Tie beams too long between foundation pads, any good?
Tie beams too long between foundation pads, any good?
(OP)
This one bugs me.
Seismic codes force us to connect column footings by tie beams (you guys may have other names) using some % of footing load as strut force for the tie below grade.
NOW
In metal structures, you have closely spaced footings in the longitudinal direction and very large spacing in the other direction since you are crossing the span with a frame with no intermediat column.
How effective will a tie beam between footings at the other ends of the span be, given all that distance say 70ft?
Regards
IJR
Seismic codes force us to connect column footings by tie beams (you guys may have other names) using some % of footing load as strut force for the tie below grade.
NOW
In metal structures, you have closely spaced footings in the longitudinal direction and very large spacing in the other direction since you are crossing the span with a frame with no intermediat column.
How effective will a tie beam between footings at the other ends of the span be, given all that distance say 70ft?
Regards
IJR





RE: Tie beams too long between foundation pads, any good?
If the foundation depth is not much, you should think of connecting the pedestal top and ground slab with the 'U' pin. That will also resist the lateral force transferred to footings at the base plate level.
RE: Tie beams too long between foundation pads, any good?
Usually except for very large span industrial buildings or warehouses we can handle the lateral force even without utilizing slabs.
So the problem is not statical, but code based. We are told in my area to connect footings to one another by tie beams treating them as struts capable of axial force 30% of the axial force in a pedestal
No commentary to clarify this one and we usually have to argue in circles.
Respectfully
ijr
RE: Tie beams too long between foundation pads, any good?
RE: Tie beams too long between foundation pads, any good?
RE: Tie beams too long between foundation pads, any good?
But my question also remains in that the beams are usually too long and require a large area excavation, because spacing between them is usually about 18ft while their length will be about 60-80ft in a typical warehouse steel frame construction. Code says do it without comments and sometimes you see this was probably meant for reinforced concrete buildings, and clients say "dont put me into unnecessary excavation-your 1ft*1ft beam what good is it huh! ". You know the feeling when you can not defend your design by any rationale
Respects
IJR
RE: Tie beams too long between foundation pads, any good?
In my area, the UBC 97 (for example) seems to only require lateral interconnection for piles and caissons for any building subject to any seimic forces. (see UBC Section 1807.2) For footings, I don't see any direct reference to interconnections so if you have spread footings, you needn't concern yourself.
Now with the piles, I believe the concern is that in a seismic event, much of the ground around the piles will become quite unstable, eliminating some or all lateral support on the piles. Thus, the piles need some measure of lateral restraint to avoid a sidesway dislocation of the pile or pile group. This would be disasterous for a building.
For your "long" distances across your building, the ties aren't intended to provide a tight restraint against lateral deflection but only a tie providing enough strength to avoid a pile/column buckling. The "stretch" of the tie may allow much larger deflections in the seismic event than you would normally want to restrict under other loads.
RE: Tie beams too long between foundation pads, any good?
We seem to agree that tying footings to one another is not an established strict requirement.
Respects
IJR