Differential Settlement

[STR04]

I have a one-story bldg (masonry/steel framed) placed on engineer fill. The level bldg pad is placed on a sloped grade. The geotechnical consultant states that there will be 1.25" differential settlement, approximately 0.05" of settlement per 12" of bldg length. What is tolerable for differential settlement?

TIA

 

[jgailla]

I would think 1.25" in 25 feet would cause the masonry to crack over time. If cosmetic considerations are important, you probably need a different foundation approach.

Why is the differential settlement so high if you're using engineered fill?

 

[STR04]

Correction .005" in 12" thus for 250LF bldg length you have 1.25" differential settlement. Sorry.

 

[UcfSE]

Typically we see differential settlement limited to 0.75 inches and definitely not more than 1 inch.

 

[SperlingPE]

That sounds high. Be sure that you understand what the geotechnical consultant is telling you or that the geotechnical consultant understands what you wanr from them.

 

[BigH]

Traditionally, it is accepted that differential be 3/4 or the total settlement; hence, as per UcfSE, 0.75 on 1 inch is 3/4 of total. A few texts out there have some very detailed permissible differentials - see, for example: Bowles. However, I believe the best discussion I have seen is in Tomlinson's Foundation Design and Construction: 6th edition. He has 7 pages on discussion. He has a nice table on limiting values of distortion and deflection of structures. For "cracking in walls and partitions" of framed buildings and reinforced load bearing walls, he gives angular distortion ranging from 1/300 (Skempton and MacDonald who recommend 1/500); 1/500 (Meyerhof); 1/500 (0.7/1000 to 1/1000 for end bays) (Poshin and Tokar); 1/500 (Bjerrum). He clearly states that it is not reasonable to design foundations to prevent all cracking due to differential settlement. He gives a cracking severity table for plaster and brickwork or masonry walls.