emporary support of patio

huangyhg

temporary support of patio
i would like to enclose my rear patio which also supports a second story walk out deck with roof.  it is a timber framed structure with 8x8 timber columns supporting 4x8 beams.  the whole thing is supported by a concrete patio slab.  i need to pour a thicker slab to keep drainage from my back yard from ponding up on the patio.  my dilemma is how to support the second story so that i can cut off the bottom of the existing columns, form and then pour the concrete.  i was thinking of using temporary steel pipe columns, the concrete would be poured around the pipes and after it cured, they could be cut off flush and grouted and support returned to the shortened wood columns.  does this sound reasonable?  
the cut pipe will rust if not adequate covered.  other options:
1. place new pads inboard of existing, replace posts, replace slab
2. build up a new temporary beam extending past the slab on both sides, temporarily support beam, replace slab, replace posts
3. leave post in place, place concrete around post
4. thicken slab in sections, placing temp posts on new section while posts are moved
i like options 2 and 4.  don't think 1 will work.  wouldn't want 3 which embeds the bottom of the post in the concrete - sounds like a recipe for rot.  see photos linked below
install angled posts adjacent to your exiting columns just outside the slab foot print. brace them to the adjacent structure and to each other. analyze the posts for axial loads. check axial plus bending if you want to go crazy, but for temporary shoring, a foot of eccentricity in 10 feet is a rule of thumb.
of course, you could replace the columns one at a time, cutting them to height and placing them on the precast deck pedastals commonly sold at home centers and later pour the thickened slab over your existing slab. how thick are you going and does the thickened slab need to be structural?
daniel
only going about 3 - 4 inches higher, mainly to keep runoff in backyard from ponding.  as you can see from the photos, the existing slab is right at grade.  even though i installed a drainage system, i still occasionally get water ponding on the existing slab.  existing slab is handling the load fine and the only increase in the load will be non-bearing walls with windows and doors to enclose the patio.  i don't plan on any additional reinforcing for the concrete.
chuck is the posts pt?
not sure if they are pt or not.  they have been there for nearly 25 years so far and appears that 1 or 2 have shown some signs of rot - but may have been repaired with epoxy injection.  another reason to cut off the ends! i have owned the property for just 3 years.  however, they appear to be in fairly good shape as i have taken a hammer and screwdriver to perform a highly scientific standard penetration test at the bottom of each timber and they seem quite sound still.  our climate is quite dry, so rain, when we get it is more of a nuisance than anything.  i believe our annual average rainfall is about 8 inches.
i think you can use dtgs idea and use the angled support from the outside.  
if you are framing in the structure just remove the posts and frame with 2x4s.
the issue here is the temporary support. unless you plan to start and never finish, i'd angle timbers from outside the foot print. if you want to be snazzy, order some ellis screw boots from ellis-ok.com or a local supplier. to be simple, jack up the support with a jack adjacent to the column, install the angles prop on the outside and remove in the column.
will you need assistance to calculate the capacity of the 4x4 or other member to prop it with? if so, you may want to look to rental shoring with known values, or go to the ellis shore page and use their published values. i would not think the 8x8 post is seeing a load which requires it to be 8x8, but having not see the situation, i do not know. use your good judgement.
the least risk probably involves removing the columns one at a time and pouring in the area or putting them on blocks. the way i would go would be with the angles supports. before i even considered growing up to be an engineer, my dad supported our 100 year old porch with a series of braced 4x4 frames so he could remove and repair the built up columns supporting the porch roof/second story.
anyway, use good judgement, and call someone local if you feel your next move will get you in over your head.
daniel