForumReport.pdf

Other links are available at US Metric Association:
http://lamar.ColoState.EDU/~hillger/#metric

You will have to stitch the multi-line URLs to get the correct links or
cut them short and browse to find the reference.
--
R. Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
*
*   This email was sent to you via Structural Engineers
*   Association of Southern California (SEAOSC) server. To
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
*   send to the list is public domain and may be re-posted
*   without your permission. Make sure you visit our web
*   site at: http://www.seaint.org
******* ****** ****** ****** ******* ****** ****** ********

```