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Heritage Building - Excessively Thin Roof Slab

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Fellow engineers,
 
        As many of you know, I am working on an important 1912-13 heritage building.  I have uncovered a situation with this building which I would like to discuss with some of you.
 
        This posting may get a bit long so those of you who are not interested in heritage buildings or in excessively thin concrete slabs may wish to skip to another posting.
 
        The floor and roof slabs in the building are deficient by modern codes in that they only have bottom steel reinforcement and that in one direction.  There is no temperature reinforcement and no top steel in the negative bending locations over the beams.  They are also thinner than generally accepted by modern codes.  All of the spans are a bit under 16' c/c supported on concrete encased steel beams that are about 1' wide (leaving clear spans of about 15').
 
        The floors, which are only 6" thick, have performed admirably over the past 93 years.  There are no visible cracks anywhere.  Even the terrazzo floors in the old corridors display no cracks over the beams; the remaining terrazzo is in perfect condition!
 
        The roof is a bit of a different story.  The roof is only 4.5" thick; the deflections are very visible; and there are clearly visible cracks at each beam (usually there are two parallel cracks 6" to 8" apart over the beams).  I have analyzed the slab using the following information and equations.
 
Data and Calculations
 
Fy (steel) = 33 ksi based on CRSI data on steel produced since 1911.  Actual tests will be performed when samples are available.
 
F'c = 3,000 psi. (actual cores plus a large number of Schmidt Hammer tests almost exclusively between 21 MPa and 23 MPa, which is in the order of 3,300 psi.  The actual spread was much smaller than I would have expected.
 
As = 3/4" smooth bars @ 8" c/c
 
M = w*L^2/8
 
Mu = 0.9*As*Fy*(d-a/2)
 
As*Fy = 0.85*F'c*a*b
 
Depth of neutral axis, kd, = a/0.85
 
delta = 5*w*L^4/(384*E*I)
 
I = cracked section transformed with N = Es/Ec = 10
 
Wu = 1.25*W(dead) + 1.5*W(live)
 
Dead load, existing 4.5" slab plus new insulation and roofing = 66.5 psf.
 
Allowable (working) live load based on strength considerations as expressed in the above equations = 51.7 psf.  The Calgary ground snow load is 1 kPa, or 20.7 psf. This seems to me to be an adequate reserve to allow for snow drifts, etc.
 
Dead load which has existed for the last several years = 86.5 psf, allowing 66.5 + 20 for about 2" of gravel ballast and a complete second roof, all of which is being removed.
 
Live load deflection based on 21 psf = 0.218"
 
Dead load deflection as calculated using 86.5 psf and the equations listed above = 0.90
 
Measured long term deflection 2.0".  This is about 2.22 times the short term elastic deflection indicated in the last paragraph.  This seems to not be unreasonable considering the effects of creep and shrinkage.  One flaw in the comparison is that it is impossible to say how much of the 2" is deflection and how much is a result of crooked forms.
 
Canadian Code Considerations
 
The Alberta Building Code in force is a derivation of the 1995 Canadian National Building Code.  Section 4 is virtually (or actually) identical, and the 1995 Structural Commentaries are specifically made mandatory.  The following two paragraphs are an excerpt from one of these commentaries.
 
Commentary K
 
Evaluation Based on Satisfactory Past Performance
 
    18.    Buildings or components designed and built to earlier codes than the benchmark codes or standards, or designed and build in accordance with good construction practice when no codes applied may be considered to have demonstrated satisfactory capacity to resist loads other than earthquake, provided:
  • careful examination by a professional engineer does not expose any evidence of significant damage, distress, or deterioration;
  • the structural system is reviewed, including examinations of critical details and checking them for load transfer;
  • the building has demonstrated satisfactory performance for 30 years or more;
  • there have been no changes within the past 30 years that could significantly increase the loads on the building or affect its durability, and no such changes are contemplated.
Serviceability
 
    44.    The serviceability criteria contained in Part 4 and referenced standards are intended for the design of new buildings.  For existing buildings, in many cases demonstration of satisfactory performance eliminates the need to apply the serviceability criteria given in Part 4 and referenced structural standards for structural evaluation.  Unacceptable deformation, settlement, vibration or local damage will usually be evident within a period of 10 to 30 years from construction.  Examples where serviceability evaluations may be required include change of use, or alteration of building components affecting the properties of the structure.
 
Discussion and Questions
 
        It's my opinion that:
  • 93 years of satisfactory service greatly exceeds the 30 years required in the structural commentary;
  • the required strength is provided;
  • the calculated live load deflection is small;
  • the measured long term dead load deflection is reasonably close to what would be predicted based on analysis;
  • the only really serious deficiency is the fact that the actual thickness, L/42, is very much less than the L/24 or L/28 that would normally be required of modern codes.
        I'm a firm believer that "If it ain't broke don't fix it."  I do not believe that a "fix" is required to correct the slab thickness deficiency.
 
        I would very much like to read your comments on this topic.
 
        Thank you in advance for anything you might wish to write.
 
Regards,
 
H. Daryl Richardson