Engineering News Formula - Timber Pile 1

[nbr1]

An old dwg prepared by USACE references the following:
'Min. length of round timber piles 30-ft; or driven to a firm bearing so as to support a load of 25 Tons, calculated by the Engineering News Formula.'

My question is whether or not the pile load referenced is allowable or ultimate. My understanding is that the ENR calculates allowable capacity (with a usual factor of safety of about 6), so I have assumed that the load referenced is an allowable load. Is my assumption correct? Any thoughts?

 

[oldestguy]

You are right. When a drop weight is used the numerator value of C is 1.0 For single acting steam hammers the number is 0.1 For other hammers and diesel that C value might be modified. The Wisconsin DOT assigns c as 0.5 for gravity hammers and 0.2 for air, steam or diesel hammers. All as allowable.

 

[nbr1]

Looks like there are several versions of the ENR formula.
Some which calculate Qu and some which calculate a 'safe load'
which I assume is Qu.

What does the original formula calculate?

 

[oldestguy]

Safe load. That was back in the days when working stress, etc. was used in design, not ultimate. For this old guy, real old, I don't see the reasoning for factoring ultimate every design. Why not establish design working stress once and use it for many jobs? Maybe too complicated for me.

 

[oldestguy]

One more comment. When you go to buy cable or rope, the manufacturer has a number for each size as "load limit" or some other term other than breaking or ultimate. Must be a reason. Dummies otherwise might use the number for what it can do safely.

 

[Ron]

OG...score one more for us "allowable stress" guys. It's simple, it makes sense and it is less prone to error.

 

[BUGGAR]

When you step into quicksand and go under, are you exceeding the allowable soil working stess? Or the soil ultimate strength considering your factored weight - but is your weight a live load or a dead load? Well, it's a dead load now I guess.

 

[BigH]

As an aside - check out some papers by Flaate and Olsen back in the ASCE Geotechnical Journal (oops, that was one generation back) when it was the Soil Mechanics (oops, two generations past) Journal. They did some nice studies of various pile driving formulas between calculated and achieved - then offered some adjustments. Gates Formula fared well and it seems to be a popular one in LRFD from what I understand.

Nice Pun, Buggar, but you only exceeded the safe working stress because the stress was buggered up due to upward groundwater seepage pressure - something that doesn't seem to come into play in LFRD . . .