Excess Capacity in Historic American Reinforced Concrete Floors

== Abstract ==

The  introduction  of  reinforced  concrete  as  a  structural  material  in  American 
practice circa 1900 created unique challenges in design and analysis. Engineers and builders 
had  to  simultaneously  determine  proper  constituent  materials  for  the  concrete,  the  relation 
between materials proportions and strength, and formulas for sharing load between steel and 
concrete.  Early  on  there  were  only  local  building  codes,  which  often  treated  concrete  as 
masonry. This was followed, starting in 1909, by the reports of the “Joint Committee” which 
served  as  a  de  facto  national  code  for  concrete;  the  American  Concrete  Institute  code  was 
first independently published in 1941 and has been updated regularly since. 
All of the early codes, specifically including the Joint Committee report and the ACI code, 
used  allowable-stress  design  with  a  linear-elastic  model  of  flexure.  Load-and-resistance 
factor design using an ultimate-strength model was first introduced in the US as an alternate 
method in the 1956 ACI code, was elevated to equal status with the 1963 code, and was made 
the standard in the 1971 code with the linear-elastic allowable-stress model as an alternate. 
Since  the  ultimate-strength  model  is  generally  accepted  to  more  accurately  represent  flexure 
in reinforced concrete, the allowable-stress alternate was little used after the transition. 
Many  details  have  changed  minimally  since  the  1910s,  including  provisions  for  T  beams 
that  take  advantage  of  the  slab  for  additional  compression  flange  area.  The  calculation  of 
shear  capacity  in  both  concrete  and  web  reinforcing  changed  minimally  except  for  the 
transition from allowable-stress to load-and-resistance-factor design. 
Re-analysis of extant concrete structures designed before the 1956 code generally shows a 
pattern  of  significant  excess  capacity  in  flexure  and  limited  excess  capacity  in  shear.  For 
members that are generally not controlled by one-way (beam) shear (two-way slabs and one-
way  slabs)  the  increase  in  flexure  capacity  governs;  for  members  where  one-way  shear  is  a 
controlling factor, the small increase, no change, or decrease in shear capacity governs.  
Much  of  the  current  literature  on  evaluating  the  capacity  of  extant  concrete  structures 
focusses  on  strength  degradation  from  material  changes  such  as  rusting  rebar  and  from 
overall material changes such as carbonation. The presence of excess capacity should also be 
considered, and it is possible based on review of the governing codes and design methods to 
determine general patterns as to how much excess capacity may be expected.

== Full document ==
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