Do you use slab mass to resist column uplift in metal structures? 4

[IJR]

Nice fresh week friends

I was just curious. Some friends use mass of floor slabs in calculating weight to counteract uplift in columns of metal structures.

Those who have used this procedure, do you want to share your experience and implications with us?


Respects

IJR

 

[Ron]

IJR...yes, I use everything I can! I have done a lot of light, rigid frame structures subjected to overturning and uplift. For those, I consider an influence area of the slab of about 5 feet on either side of the column, as you can expect a large "chunk" of concrete to be pulled upward as the column is being pulled.

For these, the configuration is usually an independent, small "box" footing with the column embedded in the footing for fixity, though the same can be accomplished with a bolted baseplate. For monolithic thickened slabs with reinforcement extending into the slab, you might even consider a larger area as contributing.

Also, if the footing is a ways below the slab and there is fill above the footing and below the slab, don't forget to include the soil over the footing.

Ron

 

[IJR]

Respects Ron

I understand you dont go into slab stiffness, you only utilize the weight of the slab, which is really good enough for home-kind of lightweight metal structures. Right?

If I have something of more span, like an industrial building, do you think I should also take the stiffness into account?

As for the soil above footing, I usually use the soil cone (30deg from top of footing pad extendingo out like in EIA-222-F). What is your opinion on this?

Thanks a million Ron

IJR

 

[Riz]

Hi IJR,
I agree with Ron.
Its usually important to mobilise weight of the floor slab not only for countering uplift forces but also to assist in reducing tension in soil when designing foundation for a long span, light weight structure.
I take a 45 degrees angle from the edge of pad for consideration of floor width. The angle is actually the idealised shear plane. Additional reinforcemnt is also added to allow for tension build up in the slab at the point/location considered.
Stiffness of the floor ? can u clarify!

 

[Ron]

IJR,
I would not consider the stiffness of the floor since you want to limit the upward movement, not accomodate it. If you consider the stiffness, you would be allowing the slab to bend as a beam, with quite a bit of upward deflection. If you consider the shear plane, it keeps the influence area smaller, forcing you to gain mass from other sources, thus limiting upward movement.

Riz has given a good rule of thumb...45 degree shear plane for the longer spans.

Ron

 

[IJR]

Thanks Ron
Thanks Riz

I see your point, beam action(stiffness accounted for) does not make sense given uplift condition. Respects Ron

And the 45 deg from pad to slab, that is very clear to me now. Great input Riz

Thanks once more and again

IJR

 

[emmgjld]

IJR

I agree with the above discussions.
The 45 degree from the top of the pad is normally considered appropriate for most soils, assuming a 'moderate' amount of backfill compaction. Using more can be risky if compaction is poor or is the soils are fine grained and/or saturated.

A point for consideration is that if more 'hold down' mass is required, the use of 'Helical Piers' should be considered. Remember, A.B.Chance first used them as tension anchors for overhead lines.

 

[IJR]

Emmgjld

Thanks and respects

Next time you visit, could you please give me a reference which mentions 45deg cone?. I usually use 30 deg which I picked from EIA-222-f; Electronic Industries Association (Guidelines for design of Communication Towers). I have been able to convince many authorities with that. Now I need to be able to do the same with 45deg

As for Helical Anchors, thanks for mentioning again, makes me feel more and more comfortable about them cause Ron and Ginger also mentioned their satisfaction with them

Regards
IjR

 

[Gourile]

JIR... I agree with your 30 degree. The 45 degree is applied when dealing with punching in concrete slabs. For the soil, the situation is always tricky. First of all, you better not count on its backfilling advantages if there is a possibility of excavation in future. For industries, there is allways such a possibility to reach underground utilities. The degree of backfill cone you use, depends on the type of excavation for manufacturing the foundation. If you excavate in a compacted soil with nearly vertical walls, the back fill will act with the wall angle. If you excavate a large area and bury foundations after construction, then you may keep your 30 degree criterion.
A tension column most of the times appears either when earthquake forces affects the structure, or as a reverse action of an adjacent long span with heavy loads (e.g. consider three columns which are connected by a beam with rigid connection with two dramatically different span lenfgthd and weights) . For the first case, you will be under the trouble with your overturning checks most of the times. If second situation is made a tension column, it would be a good idea to see if a vertical roller type of beam-column connection can be made, so that it gives the beam to move slightly upwards, but not downwards. Obviously you have to check for the frequency of such upwards movements and its acceptability

 

[IJR]

Thanks Gourile, that bit took your time but I promise you I went through every word with tons of discipline and respects.

Thanks for opening my eyes even wider on the subject.

Respects

IJR