Expanding an existing deck

[Star.1]

There is an existing deck with width 5', and the deck needs to be extended on one side by 2 ft. The old deck has its cassion designed for 40 psf live loading. However, as per new criteria, deck is to be designed for 60 psf live load. For 60 psf live load, the existing cassion size on the side A is obtained to be inadequate by calculation. What is done in such case? The existing cassion on the side A has no change in tributary area, but the loading has increased. Does city asks for new cassion size for Side A cassion too? Attached is the sketch showing the case.

 

[jayrod12]

Often we'll see if we can get a geotechnical engineer to give us more refined values for the caisson capacities. Sometimes that works, sometimes it doesn't.

 

[KootK]

In general, I don't feel that it's reasonable to insist upon a foundation upgrade in a situation like this. Unless the AJH discourages me from doing so, I'll explain to the owner that there is some risk of settlement with the existing foundations left as they are but that is the only real risk of "failure" for such a situation, most of the time. Then I'll leave it to the owner to weigh the risks and decide if they want to upgrade the foundation or not.

The genesis of that 50% increase in deck live load has to do with the demonstrated weak spots in most residential deck systems:

1) Overall stability.
2) The framing.
3) The framing connections.

It had nothing to do with foundation issues.