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
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
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
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
Data and Calculations
Fy (steel) = 33 ksi based on CRSI data on steel produced
since 1911. Actual tests will be performed when samples are
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
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 =
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
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.
Evaluation Based on Satisfactory Past
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,
- 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.
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
- 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.
H. Daryl Richardson