Load Calclulations - Units 2

[VVang]

I am trying to calculate the load applied on a structure by water. Using hydrostatic pressure I can calculate the pressure and force applied. My question is, is that force in terms of lb-mass? would I need to convert that to lb-force for my structural analysis?

 

[JStephen]

For hydrostatic loading, you'd generally work with unit weight (not mass) of water as 62.4 lbf/cuft, depth of water in feet, and pressure as lbf/sq.ft.
If you get into more involved liquid calculations (involving flow), then you'll wind up with mass density instead of unit weight, and need gc conversion factors,etc.

 

[VVang]

Does that mean instead of using rho (fluid density) I can replace that with unit weight instead?

 

[Stick.]

If you start with a unit weight in lbf/ft^3, you can swap that for rho if you don't have a gc in there as well. So instead of rho*gc*h, you'd just have (unit weight)*h.

The more complex your calcs get here (like flow calcs that JStephen mentioned), the wonkier it gets to use both pound mass and pound force. I typically sidestep this and just do the calc in metric (and maybe convert back after), or I'll remind myself what a slug is and use those for mass/density instead.

 

[BridgeSmith]

As long as project is somewhere are the face of the earth lb-mass = lb-force, all day long. If you're designing a container for water on the Moon or Mars, then it would matter.

 

[rb1957]

well, 1lbm has a weight of 1lbf.

by fundamental units 1 lbf = 1 slug * 1 ft/sec2 = (in less fundamental units) 1lbm * 32.17 ft/sec2

so pressure = 1/2*rho*g*h, where rho is in mass/ft3. The mass density of water is close to 2 slug/ft3, so pressure = 1/2*2*32.17*h lbf/ft2
if you're using weight density the pressure = 1/2*(rho*g)*h ... you've already accounted for "g" = 1/2*64.34*h lbf/ft2



"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[CANPRO]

lbf and lbm? I'm standing on my metric pedestal right now and casting judgement.

 

[rb1957]

kgf vs N ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[CANPRO]

I knew the kgf force comment was coming. That is reserved for the truly insane. But even then at least the two units aren't easily mistaken for each other.

 

[IDS]

kgF?

What is this kgf thing of which you speak?

Checks Wikipedia:
"The kilogram-force (kgf or kgF), or kilopond (kp, from Latin: pondus, lit. 'weight'), is a non-standard gravitational metric unit of force. It is not accepted for use with the International System of Units (SI)[1] and is deprecated for most uses.[citation needed] "



Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[human909]

kgF is kilogram X Farods which is certainly an interesting combination of unit by still Si compliant.

kgf is kilogram X feet which is another interesting combination

 

[CANPRO]

kilogram x feet is going to keep me awake tonight. Thanks, human909.

 

[human909]

kilogram x feet is going to keep me awake tonight. Thanks, human909.

People might be mistaken to think that Europe is a bastion of the metric system and SI units. But really I've seen plenty of (IMO) horrible abuses of the SI system. While some are formally correct I still see them as a bit of an abomination.

I regularly get loads given in:

daN - deka-Newtons
T - Tonnes (for a force)
Tm - Tonne-meters (for moments)

Of course dealing with these isn't that painful to convert to kN or N. But I still find it odd to give moments in Tm. Europeans in general seem to enjoy making liberal use of the 'centi' and 'deka' prefix. I don't see 'hecto' (except hectar) or 'deci' (except decibels) used much.

Down here in the land of kangaroos we've enthusiastically adopted the metric system and formal SI units without too much abusing of the system. I'm sure there is a few quirks that I can't think of at the moment though. Everywhere in construction still has non rounded sizes due to using timber and steel member cut/rolled in sizes from the imperial days. But it isn't too bad. It would be nice to have clean European sizes in hot rolled steel though.