huangyhg

maximum number of bars in beam
does anyone have something handy to find out what will be the maximum number of bars in beam as per aci 318-05. in case. they do not fit in one layer than what are the if and buts for second or higher layer, stirrups requirements and detailing.
thanks.
aci spells out the clearance you need between bars based on the bar sizes and aggregate size (about 1" min clear). you'll also need to look at what rearranging your bars does to the effective depth. bar bundles may be a possibility.
how many bars in what size beam are you looking at?
crsi handbook, under beams.
jlnj
18" wide beam, 9 - #9 bars.
i have not used it for many years (since aci 318-89), but at one time aci had a design aid containing tabulated information about various bar sizes, quantities, single layer, bundled bars. i don't know if that design guide is available, but the aci website lists an sp-17 design guide that may have the same information.
from the copy of the crsi table c-2 i have on my bulletin board, you can put 6 #9's in an 18 inch wide
the number of rebars are controlled by two considerations:
1. the physical constraint due to beam width, clear concrete cover, bar size and bar spacing.
2. maximum reinforcing allowed to achieve ductile failure mode (<0.75 balanced reinforcing ratio. for practical concerns, 0.5 is usually used in lieu of 0.75.)
when more than one layer of bars are to be placed, the effective depth reduces, the strength gained by adding bars sometimes are not very significant compared to the loss in ductility, and a lot of headaches such as difficulty in rebar placing and concrete pouring/consolidation. also to be noted, the study on behavior of concrete beams with layered/boundled bars is very limited, if any. so, try to increase the beam size first if you can.
the use of compression steel could also be considered, it increases strength and ductility.
9 #9 - that's quite a lot. hopefully you have a rather deep beam or you will be up against the max. use the bar dia for clearance between bars (including different layers) play around with it and see what works. it shouldn't affect your stirrup layout or detailing mcuh; however, the more shallow the beam the less effective those upper bars will be.
you can indeed theoretically get 6 #9s across but laps or column crossings might pose additional challenges if all your clearances are set to the minimum. sometimes going to fewer bigger bars helps, sometimes it doesn't.
i have been guilty of specifying bars thinking that there was plenty of room and then getting to the field to see what a challenge the bar arrangement turns out to be.

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