Concrete is great at absorbing uniform loads up to its failure point. It
doesn't
cope with shear or tension very well (esp tension). It crumbles beyond its
failure point.
The problem here is that the load is *not* uniformly distributed but rather
will be *very* localised by a bolt head, metal edge etc. The localised load
may well cause stresses well beyond the failure stress (20kPa for structural
concrete if I recall correctly). A localised crumble will probably cause
internal
tension stresses and the block will crack and break.
If you *must* use concrete blocks, then use a piece of 4x2 between the block
and the car. The point loads will be braodly distributed.
However, I do *not* recommend this - use stands (no liability etc etc)
Mike
----- Original Message -----
From <DLancer7676 at cs.com>
To: <Bushwacker4@prodigy.net>; <spridgets@autox.team.net>
Cc: <DLancer7676@cs.com>
Sent: Monday, January 15, 2001 10:20 PM
Subject: Re: Fw: Fw: engine pull...
> In a message dated 1/14/2001 11:22:07 PM Eastern Standard Time,
> Bushwacker4@prodigy.net writes:
>
> << Remember, they make buildings out of
> those things. The strength is there. I have used them hundreds of times.
>>
>
> True Kent, they do withstand tremendous pressure in a building situation.
> The forces exerted in that situation are spread along the entire surface
of
> the block. An automobile tends to exert point pressure that building
blocks
> may or may not withstand. I just don't wanna be (and I don't want YOU to
be)
> under the one that won't withstand the force.
>
> Jack Stands are cheap and proper for holding automobiles up. Why not play
> safe and use them instead?
>
> --David C.
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