Grillage Foundation

[capablanca]

On electric transmission towers grillage foundations are
usual.But on telecomunication towers it seems not.Why is
this? The main restraint is similar:structures on hills where concrete transport is expensive.In many telecom steel towers I know the concrete was poured from an helicopter.Why not use steel grillage instead?
Does anybody knows a practical way to calculate such grillage found? Thanks.

 

[Focht3]

What do you mean by grillage? Please describe.

The loads are generally a lot higher for transmission towers than for monopole cell towers of similar height. The additional load comes from wind and ice loading on the conductors and shield wire (if used.)

Most transmission lines take years to build; most cell towers are designed and constructed in less than six months - sometimes in less than one month. Cellular providers may want to erect a new tower to relieve "congestion" on nearby towers - in this case, "time is money." The increased construction cost is outweighed by the increased revenue stream. And sometimes it's a regulatory issue - the provider's license is about to expire for a particular area unless they show progress in providing cell service by a specific date. The increased construction cost of airlifting concrete is insignificant compared to the cost of buying the license again.

Using a helicopter for construction can be very cost effective. Especially for small amounts of concrete - like backfilling around direct embed poles. Each circumstance is different.

 

[capablanca]

Thanks for your info.
I mean a spread foundation where the "slab" is a steel grillage or plate.The slab is connected to the tower legs by means of an interface steel structure.No concrete.See for example IEEE Std 977-1991.What I am exploring is to
supress the interface embedding the telecom tower in the soil,say2-3 meters.The grillage is connected directly to the base plates of the tower.Perhaps for some locations and tower heigts this solution is economic.Another point
is to use the steel in contact with soil for earthing.
I have not found in the technical literature how to analyse
and design such foundations .But using common engineering
assumptions the solution seems feasible.¿Any comments?

 

[Focht3]

Okay, I think I understand what you mean by a grillage foundation. (I don't have access to the IEEE standards.)

Grillage foundations (steel footings without concrete) are not common in U.S. construction of any type. The main reasons have to do with total cost and the site area required for a large, lightweight footing. Concrete is fairly cheap in most of the U.S. and can be obtained from local sources. The dead weight aids in resisting the overturning forces on a tower, particularly when the foundation is a spread footing. This helps keep the area needed for the footing much smaller than would otherwise be needed if a grillage foundation were used for the same structure and loads.

Keep in mind that many of these towers are 25 to 50 meters tall, are not guyed and are designed for wind speeds of over 145 kph (or more); the foundation carries all of the loads. A grillage foundation would probably be too large for a typical 50 meter by 50 meter cell site (and many sites are much smaller than that.) You can't use the weight of soil over the footing to increase the footing weight unless you take special precautions to be sure that the soil is not removed or eroded after the structure is built. (This could be done for communications towers (which are fenced), but is impractical for transmission towers.)

A grillage foundation could be used to support a communications tower when it is guyed; but would not be useful at all for the anchors - which are the real foundation challenge for these types of structures.

A grillage foundation might be useful in providing grounding for a tower; but it would have to be embedded (i.e. buried) in order to assure contact with the ground during dry periods. You would need a resistivity survey to determine your grounding requirements.

And don't forget to evaluate corrosion of the grillage and embedded portions of the structure -

 

[RiBeneke]

I recently designed a small 8 m high portable retaining wall using a grillage base. It is used at the loading ramp of a mobile crushing plant.

In trying to determine the capacity of this base, the main problem related to spacing of the grillage members and whether they would effectively present a 'solid' face to the overlying backfill or soil.

I made the assumption that if the gap between the members is less than 3 to 4 times the typical particle size of the backfill (we used a rock rubble backfill) then the backfill would not 'flow' around the grillage because arch action would develop.

I could not find useful research on this issue, and would be interested to hear of other experience.

 

[capablanca]

I heard that this type of foundation is covered in one of the Bureau of Reclamation codes.But I dont know the complete reference.

 

[Focht3]

I guess I should clarify my previous statement: I am not aware of the use of steel grillage foundations for use as permanent foundation elements for "conventional" structures in the U.S. They may be employed as temporary structures in some construction and mining operations, but I have not seen these applications. I would appreciate hearing any other comments on local practice with grillage foundations, site conditions, reasons for use, etc.

RiBeneke - your 3 to 4 times particle size for maximum gap size is rational, and is supported (I think) by the empirical design of wellscreen slots and other filter problems. You might check out Harry Cedergren's textbooks (including pavement drainage) for research that supports your approach. My only concern would be (substantial) upward flow of water through the granular material supporting the grillage, with the potential for loss of support and stability. This should not be a problem for a mobile crushing plant, but could be for an aggregate mine that extends below the groundwater table.

An 8 meter retaining wall is fairly tall for a cantilever section. What were the possible consequences of retaining wall failure? What did you use for your equivalent fluid pressure? How large were your ramp loadings? Where did you find the grillage materials, and what did you use for design guidelines?

[You're never too old to learn...]

 

[RiBeneke]

to Focht3
Thanks for your comments and references on my assumptions. I will follow them up.

The retaining walls are used at an opencast platinum mine here in South Africa, on the level ground outside the pit. There is no groundwater problem as the ramps are built up from the level stockpile terrace, and the area is very dry.
The walls are modules consisting each of a grillage base that extends under the backfill, a steel column and a raking back tie that forms a stiff triangle with the column.
Infill between the columns is done with loose steel panels.
Design pressures based on at-rest pressures plus braking loads from the trucks (Cat 769 or 40-ton ADT).
Grillages were fabricated as far as possible from second-hand steel sections (H and C-sections).

 

[QSE]

Grillages can be used for telecom latticed towers when the conditions are right, for instance
1-Develop a standard grillage for a tower type and soil condition. The same grillage can then be used many times, same idea as a standard tower. In the steel business, we try to avoid design/detail/trial assembly a new design that is going to be used only once.
2-Mass production of steel grillages for economy. Nobody likes to fabricate small quantities

Technically, transmission line people have used USBR #10 for criteria in sizing up the foundations. These are true for all type of foundations, concrete or steel. Steel elements are then designed based on normal structural engineering practice. Also check with IEEE, they have a few reports on full scale tests on tower foundations.

Oh yes, steel grillages are still widely used in transmission lines (latticed towers) today. Focht3 is right, Cell people are always in a hurry, dig and pour concrete is a lot faster than waiting for steel supply.

hope this helps

 

[Larnie]

I am a Project Manager and am currently involved with tower grillage foundation refurbishment. I am not an engineer but have been involved in grillage refurbishment for the last 12 months in New Zealand.
Currently we use a propping system to support the towers to carry out the foundation replacement, this is a time consuming exercise and we are looking at modifying our efforts.
Is anyone able to suply details of other propping systems in use around the world.I would be interested to hear from others involved in this work, and discuss methodologies and current practices.

 

[TYL]

I have never heard of any permanent grillage foundation that is not encased in concrete and generally sitting on or part of a reinforced slab or mass concrete foundation. In other words you have to pour the concrete anyway.It is relatively easy to design a grillage, even by hand ! All structural programmes will handle it.