1946 Concrete Construction 1

[PEnSE]

For those of you with experience in dealing with older concrete construction, I would appreciate any input/advice you might have.

I have a concrete structure that consists of a one-way 4" thick elevated slab with adjacent concrete joists about 6'-8" o.c. and concrete girders @ 20' o.c. The bay spacing/columns are 20' o.c. in both directions. The girders are in a 2-span condition (40' total length).

My client would like to make large penetrations in the 4" slab to accommodate new ductwork...the duct sizes vary with a maximum of 58"X20". The long dimension of the cuts will be parallel to the slab span (perpendicular to the joists).

I have the original 1946 plans, but they do not indicate: design load ,f'c or fy

As it's original intent was light equipment loading, I've assumed:
LL = 125 psf
DL slab = 50 psf
DL misc = 35 psf
f'c = 2000 psi
fy= 33 ksi

I have 2 questions/concern:
1. Reinforcing development length - Was the reinforcing used in 1946 deformed and development lengths similar to today? I will be cutting multiple slab bays and will lose my neg moment at the top of the slab. I have looked at the reinforcing which appears to be #3 @ 5 1/2" o.c. in the slab and using wl^2/8 for the bottom flexural reinforcement - the existing reinforcing appears to be adequate. My concern is development length.

2. T-Beam and Existing Shear Capacity: - My client must make these cuts adjacent to the main concrete girders. If the girders were designed as T-Beams I will lose 1/2 of my compression flange. Using a 2 span condition the maximum neg. moment is over the support at wl^2/8. The existing reinforcing over the supports appears inadequate for this loading. If I consider only pos reinforcing and wl^2/8 at the bottom I came up with a capacity of about 47 psf LL

The existing beams appear to be grossly under-designed for shear. I am not sure if they would have considered the concrete slab as contributing to the shear strength of the T-beam???

I will try to upload pictures

Thank you in advance for any help!

 

[PEnSE]

Slab Drawing

 

[PEnSE]

Joist Drawing

 

[PEnSE]

Girder Drawing

 

[PEnSE]

Beam Schedule

 

[JAE]

I have the original 1946 plans, but they do not indicate: design load ,f'c or fy

I would assume for 1946 an f’c of 3000 psi unless you take cores to justify a higher value.
f’c doesn’t affect flexural design all that much but does affect your shear capacity. So if you have shear issues then perhaps core samples might be validated.

LL = 125 psf
DL slab = 50 psf
DL misc = 35 psf
f'c = 2000 psi
fy= 33 ksi

I wouldn’t assume a live load. Instead I would back calculate the actual/original live load capacity based on various analyses. f’c of 2000 seems too low.
fy = 33 ksi may be correct. There is a CRSI table (I’ll see if I can post it) that outlines historical values.

Was the reinforcing used in 1946 deformed and development lengths similar to today?

I think that development lengths have undergone significant changes over the years. What I’ve found in old structures like this is that typically the top, negative moment flexural bars were rarely extended past the columns far enough based on current code provisions. If the bars are deformed they would generally exhibit the same development lengths calculated today based on ACI 318. For older bars which were smooth or simple square-twisted bars, the values may have to be adjusted.

The existing beams appear to be grossly under-designed for shear. I am not sure if they would have considered the concrete slab as contributing to the shear strength of the T-beam???

Shear has always been a problem for older concrete structures that I’ve checked. You can count on the “crank bars” diagonal legs to help the shear but only in the areas where the diagonal occurs.

They would NOT have counted on the slab to add to shear capacity. You can perhaps count on any variable width of the beams (sloping forms on the beam sides) to add somewhat to the shear area.

Sometimes with holes making systems into Swiss-Cheese floors you have to bite the bullet and add supplemental steel framing to allow for the holes.


Check out Eng-Tips Forum's Policies here:
faq731-376

 

[JAE]

See Table 4 of this CRSI document:


Check out Eng-Tips Forum's Policies here:
faq731-376

 

[JAE]

Or perhaps this one is more appropriate:



Check out Eng-Tips Forum's Policies here:
faq731-376

 

[msquared48]

I agree with the 3000 and 33 ksi for the time. I also agree with back calculating the live load to determine the live load capacity. Be very careful of the beam rebar drawings. Nomenclature was not consistent at that time.

Mike McCann, PE, SE (WA)

 

[bridgebuster]

I agree with JAE about shear. We have the same problem with older bridges - pre-1970's. The shear values used then were about twice what we use today.

 

[SlideRuleEra]

Contractors sometimes referred to the rebar that is bent upward and over for negative moment reinforcement as "galloping steel". They did not like it, since it could be difficult to guarantee that it would remain in place during concrete placement. If you decide that the design is adequate and proceed with the modifications, try to verify that as-built construction is in accordance with the plans.

IMHO, I would not trust the dated design... too many known problems. Using supplemental steel as JAE recommended would be best.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[PEnSE]

JAE...thank you so much for your thorough post and thank you for the attached files.
Msquared48,bridgebuster and SlideRuleEra thank you guys as well for your response. I will definitely look in to supplemental steel. I have not looked at the structure yet, but plan to go look at it next week.

Thank you!!!

 

[Ron]

Sometimes with holes making systems into Swiss-Cheese floors you have to bite the bullet and add supplemental steel framing to allow for the holes.

My thought as well.