Column Base Plate Design Help

[Shaylon]

Hello all. I am trying to design a steel base plate for the column connection of that same canopy I have been working on, but I am having trouble doing so. If anyone could possibly shed some light as to what to do with this I would greatly appreciate it. I have the following condition...

PU= -11.173 KIPS
MU= -19.676 KIP-FT
4 ANCHOR BOLTS
3000 PSI CONCRETE
PROPOSED 12x12 BASE PLATE (3/4" THICK)
COLUMN TO BE 8" DIA. XS PIPE

Again, I am a recent graduate and I am turning to the knowledge base of this forum for a bit of guidance and direction. Any and all help is greatly appreciated. Thanks.

Shaylon

 

[msquared48]

You are simplistically looking at P/4 + M/d, plus some extra for prying action if appropriate (assuming your "P" load is tension). Guestimating, you could be looking at 20 to 25 Kips per bolt. Apply the force locally to the plate over a stip, say 4" wide and see what thickness you need.

There are more complicated and more precise methods available, but this would be a good ballpark check.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[transmissiontowers]

You can get a free HILTI program called Profis that does the calculations for the anchor bolt load and plate bending. If you're using leveling nuts under the BP, the answer will be a little different and if you have a gap between the bottom of the BP and the top of concrete of more than 2 bolt diameters, you need to consider bending in your anchor bolts.

I usually determine the A-Bolt loads and find out what bending moment they produce at the face of your column, then limit the bending plane to 12 times the plate thickness (Omer Blodgett book recommendation).

You can spend a lot of time doing a FE analysis of a base plate or use some simple hand calculations to determine plate thickness.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[weab]

You need AISC Design Guide 1. This is the primary reference for base plates. Even if this particular base plate design is fairly simple, you will need design guide 1 often.

 

[csd72]

shaylon,

Design of these is fairly similar to that of a reinforced concrete beam in bending and axial load but there are bolts instead of tension steel and there is a triangular bearing stress instead of the (usually) rectangular stress block.

As a preliminary design it is definately adequate to treat it as msquared48 has suggested but for the final design I would suggest you follow design guide 1.

 

[ToadJones]

moment baseplates are difficult to do with a typical baseplate set up; especially for a cantilevered single column as deflections can be high. I'd suggest using wing plate or similar design

 

[a2mfk]

Listen to Mike and CSD. I've designed a ton of these in my career and I am not that old, especially if you work on small to mid sized projects in higher wind areas you will do more.

All of AISC's guides are worth the price of membership for a year at a minimum, where you can download all of them for free.

 

[WillisV]

@transmissiontowers - where did the two bolt diameter = no bending rule come from - reference?

 

[transmissiontowers]

WillisV; I was on the committee that authored ASCE 113 and it is in there. I also believe it is in ASCE 48. It might also be in ASCE 10.

The reasoning is the the leveling nut on the bottom and the nut on top clamping the BP make it rigid and since the nut is usually the bolt diameter high, the gap between the bottom of the leveling nut and the T.O.C will be one bolt diameter. We felt that the concrete was fixed and the bottom nut was fixed so only the shear will be transferred and there is not enough room for the bolt shaft to bend. I know from a pure statics point of view, the bending will be there, but in practice it should be small and can be neglected.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[csd72]

Bending is always there though, even in a supposed 'shear' failure of bolts they are almost always a bit bent. So what you think of as a nice clean shear capacity is really the capacity of the bolts to take a combination of bending and shear as applied by two test plates sliding over each other.

 

[transmissiontowers]

csd72;
Yes you are correct about the bending being present and we don't say you must neglect it but if you want to, you can. It is fairly common practice in our industry.

Our safety factors are very low compared to the bridge and building industries. Our structures are not inhabited during a storm that produces the design loads (hurricanes and ice storms) so we only want to make the structures a little stronger than necessary and we are willing to accept a few structural failures in the interest of cost savings. People are already griping about their summer electric bills so we don't want to make them a lot more expensive.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[a2mfk]

Huh? Replacing one or several transmission towers or power poles in possibly remote areas can be a huge expense compared to bumping up the size of the anchor bolts or base plates. And though the risk of loss of life is not the same as a bridge, tell that to someone who lives next to a giant power pole. Not sure I get that justification, though I understand your analysis explanation and accept that.

 

[ToadJones]

...if I recall correctly, I think the allowable tensile stress and combined bending + Compressive stresses in ASCE 48 actually go up to yield in some cases...
Is this correct TransmissionTowers?

 

[transmissiontowers]

I get long winded sometimes so bear with me, but if you want to know, we live on the edge of failure.

We use a form of LRFD where we apply overloads to our loads and we go all the way to yield. Our overload on the hurricane wind is 1.1 and the overload on the wire tension is 1.2 during a hurricane. We use the ASCE 7 wind map and design for a 50 year MRI storm. So for 98% of its life, the stresses in the poles and towers are under 15% of capacity. We use our own buckling formulas for single angles in compression based on testing many years ago. For cold parts of the country the NESC prescribes the overloads and ice accretion on the wires. These are much higher at 2.5 for wind, 1.65 for tension and 1.5 for weight.

When we design a new tangent lattice tower or pole we usually do a full scale test and usually test to destruction (not many bridge engineers can do that) to show proof of concept. We may fabricate the same tower 2000 times over its life so if we can save 500 to 1000 pounds per tower at $2 per pound, we feel it is worth it. The heavy angle towers and poles use a little more overload since not many test facilities have the capacity to pull one over.

It is usually not the wind that brings down a pole or tower, but wind borne debris that wraps around the structure or wire that increases the wind force dramatically.

Some call for all lines to be put underground but it is very expensive at the transmission voltages of 138 kV. We recently completed a 6 mile UG line that the State paid for that cost $10 million per mile which is 8 to 10 times as expensive as overhead lines. You have to dissipate a lot of heat into the ground when you go UG.

AFA living next to a large tower or pole, we use wide ROW where the fall distance would not hit any house outside the ROW. When they do fall it is not like a tree but the crumple down upon them selves.

I hope you guys enjoyed my dissertation, but only structural engineers that do lattice towers are crazy enough to use single angles in compression and we have some 12x12x1.25 angles welded up from plates to handle some very large leg loads.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[a2mfk]

Very interesting, an area of structural engineering I know almost nothing about. All good to know, and I have thought about the underground vs above ground thing a lot, but you just answered that. The opposite of common sense knowing nothing about the subject, just looking at these massive steel towers with guy wires and thinking why not on or near the ground, maybe a few feet off the ground like a gas line behind a fence of course.. But I did not consider the heat issue.

I'll take your word on the ROW where you are working, but I just went to a house for a forensic job and she pointed to a steel power pole about 2ft in diameter or so installed about 20 feet away from her house, because she was concerned that had some effect on sinkhole activity. I told her to get a good night's sleep and stop worrying about everything.

 

[transmissiontowers]

2 foot diam sounds like distribution class under 35kV and yes, distribution by necessity is closer to houses because land is expensive and distribution has millions of poles while we have only 1000's. Some of our larger steel poles are almost 300' tall and close to 10 feet in diameter with 3/4 inch wall thickness.

AFA close to the ground, the conductors are not insulated either at distribution voltages or transmission voltages because of heat dissipation. We run our wires at 250°C in an emergency and they sag like crazy at the high temps. You rarely see birds on the conductor because it burns their feet.

The safe approach distance at 345kV is about 10 feet which means that under the right conditions you will get electrocuted just standing within 3 to 4 feet of a wire. We want to be at least 22' above ground at the max wire temp which means on a cold day with low power flow, the wire is 30 to 40' above ground depending on span which is typically 500' to 1000'.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[ToadJones]

Thread officially hi-jacked.
But its a good one!

 

[transmissiontowers]

Yes, sorry about the drift/hijack. We could go over to the transmission structure forum and continue but I'll post answers/opinions here if you want to ask more.

http://www.eng-tips.com/threadminder.cfm?pid=608

This aspect of structural engineering is very specialized and there may be a few hundred engineers that do it in the USA and we are usually passionate about the topics.

Back to the base plate topic, we used to grout under our base plates but when we saw some rusting of galvanized a-bolts we got away from using grout. Even a flat plate with no holes other that bolt holes will trap water if the grout does not completely fill the gap. The water gets between the nut and the plate and stays in the annular space and rusts the anchor bolts. We had about 30% loss of material on badly rusted bolts.

I have seen hollow tapered columns with 6' of water trapped inside because there were no drains. The column walls had rusted through on a couple of others and the water drained out but the damage was done.

Without inspectors to watch, the construction crews just pack grout around the perimeter of the BP and make it look good because it is hard to get it to flow underneath.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[BAretired]

Shaylon,

Assume your four anchor bolts are 9" apart. Mu = 19.676'k (20'k). Do not presume to know loads or moments to five decimal places. You are lucky if you know them to two. I assume that Mu is ultimate moment.

With no axial load, Tu (2 bolts) = 20*12/9 = 27k (13.5k each). Use 4 bolts with ultimate tensile capacity of 13.5k each.

Check baseplate. Diag. distance between bolts = 1.414*9 = 12.7" which means that cantilever m = (12.7 - 0.9*8)/2 = 2.76" and Mu(plate) = 13.5*2.76 = 37"k.

Plate is 12*1.414 = 17" corner to corner on the diagonal. Effective width of plate resisting moment = 17-0.9*8 = 9.8" (at 45 degrees). Thickness = 3/4".

Z = 9.8*0.75^2/4 = 1.38in^3

Yield moment of baseplate = 0.9*Z*Fy = 0.9*1.38*50 = 62'k > 37, so the plate is okay (assuming 50 ksi for baseplate).

The addition of 11.2k axial load cannot make the situation more critical, so it is not necessary to check for combined axial plus bending.

In the above, I assumed the moment was parallel to one of the sides of the baseplate. You should confirm that a moment at some other angle does not produce a more critical result.

BA

 

[csd72]

transmissiontowers,

My point on the bolt bending was aimed at the fact that a small amount of bending would have been included in the original shear tests on the bolts rather than just saying that it has been neglected.

As the safety factors stated refer to yield stress then there is generally a significant safety factor above that in regards to rupture strength. The difference is usually in the second order effects which I would not expect to be particularly large in a transmission tower.

If there is no grout under the base plate then these can then be treated as discrete points of support and analysed by the simple formula stated above.

Just my $0.02