Transfer Columns and Slabs

[Galambos]

I was reading about staggered trusses in AISC's Steelwise article (see attached), and came across a paragraph regarding column transfers. The author mentions that 3 to 4ft thick transfer slabs are often required.

I have never heard of this and wondered if these thicknesses were to transfer lateral shear into a vertically discontinuous lateral system. I also pictured the columns bearing on a transfer truss, while the lateral forces were resolved with a horizontal transfer truss. is this not the case, typically?

for those in the know, i would appreciate your comments.
Details or articles would be really helpful!

 

[ishvaaag]

Everything needs meets their structural requirements. I think AISC emphasizes the cases where thick slabs are used against the advantages of the proportionally slender-member staggered trusses. I of myself in our mid-rise works, mainly RC buildings didn't built maybe a single column with load to be transferred. I have had, this notwithstanding, the experience of working 4 years with a team less stringent in direct path to the foundation and in these 4 years transfer beams for columns for midrise were like mushrooms in autumn. In the entire practice, most already built, never a thing like a 3 ft thick slab was required, far less 4. Slabs were as usual and transfer beams were used ranging from really small protrusions from the slab to maybe 4 ft structurally deep beams. These were not buildings with the easance to use whole levels for transfer so it seems was an adequate option.

There's also the issue of way of construction to find the maximum load for the "transfer" beam. For if you allow the whole upper frame to be built upon temporary columns, you may then make such upper frame later as a full height vierendeel-like framing (or braced, if allowed), then of course minimizing the required sizes of the beams at the "transfer" level. But this is usually not chosen because brings the shadow of the doubt on how at the moment of putting the whole frame in its definitive way of behaviour what in place will be affected (not that it can't be analyzed).

Respect lateral transfer systems were not critical at the midrise (here, say just 10 to 12 stories above ground) buildings this practice refer. But if significant it is clear they will have to sort the discontinuity either piercing it or by transfer with rigidities of the same order provided in whatever the way.

 

[Teguci]

AISC has been trying to sell steel parking garages in the Northeast for some time with little success. I've seen a diaphragm thickened to 8" in order to handle the additional lateral load transfer from lateral bracing discontinuities. The 3 to 4' slab would be replaced with the 8" thickened slab and up to 9' deep transfer girders.

As for the parking layout, angled parking would cut the 60' span down to approx 55' or less.

Steel requires fire proofing and, unless well detailed and protected, will exhibit unseemly corrosion in a parking deck.

 

[hokie66]

The use of 3 to 4 ft or more thick transfer slabs is common in Australia for the purposes listed in the article. The columns in the carparking levels below have little relationship with the loadbearing walls or columns above. Most of these transfer levels are post-tensioned, with the stressing staged as the construction above progresses.