Rotational Fixity of a Pad Footing
Rotational Fixity of a Pad Footing
(OP)
Hi guys, there has been a couple threads lately about semi-rigid design of columns, thread744-260012: Base Stiffness and AS4100 and thread507-261922: Fixed base on tube steel colum, and now I want to put the theory into practice. I have a steel-frame building which relies on frame action to brace the structure in one direction. I want to reduce the drifts for service conditions and reduce the column moments for ultimate conditions, so I am designing a semi-rigid base. Please note that I only intend on resisting a minor amount of moment using a rotational spring stiffness at the base of 0.1*4*E*Ic/Lc.
Because this is a steel-framed structure, there is not a great amount of dead load compression force on the column and thus uplift does result. What would be the most ethical approach to take out the base moment (referring to the attached sketches). Would it be best to assume an unbalanced slab-on-ground load or would it be assuming some passive soil pressure is developed on the pad footing to resist applied moment. My thoughts are that I should look at designing the slab-on-ground to span onto the pad to provide additional hold-down resistance. This will mean designing the slab on ground with bottom reinforcement locally around the column.
All help will be appreciated.
Because this is a steel-framed structure, there is not a great amount of dead load compression force on the column and thus uplift does result. What would be the most ethical approach to take out the base moment (referring to the attached sketches). Would it be best to assume an unbalanced slab-on-ground load or would it be assuming some passive soil pressure is developed on the pad footing to resist applied moment. My thoughts are that I should look at designing the slab-on-ground to span onto the pad to provide additional hold-down resistance. This will mean designing the slab on ground with bottom reinforcement locally around the column.
All help will be appreciated.






RE: Rotational Fixity of a Pad Footing
RE: Rotational Fixity of a Pad Footing
Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it
RE: Rotational Fixity of a Pad Footing
I consider the point of fixity at the top of the slab and check for localized crushing of the concrete at that point. The footing is designed to take the entire moment of the column (and lateral load), with the slab providing uplift resistance only.
RE: Rotational Fixity of a Pad Footing
I have, with light weight structures, with high overturning loads, put grade beams between the footings and included them in the calculation/model. the foundations would be smaller in plan but a little deeper than yours. This gains in Pp to counter the horizontal loads and gives an almost uniform soil pressure under the vertical loads.
It is all a balancing act between the weights and forces and soil pressures.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Rotational Fixity of a Pad Footing
BA
RE: Rotational Fixity of a Pad Footing
For another example when I do have bearing under the footing (compression load). Can a varying pressure distribution be assumed to balance the applied moment. For example, say I have a 150kN (34kips) compression load and applied moment of 20kN-m (15kip-ft) applied to a footing 2100 x 2100 (7' x 7'). To balance these applied loads the bearing pressure distribution varies from 47kPa to 21kPa (1kips/ft^2 to 0.45kips/ft^2). Would it be acceptable to say that this resists to moment because I would believe that the foundation settlement would vary across the footing with varying pressure distribution and this differential settlement would be a source of rotation which would alleviate the moment attracted by the foundation.
Any opinions?
RE: Rotational Fixity of a Pad Footing
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Rotational Fixity of a Pad Footing
DaveAtkins
RE: Rotational Fixity of a Pad Footing
RE: Rotational Fixity of a Pad Footing
0.6D + W
I use a rectangular footing to resist the uniaxial moment/rotation. I do not rely on active/passive pressure on the sides of the footing. I use a trianugular bearing pressure on the bottom of the footing.
For uplift, I would suggest a) larger/heavier footing or b) embedding the footing deeper and using (overburden + footing weight) to help resist uplift.
RE: Rotational Fixity of a Pad Footing
For uplift, I'd say lower the footing and add a pier. Trapezoidal soil pressures are pretty common.
RE: Rotational Fixity of a Pad Footing
BA
RE: Rotational Fixity of a Pad Footing
IS it necessary to model this type of footing?
RE: Rotational Fixity of a Pad Footing
BA
RE: Rotational Fixity of a Pad Footing
RE: Rotational Fixity of a Pad Footing
A source of guidance on default soil moduli:
http:
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Rotational Fixity of a Pad Footing
The units of rotational spring stiffness are Moment per unit rotation or M/Θ. The Modulus of Subgrade Reaction, 'k' should be available from the geotech. It is usually expressed as pressure per unit of settlement MPa/mm or psi/inch.
If the footing undergoes a rotation of Θ one edge drops Θb/2 where b is the width of a square footing. The opposite edge rises by the same amount. This corresponds to a stress of +/-kΘb/2 which can be converted to a moment for a particular footing size.
The rotational spring stiffness will be a function of both k and b.
BA
RE: Rotational Fixity of a Pad Footing
RE: Rotational Fixity of a Pad Footing
Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it
RE: Rotational Fixity of a Pad Footing
RE: Rotational Fixity of a Pad Footing
Michael.
Timing has a lot to do with the outcome of a rain dance.