Strenghtening Existing Spread Footing 1

[Eng_Struct]

Hi Group,

I have a 2.7m x 2.7m wide-spread footing with a 0.6x0.6 pier. The client is adding a rooftop unit that results in bearing pressure greater than the allowable based on the record drawing.

In the past, I have underpinned the existing footing by pouring a larger pad below the existing footing to distribute the load over a larger area to reduce the applied bearing.

Just wanted to ask here if there are other less involved solutions that can be implemented.

Regards,

 

[Enable]

How much are we failing by if we use the existing? Geotechnical engineering is not that exact and if we're talking a minimal increase then I might lean towards do nothing. However, even in the do nothing case I'd probably request a site visit by a geotech to confirm capacity and see if they were willing to provide an increase to the amount that you need for the new RTU. In my experience for small stuff like this if you say you want 200kPa, and soil is say reasonable for that load, they'll say yup 200kPa max capacity...but they'd also do the same if you asked for 150kPa or 250kPa.

 

[HTURKAK]

Do nothing would be definitely less involved solution . It will worth to dig to learn the type of soil, settlement sensivity of the structure and the FS applied. Probably the FS applied is 3.0 which is typical and if the SF reduces to 2.5 ,probably nothing will happen.

I have copy and pasted relevant paragraphs of BOWLES ;
........
The recommendation for the allowable bearing capacity qa to be used for design is based
on the minimum of either
1. Limiting the settlement to a tolerable amount (see Chap. 5)
2, The ultimate bearing capacity, which considers soil strength, as computed in the following
sections
The allowable bearing capacity based on shear control qa is obtained by reducing (or dividing)
the ultimate bearing capacity qult (based on soil strength) by a safety factor SF that is deemed
adequate to avoid a base shear failure to obtain

qa = qult/SF

The safety factor is based on the type of soil (cohesive or cohesionless), reliability of the soil
parameters, structural information (importance, use, etc.), and consultant caution. Possible
safety factors are outlined in Sec. 4-15

..and relevant table suggests SF = betw .2-3 .



He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[Roukkia]

What I have done before is increase the plan size of the footing by pouring concrete around the footing on 2 or 4 sides, and doweling bottom footing bars into the existing footing that then get 90deg hooked to the rebar in the new portion of the footing.

 

[DaveAtkins]

The problem with increasing the footing size in plan without thickening it is the original footing may fail in bending or shear. Also, you need to make sure the new dowels lap with the bars in the existing footing by a sufficient amount so that the new portion of the footing will not "pull off" of the rebars in the original footing.

DaveAtkins

 

[Eng_Struct]

DaveAtkins/Roukkia,

Lapping the new dowels will require about a 500mm hole into the existing footing close to the existing rebar location on all 4 sides. I think there will be issues with bars at the corners hitting each other and also, concrete splitting.

I am thinking of providing dowels with 200mm embedment depth in the existing footing and taking the portion of the bearing pressure acting onto the extension through the shear interface between the existing and new footing considering the dowel contribution is limited to their pull/breakout strength. Once the load gets transferred to the existing footing, the existing rebar is sufficient to resist the overall bending. What are your thoughts on this?

 

[BAretired]

200" embedment depth? That's more than 5m.

 

[Eng_Struct]

BAretired, It was a typo. In my mind I was typing 8" and then changed the number to 200"mm".

 

[BAretired]

Okay, we all make typos.

If the existing reinforcement is still okay, it must have been more than required for the 2.7m square footing because, unless you change the pedestal size, the moment must be greater with the additional area.