DWG for isolated footing

[king12]

DWG for isolated footing

info:
This is drawing for isolated flat foooting.

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[enriquevv]

connection beam is not attached to the footing?????

[Dell_Brett]

Dear enriquevv,

Not necessary! If the foundation beams restrain all column moments in both directions (X and Y), so, all moments will be absorbed by these beams and can be considered null the moments in the footing.

[chigozie]

enriquevv writes:-
..connection beam is not attached to the footing????? ..
Comment:-

I think that the connection beam i.e. the floor or tie beam should run between the footings at the foundation level (and not at the level that it appeared in the drawing) as was pointed out by enriquevv because:-
· The connection beams, apart from serving as the supports for eventual walls that might run on top of them , serve mainly to redistribute loads between the footings. In so doing, they also serve as controls against differential settlement.
· In an event of an earthquake, the structure responds differently from the sub grade (soil that supports it). This is due to the fact that the structure has a different period from that of the surrounding earth- thus vibrates at different frequency from the supporting soil matrix. This results from the inertia of the structure (which is different from that of the soil), the damping coefficient of the structure (which is different from that of the soil etc.). As a result, there is a relative movement of the soil with respect to the structure, which due to her larger inertia than that of the earth (loose material) is more reluctant to move than the earth. This relative movement of the soil discharges a large force on the substructure (the footing and the submerged part of the column). This force in combination with that generated from the earthquake that is transmitted to the foundation in the form of wave, impact stresses on these parts of the structure. Due to variations in the soil matrix (none homogeneity), none uniformity in the respective footings (none homogeneity, error/variations in craftsmanship), spatial distribution of the footings, each footing will respond differently with respect to the other. This phenomenon has the net effect that it amplifies the destructive effect of the externally imposed load (the seismic wave and the impact on the substructure due to the relative soil movement. etc). So it becomes apparent that these forces generated in the structure should be contained as not to attain the destructive level. To do this, all the facilities available have to be mobilized. This includes the placement of the connection beams that redistribute the loads (as mentioned above). The connection beams also force the individual footings to act as a unit, thus avoiding the differential movement of the footings that could lead to the disintegration of the substructure-thus the superstructure as such the whole.
· If the connection beams is constructed as it appeared in the design, it will act as a fulcrum to the lateral force imposed to the footing at the foundation level- thus lead to the rupture of the column at the column/beam connection level as such lead to structural collapse. In this case, the beam/column footing arrangement is like that of a cantilever loaded at the extreme end which is one of the most dangerous loading that could be encountered (for the fact that the moment generation is maximized). In this respect (as to shade more light on this discussion) , let’s make a rough estimate.

Assuming that the foundation is 1.5m deep and the connection beam is locate at the ground level i.e. the distance between the footing and the connection beam is 1.5m. If we employ 300mm x300 mm concrete column and if grade 30 concrete is used for the footing/column and the connection beam. Let typical column be reinforced with 3/12mm diameter high yield steel (characteristic strength = 460 N/mm^2)on each face. Then resistance offered by the column in bending (if we neglect all the other loadings coming on the column) = 0.87 x 460 x 3 x 113 (since on the attack face of the column or the side subjected to tension, it is assumed that the concrete has cracked as such of null effect) =F = 135667.8N = 135.6KN. If we assume a cover/link space of 50mm on all the faces of the column, then center to center distance between the opposite steel array (distance between the steel in tension and that in compression)= 300 - 2(50 + 12 ) = 176mm. This implies that the moment of resistance of the section = 176 x 135667.8 = 23877532.8Nmm. Therefore a force P applied at the footing level will rapture the column if it could mobilize a moment that is greater than 23877532.8Nmm. If we should go further and analyze the column at the failure point, then P x Z = P x 1500 (Z = distance between footing and the connection beam, which I would like to refer to as the lever arm) > 23877532.8Nmm or P > 15918.3552N (15.92KN) will cause the column to rupture at the point of beam attachment to the column. But if the beam is attached to the footing (i.e. at the foundation level), there will exist no fulcrum (the lever arm Z = 0) as such no moment will be developed for which the structure will be safe as far as the fulcrum effect is concerned.
Regards
Teddy

[Dell_Brett]

Dear Chigozie, Dear friends,

Congratulations for your answer!
This question and answer open an important point: -The difference between seismic resistant and the design in non seismic area. All that you post is perfect and technically correct for seismic zones.
For designers in non seismic zones, this kind of care isn,t necessary!
This is the importance that all questions must be complete (All information to design) and, the most important, all colleagues that read the answers must have a criteria to analyze if the answers is specific for determinate situation or a generic issue.

Thanks again,

with regards

Dell Brett
(Greetings from Brazil)

[seb3k]

Thanx for a very informative post.
My question is - how would you model this beam in a software?
Would you use it as a linnear support(a narrow strip foundation) and assume it carries some gravity load to the ground
or you would only use isolated footing as a support and model the beam as just a connection between them.
Im guessing if there are walls on top of the beam it would be a strip footing, but what if there are no load bearing walls eg. steel industrial hall
Regards,
Ivan.

[chigozie]

seb3k wrote:-
...My question is - how would you model this beam in a software?..

Comment:-
If you are using a software such as Sap 2000, you do not have to do any other thing than to model your structure and input the right loadings; then the software will take over from there.
Dell wrote:-
....This question and answer open an important point: -The difference between seismic resistant and the design in non seismic area. All that you post is perfect and technically correct for seismic zones.
For designers in non seismic zones, this kind of care isn,t necessary! ...
Comment:-
Yes. Like I mentioned in the post, this situation is only necessary when you expect a condition in which the footing is to witness transverse or lateral loading. This will only occur if the structure is located in an earthquake zone. If not (like in my country-Nigeria), it is not necessary as pointed out by my Dear DELL
Regards
Teddy

[jonamcivil]

No Isolation joint required between column and slab on grade??