more on base plates
i was looking at this old thread:
blodgett has some equations to account for the prying action you are describling in designing base plates. years ago, i had to go to an old math book to help develop the solution to some of the equations given.
mike mccann
mmc engineering
i will have to post twice, but here is my solution i did some 26 years ago.
mike mccann
mmc engineering
and the second page...
mike mccann
mmc engineering
let's try that again ... i obviously have the fabulous flying fingers syndrome here...
mike mccann
mmc engineering
without refer to dg, t & fc can be solved by the static equilibrium equations:
sum f = 0, p = t+c
sum m = 0, m = c*d = t*d
as long as the e (m/p) falls within the plate, there is a unique solution, which can be obtained through a few iterations. (note: the last responder on the referred link has pointed out correctly)
note the second term contains mistake, the moment shall be taken about t, or p, with sum m = 0.
msquared48:
thanks mike, very nice concise two page summary. from 1983! you're a pack-rat like me, it's hard to throw stuff away - you never know when it will come in handy again.
i'm seeing part of my problem, the columns have small vertical loads compared to their moment loads and the e=m/p
eccentricity lands outside the length of the plate. i'm wondering now about adding a horizontal brace to the frame to take the lateral load out of the base plate design.
mike tackled the problem from a purely elastic perspective following the method of blodgett.
using limit states design (lsd), p and m would be factored loads with m = p*e.
if e < 1/6 the load is inside the kern of the baseplate and there will be no tension in the anchor bolts.
if e > 1/6, the problem may be solved by moving the load, p to align with the anchor bolts and adding a moment p*f where f is the same as in the blodgett solution, i.e. centerline of baseplate to bolts. the moment about the anchor bolts is then p(e + f).
estimate a uniform stress block and, on that basis, calculate t' (tension in anchor bolts due to p(e + f)). adjust the stress block as required to comply with the code and recalculate t'.
finally, t = t' - p where t is the ultimate tension in the anchor bolts due to factored loads.
i think this may be the same procedure as kslee was talking about.
ba
without get into details, just by a quick glance, i sense mike's method is little deeper than the simple static equilibrium solution. looks like it incorporates the deformations of the concrete and the anchor bolt, so on top of the simple static equations, there is a strain compatibility component to look at. interesting is it? kind of lazy to do the investigative work, wait till someone to validate it, and pointing out the benefits of that method.
wow!!! calculations from the day i was a year and half old .
2009-09-10, 03:12 PM
more on base plates
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