Impact to footing from crane loads

[aardy_wolf]

I work at a firm where we design crane plans (mainly mobile cranes, however we do some tower crane, suspended scaffolds, mast climbers, and pile drivers) and one of the main design considerations is the presence of voided/vaulted spaces below grade. We usually follow the 1:1 (also known as 45 degrees) ratio rule when operating near foundation walls. This usually means that we keep the distance (as seen from plan view) from the outer edge of the crane's outriggers (and if it's sitting on dunnage/plate, the outer edge of that) to the edge of the foundation wall equal to the height of said foundation wall. So for example, if the basement of a building has a depth of 10', we usually restrict the crane's outriggers to a 10' minimum distance away from the foundation wall, as seen from plan view, in order to avoid impact to the foundation wall.

However, a very important question came up amongst my team regarding the impact that the crane loads have on the footing. Since many of the buildings in our jurisdiction are old (majority are pre 1930s), we do not have all the details for the foundation wall design or footing design, so an analysis is difficult to do (however we do analysis with conservative assumptions when required). So we usually just follow the 1:1 ratio rule as a good standard of practice (and if 1:1 distance cannot be maintained, we do impact calcs to see if foundation wall can sustain crane loads). Following the example as stated in the previous paragraph, lets assume the depth of the vaulted space (e.g. basement) is 10'. Without plans or specifications, we do not know the depth of the footing. So if we were to limit the crane's outriggers to a minimum distance of 10', the impact from the crane loads would technically be impacting the footing. For simplicity sake, lets assume the footing is 1' deep. My questions is:

a) Is the crane loads really impacting the footing? and​

b) If so, how can the impact to the footing be measured? For impact to foundations walls, we usually follow a terzaghi method to find the lateral loads and then compare that to the design strength of the foundation wall. Would we follow a similar method? and​

c) What would be a safe assumption for the depth (or height) of a footing? I know there are many answers to this, but assume a building height of around 6 stories (or 60 feet) designed and built around the 1930s​

I would assume that the crane loads truly impact the footing, but that the impact is negligible at best due to the fact that there is usually a slab adjacent to the footing (imagine a section view of the foundation wall) which provides the footing with more strength to resist lateral loads. Since the slab is also acting as a diaphragm, and that there are no voids to either side of the footing (imagine a section view of the footing, with the left side being the slab and the right side being soil), the footing is acting primarily as a member of the buildings' foundation to support the foundation walls in bearing. I doubt that footings are designed with lateral loading in mind, however I am not that well versed in footing design and considerations. I of course would prefer the most conservative practice when developing crane plans and would like to avoid impacting the footing.

Any help would be wonderful! Just looking for opinions, as my background is not in foundations, but is primarily structural. Please let me know if I phrased anything incorrectly or if I could have phrased it better or shorter, as I wish to improve my technical writing.

 

[dik]

Unless your loads are significant (equal to a significant portion the structure load, for example) and sustained, I would think the transitory loading would have little effect on the foundation and it is unlikely that failure would be catastrophic. The effects can be easily monitored. Same with lateral loading as long as the structure can withstand the loads. The client should be aware of the shortcomings and possible remedial action to be taken.

Dik

 

[Lomarandil]

I think your use of the term "impact" may be throwing dik off the track a little.

A well operated crane should see little or no dynamic/transitory/impact loading. (And any dynamic loading is going to dissipate quickly in most subgrade materials). However, crane loads can certainly impact/affect an adjacent foundation.

I agree that for a footing 10' below grade, the effect of the lateral load will be negligible in most cases. Much of your surcharge pressure will have dissipated by then (although for a crane on dunnage, you may still be within the zone of influence), and the footing will be often restrained by some sort of slab.

There's always the risk that the footing is some distance below the slab/lateral element, and therefore the footing column/wall is exposed to bending. That is mostly a function of the local building practice there. Where are your jobs located?

For shallow footings/large outrigger loads, the lateral load will sometimes not be negligible. I'd go with a Bousinessq solution to find the effective lateral pressures from surcharge loads (for walls, remember that you need to approximately double the result for an elastic half-space solution), then analyze the foundation structurally. Nothing too crazy there, just approach it rationally and remember that in the world of construction loads and geotechnical engineering, there's no sense sharpening the pencil too much.

----
The name is a long story -- just call me Lo.

 

[SlideRuleEra]

...how can the impact (influence) to the footing be measured?

One old and simple approximation that is consistent with the 1:1 zone of influence rule is to distribute the load over the assumed soil contact area at the desired depth. I'll push my isometric projection sketching ability to the limit and put this in an example:

As shown, the original 32,000 lb point load (on 4' square dunnage) has dissipated to a negligible soil contact pressure of 56 lb/ft² at 10' depth.

Note that even without dunnage, a 32,000 point load will be distributed on an assumed area of 400 ft² (20' x 20') at 10' depth... 80 lb/ft², still negligible.

http://www.SlideRuleEra.net

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[aardy_wolf]

Unless your loads are significant (equal to a significant portion the structure load, for example) and sustained, I would think the transitory loading would have little effect on the foundation and it is unlikely that failure would be catastrophic. The effects can be easily monitored. Same with lateral loading as long as the structure can withstand the loads. The client should be aware of the shortcomings and possible remedial action to be taken.

To dik: Thanks for the response! I definitely do agree with your conclusion.

I think your use of the term "impact" may be throwing dik off the track a little.

A well operated crane should see little or no dynamic/transitory/impact loading. (And any dynamic loading is going to dissipate quickly in most subgrade materials). However, crane loads can certainly impact/affect an adjacent foundation.

I agree that for a footing 10' below grade, the effect of the lateral load will be negligible in most cases. Much of your surcharge pressure will have dissipated by then (although for a crane on dunnage, you may still be within the zone of influence), and the footing will be often restrained by some sort of slab.

There's always the risk that the footing is some distance below the slab/lateral element, and therefore the footing column/wall is exposed to bending. That is mostly a function of the local building practice there. Where are your jobs located?

For shallow footings/large outrigger loads, the lateral load will sometimes not be negligible. I'd go with a Bousinessq solution to find the effective lateral pressures from surcharge loads (for walls, remember that you need to approximately double the result for an elastic half-space solution), then analyze the foundation structurally. Nothing too crazy there, just approach it rationally and remember that in the world of construction loads and geotechnical engineering, there's no sense sharpening the pencil too much.

To Lomarandil: I work primarily in downstate NY. Lots of old buildings in the metropolitan NYC area, and not so good record keeping. Following your recommendations, I'll do some analysis of lateral loads on footings, compare it with the strength of the footing, and see what my conclusions are, especially in regards to footings which may experience bending.

One old and simple approximation that is consistent with the 1:1 zone of influence rule is to distribute the load over the assumed soil contact area at the desired depth. I'll push my isometric projection sketching ability to the limit and put this in an example:

As shown, the original 32,000 lb point load (on 4' square dunnage) has dissipated to a negligible soil contact pressure of 56 lb/ft2 at 10' depth.

Note that even without dunnage, a 32,000 point load will be distributed on an assumed area of 400 ft2 (20' x 20') at 10' depth... 80 lb/ft2, still negligible.

To SlideRuleEra: Your sketch looks great! I do agree with your conclusion, which you substantiated with the soil contact pressure calcs.

Btw, on a separate note, I visited your website listed on your signature and I think its such a great resource! Working in downstate NY, we often work with very old building codes (I've seen some from pre-1900s). It is very interesting!