Overturning Check for Industrial Shed Footing

[MSUK90]

Dear all,
I have to clarify the load combination to check overturning affects for an industrial shed footing. The reaction for different loads provided by steel designer are attached in the picture. I just need to confirm that when I use a 0.6D+W combination, do I apply "0.6" factor to horizontal loads and moments also? Also, is there a possibility for the steel designer to consider the supports to be hinged/pinned(to avoid moment) and design steel structure for lateral loads too(using some sort of LLRS)? The steel designer says that he doesn't have any other option but to use 'Fixed Support' in his calculations(especially for pipe section columns supporting a pipe truss). I am getting footings of very big sizes which the client obviously doesn't like. Sorry if something here is very general as I don't have much experience with steel design.

 

[phamENG]

Yes, you apply the factors from the load combination to all load effects.

As for the base fixity, that's not uncommon in the cheapest of pre-engineered metal buildings. By putting the challenging work of providing stability to the building off on the foundation designer, the building manufacturer can promise unbelievably cheap superstructures. The massive footings are the norm in this situation, though the contractor and owner are likely to rip your head off for them.

 

[1503-44]

It is unlikely that horizontal loads are dead loads,but if they are, as it appears to be for the entire list of loads in line 1,you would then apply 0.6 to all of those and all other dead loads you may have from other sources. Do not apply 0.6 to any wind loads.

I think the steel designer has options that HE does not want to consider, as pinned bases will shift frame moments toward the column-truss joint, necessary for a stable structure if pinned based are used. I imagine he doesn't want to have any moments there, because it increases cost of that connection and probably the member sizes of the truss.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[haynewp]

Horizontal reactions from dead load would be from thrust when using a fixed top of column connection.

 

[DaveAtkins]

Are you using hairpins into the floor slab, or continuous reinforcing steel in the floor slab across the building? That will take care of any horizontal thrust at the column base. This means the horizontal thrust won't add to any moment the footing sees. You still need to design the footing for the moment induced by the column.

DaveAtkins

 

[JoshPlumSE]

I apply the 0.6 factor to all dead load effects. With the caveat that any vertical earth pressures get the 0.6 factor, but the lateral effects of earth pressure do NOT get the 0.6 load factor.

 

[MSUK90]

Thanks everyone for the reply. So I conclude that I have to multiply all sorts of reactions due to dead load affects by a factor of 0.6.

 

[MSUK90]

The massive footings are the norm in this situation, though the contractor and owner are likely to rip your head off for them.

Yes, the size of footings are pretty big and they are really doing the thing you said!

 

[MSUK90]

Are you using hairpins into the floor slab, or continuous reinforcing steel in the floor slab across the building?

No hairpins used as it isn't a norm here. Didn't get your second point with respect to the horizontal reaction though.

 

[DaveAtkins]

Well, if your footing is located some distance below the bottom of the base plate (for example, 4 feet), the horizontal reaction will cause a moment due to its location above the footing (in our example, the moment would be the horizontal reaction times 4 feet). This moment would need to be combined with the moment induced at the base plate by the frame.

DaveAtkins

 

[MSUK90]

Well, if your footing is located some distance below the bottom of the base plate (for example, 4 feet), the horizontal reaction will cause a moment due to its location above the footing (in our example, the moment would be the horizontal reaction times 4 feet). This moment would need to be combined with the moment induced at the base plate by the frame.

Yeah, that has already been considered in designing the foundation. But this moment with the moment from the superstructure also require top reinforcing mat in footings as per my calculations. Is that a norm or am I over-killing the design?

 

[1503-44]

It is usually more convenient to design a plinth /footing to support columns independently of the slab, but it sounds like you need to do something entirely different. If you could post a detail of your design it would help us understand your particular situation.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"