Foundation Design for Large Lateral Load

[MikeMech]

Does anyone have any references or suggestions regarding foundation design for industrial or marine equipment with relatively large lateral loads, such as winches or mooring bollards? Many installations are retrofit. "Deep foundations" are not acceptable, generally they use a very thick reinforced shallow foundation.

I'm trying to develop a general set of foundation designs for a variety of winches, fairleads, and other equipment with very large lateral loads.

Any help is appreciated, thanks in advance.

Mike

 

[jdmm]

Depends on the magnetude of the loads you are talking about and the soil conditions and whether we are talking on ground or in water.

For marine environments batter piles or vertical pile groups with a stiff pile cap are frequently used. If you are on good ground then you can use passive resistance in front of the foundations.

To develop general guidelines you would have to specify the gound conditions that you are developing the guidelines for.

 

[MikeMech]

jdmm,

Thanks for your response. Lateral loads can be up to 300 kips (breaking strength of wire rope used) about 6" above ground level, footprint of units are no more than 3 foot square. Weight of each unit is less than 1,000 lbs. Units are on a steel base plate with anchor bolt holes.

Units are installed in ground, typically in an industrial environment near railroad tracks or even inside a building where a pit must be dug thru the existing building foundation.

The soil type is unknown, as it will vary from installation to installation. The only constant is that the equipment will be installed next to railroad tracks or an existing industrial building. Are there minimum soil properties that can be used in this case?

 

[BantrelStructural]

A 300 kip lateral load (even assuming it is 100% horizontal with no additional vertical uplift component) will require a very massive AND deep foundation. Using a combination of passive pressure, base rocking and base friction and ignoring several limiting factors (such as the suitability of soil near the surface to provide lateral resistance, and the soil beneath to provide adequate bearing to resist foundation rocking), a 20 ft. x 20 ft. x 20.5 ft. deep block might meet your needs. And this assumes that the actual horizontal and vertical soil deformations are compatible enough to allow development of the assumed resistances (i.e. passive pressure and base rocking might require different amounts of rotation of the block to develop their assumed capacities).

To keep it shallow (say 5 ft. thick would require a 46 ft. x 46 ft. pad).

Both answers lead me to ask if the 300 kip lateral load is a realistic design criteria. While you state that deep foundations are not acceptable, a battered pile(s) might be the most cost effective, and only feasible solution.

 

[MikeMech]

300 kip is the breaking strength of the wire rope involved in the largest unit, typical working loads are about 5 times less than the wire breaking strength. Overloading would happen only in event of an accident.

Bantrel, you may be right in questioning the desired load rating. I've seen similar units installed in shallow foundations that were much less than 20 ft cubed. I often run into the problem of safety factors being compounded unnecessarily due to liability concerns of the different vendors involved, this may be a similar case.

In this case, though, would it be appropriate to design around the wire working load? The anchor bolts are designed to withstand the wire breaking load, as is the rest of the unit. Would overloading the foundation by a factor of, say, 2 or 3 cause immediate catastrophic failure? I'm not as worried about the cost of a new foundation as the safety of the surrounding personnel, as wire ropes can store a lot of energy.

 

[BantrelStructural]

With liability concerns, the presumably third party designer of the foundation, would not design his foundation for anything less than the capacity of the anchorage.

 

[BantrelStructural]

Typical working loads at 1/5th. of the wire rope capacity is similar to lifting lugs which are designed for 5 times the specified lifting capacity to account for multiple handling and abuse such as impact damage from being dropped small distances. But since docking ships is not done under a "docking study" or under tight controls, unlike most heavy crane lifts that require an engineered lifting study, and careful pre-lift planning, there is a far higher risk of the mooring receiving an accidental full capacity load from the cable. As such I can't see that any authority would permit the foundation to be designed for anything less than the cable capacity. Or that any consultant's insurance company being happy with the degree of risk being assumed by designing for a lower load.

Just to give you a reality check comparison, a recent structure that I was involved in the design for major earthquake (west coast of North America), which was 90 ft. high, 52 ft. long and 26 ft. wide, complete with heavy petrochemical process equipment, only had a lateral foundation design load of around 400 kips, and we had 30 - 24" diameter piles and a 52 x 26 x 3.5 ft. pile cap!

 

[kme]

What about a helical anchor pile such as a Chance anchor?

 

[MikeMech]

I agree with your point about liability being the driving factor for the foundation, which is why I initailly specified the design load at 300 kips. The type of equipment I'm working with are heavy duty electric winches and capstans, wire rope guide sheaves, and cast steel dock bollards. All of these items have the common foundation problem of very large lateral loading (10 tons - 250 tons) in comparison to their weight.

For new construction of a mooring pier for example, anchor bolts are bolted (when possible) directly to large structural members within the pier, which eliminates a lot of problems. Installation in new ground, or in a retrofit situation, is where I'd like to learn more. Too many end users, for cost reasons, are just digging a hole that looks "big enough", throwing some rebar and concrete in and calling it a foundation. I'd like to change this tendancy...

 

[BantrelStructural]

I don't think that you get very large capacities out of Chance anchors (10-20 kips I believe). So assuming that you could install them at a 45 degree angle you would need 20 to 40 anchors. With group effects spacing requirements you would need a large area, and a big cap to ties these together.

Unfortunately there are some problems that have no nice and simple solution. This is probably one of them. Tieing into another structure is certainly one way of solving the problem, but that structure often isn't there.

 

[Focht3]

BantrelStructural has given good advice. Chance anchors can't develop enough resistance to be of real use for this problem.

Your loads are really too large for a "canned" design to be economical unless you are only planning to install one or two in open areas away from other structures, and excessive movement won't matter. My advice (echoing that of BantrelStructural) is to solve each problem individually; you will develop (over time) a series of solutions that will help you prepare design guidelines. If space is a constraint, then a drilled shaft or driven pile may be the only viable answer.

 

[MikeMech]

Thanks, all, for your input.

 

[TYL]

Don't forget that you also have a large overturning moment !
The problem sounds as though it needs a properly engineered solution.

 

[MikeMech]

If any of you would be interested in working on this problem more formally, please contact me at mike@usmech.net. Ideally, my goal is a foundation drawing for each type of equipment that will be suitable for most installations. Local engineers will, of course, have to review (and modify if necessary) the drawings before installation.