Concentrated Load on Existing Composite Slab

[dcStrucEng]

I have come across many threads on this site regarding concentrated loads on composite slabs. But I have a slightly different twist on things on which I am hoping to get opinions. I have reviewed the concentrated load example in various deck catalogs as well as reviewed the information in SDI’s Standard for Composite Steel Floor Deck Slabs. For my particular case, much is known about the existing composite slab because I have existing drawings. The one thing that I am not confident about is the concrete cover dimension over the welded wire reinforcement.

When analyzing the slab for a known concentrated load over a known contact surface area, the slab has enough capacity for strong axis moment, one-way shear, and punching shear. The only limit state in which capacity is exceeded is weak axis moment. I am hesitant to proceed with a slab strengthening remediation to the existing slab solely for this lone limit state not meeting the required demand.

If I were to assume zero welded wire fabric in the slab and analyzing the weak axis direction as plain concrete also does not provide sufficient capacity. The equations and notes in the deck catalog example and SDI Standard do not state that the distribution of the concentrated load in the directions parallel (W) and perpendicular (be) to the flutes vary with or without WWF. Although I agree that distribution steel helps.

So even if the weak axis moment demand exceeds capacity, how will this really impact the slab? If this causes the slab to crack parallel with the flutes, the slab will act even more like a one-way slab. Anyway, I’ve spent significant time pondering this issue and would appreciate any opinions/advice from others. Thanks in advance!

 

[hokie66]

What capacity do you need in the direction perpendicular to the ribs? Generally, composite deck slabs are only considered to span in one direction. Is your problem that you need to engage several ribs, with your concentrated load needing to act as a line load? Welded wire reinforcement is only intended to provide nominal crack control, not for lateral load distribution. It is most commonly placed directly on the deck.

 

[dcStrucEng]

Hokie,
I agree that composite deck slabs are only considered in one direction. But following the concentrated load example in the deck catalog and the requirements of SDI (2017 version currently on their site), one of the design checks is for the weak axis moment of the slab (perpendicular to the ribs). This is the check that I am struggling with. I haven't been able to find a basis for this equation. Neither the catalog example nor the SDI Standard are going to state that this design check can be overlooked, but in reality, can it?

 

[MotorCity]

WWF in composite slabs is usually not provided as flexural reinforcement, it is for shrinkage steel. If you have not already discovered it, you probably will find that the WWF provided does not suffice as flexural reinforcement. I am not following what you mean by the weak direction, but I don't have that publication in front of me now.

 

[dik]

If the load is cyclical... not a good idea for composite topping slabs.

Dik

 

[dcStrucEng]

I hear you about cyclical loads. For the purposes of this thread, let's take dynamic/cyclic/moving loads off the table and focus just on a static, concentrated load bearing on top of the composite slab. For those not familiar with the deck catalog's example nor the requirements in SDI, once you become familiar with them, I hope my concerns will become apparent (both are free).

Concentrated Load example -

https://www.canam-construction.com/wp-content/uploads/2014/11/canam-example-calculations.pdf

SDI Composite Slab Standard -

http://www.sdi.org/publications-2/standards/,

(you enter your email address and they send you a link to download); section 2.4.B.11.b, page 15.