Connecting a Roof System to a Concrete Slab

If there are transverse steel members as part of the slab support, could you extend them to the edges and make suitable moment joints to support the roof columns, which presumably would be integrated with the railing structure? (Just trying to kill a few birds with one stone.)

The short answer is "Yes". The width of the strip should be the width of the baseplate plus one or two times the thickness of the slab, some engineers say one, others say two.

Having said that and not knowing what instructions drive your design, I would not have a situation where the steel erector had to stop work to let the concrete contractor place and cure the slab, and then return later to complete the steel. I also have misgivings about the stability of the girders during construction. To get a look similar to yours, I would try for two girders with crossbeams at suitable intervals, preferably sitting on the top flange of the girders, cantilevering to pick up the roof columns. I would make the slab one way, spanning between the cross beams, preferably using metal deck forming. The cross beams would stabilize the girders and the metal deck would stabilize the crossbeams. The steel erector could complete his work before leaving the site and the concrete contractor could overlap without impeding.

excellent answer. a cross member at every load point is a must. a cantalever extending from the cross member though the girder to the load point is the norm. you can't ask a steel erection company to come back for someting as minor as this. they'll back charge the general contractor and laugh in your face to boot.

Thank you all for your helpful posts! I am glad I found this website as you all are very knowledgable about bridge design. Having just completed Steel Design and Concrete Design classes during the fall semester, I am still in the preliminary stage of the design; however, I believe I am going to move my steel girders to the edge of the slab and directly connect my roof system columns to the girders with a moment connection. Also, the structure is going to be enclosed due to the lack of room to construct a large enough ramp to accomodate snow removing aquipment.

As to cross bracing most of you have commented on, I have learned alot. I will determine the amount required as I dig a little deeper into the AASHTO manual. Never having had a class structured for bridge design, I am mostly having to learn as I go, so many more questions may come in the future. But again, thanks to each of you for your information!

BTW, welcome to CR4. Your question was a good one, and drew forth good responses. Note that the roof may have a snow load, and the whole structure some wind and seismic loads. Since this seems to be a preliminary project, you might not need to account for every one of those just now, but you can anticipate adding them later as you flesh out a total design. You're moving in the right direction; best of luck and success in your further studies.

Dear wiljhard,

You're very welcome. I'm sure I can speak for the other forum members wishing you continued success in your studies. It has been a pleasure assisting you, and we welcome any further questions that you may have.

I'm also glad that you have revised your bridge design such as enclosing it as well as moving the girders outward to accommodate the roof columns!

[Another suggestion that I forgot to discuss with you in my previous post was that the secondary framing (lateral framing) that's between the two girders can be moved up to the top flanges of the girders. The ends then can be cantilevered over the girders to pickup the roof columns directly. You'll need to provide column baseplates (see AISC Steel Manual). This will help facilitate lateral restraint of the compression flanges (top flanges) and help reduce lateral torsional buckling of the girders as well as the size of your girders. All connections can be bolted to avoid field welding, which is costly.]

moving the girders to the edge will increase the thickness / reinforcement of your slab which is alot of weight for the overall bridge.

it might be simpler to add a stub from the girder to the edge of the slab to pick up the post. you should be using the slab and the girder as a composite section anyway for the full span of the bridge in the longitudinal direction and so to use it compositely in the transversal direction isn't that much additional effort for the engineer.

the dead load moment due to the cantilever will be equalised through the tension in the top of the slab. the unequal moments will be taken by the moment resistance of the slab. if these moments are to big, you can introduce the full cross member or increase the slab reinforcement.

Wouldn't the top of the slab be in compression rather than tension?

(It could be either way, depending on the overall structure, but typically....)

I'd like to make a few additional comments here.

First, since the overall width of the concrete slab will most likely remain constant the additional weight issue of moving the girders outward is in my mind a non-issue. The only thing that may change would most likely be the rebar requirements for both Positive & Negative Bending Moments + Shear, which may or may not increase either the rebar size or decrease the rebar spacing, not increase the slab thickness. We're probably only looking at a 100 PSF Live Load here for pedestrians. Since there won't be any snowblowing equipment on the bridge due to a lack of sidewalk ramps, this too is a non-issue. Most likely there will be only enclosed stairwell towers or outside stairs, and/or small elevators (for the Handicap assessability laws) provided at each end of the span.

Another thing for our young engineering student to consider in his design would be the use of 4-inch or 6-inch think Prestressed Concrete hollow-core planks in lieu of cast-in-place reinforced concrete slab with shear studs. The underside of the plank ends can be furnished with several embedded steel plates (with industry standard anchorage studs or wire hoops) at each end of the planks to facilitate the welding to planks to girders (at the inside edge of the girder top flanges). The utilization of these planks instead of a cast-in-place slab has several distinct advantages, such as: less weight, significant less cost, and increased speed of construction.

Utilization of these Prestressed concrete planks may actually reduce the size and weight of the two main girders, resulting a goodly amount of principal costs in the project.....less steel material, less steel preparation, less welding, less primer & paint (unless Cor-Ten steel is used), less cast-in-place concrete costs, and less erection costs! A thin (2-inch min.) lightweight latex-modified concrete overlay with weld-wire mesh will need to be installed atop the planks to fill any gaps between them. I envision that the plank erector can get these planks alone laid within a single working day. The provision for the secondary steel framing will still be required for structural considerations such as: bridge stiffness; wind loading; and, lateral stability, particularily at the compression flanges. As an added benefit, the amount of positive camber can be increased due to a lessening of the slab weight (dead load).

Also, provisions for expansion joints would be simplified as well. With this type of slab approach you only need to provide a 1/4-inch thick vertically-oriented closure plate that has been welded to the outer girder top flange edge in the fabricator's shop, thus avoiding expensive and time consuming field welding and increasing QA/QC. It'd also act as a concrete screen when the concrete topping is placed. Also, I strongly suggest that the along the outside edges of the girder top flanges that shear studs (like Nelson studs) be provided in the cast-in-place concrete pocket (located between the continuous steel closure plate and the ends of the planks.....ends of planks must be stuffed with red rosin building paper to prevent concrete disposition inside the open plank cells, unless of course rebar and concrete is to be provided in the ends of the planks for additional plank tie-in requirements). I wouldn't use anything less than a 6-inch wide full slab depth concrete pocket along the outter edge of the top flanges, so as to provide adequate pullout strength of the slab, better bonding around the studs, and lastly it would promote ease of construction. Use of a single row of these shear studs will undoubtedly NOT help with regard to composite action, but rather they will promote tying together of the various structural elements.

In regard to compression or tension of the cantilevered concrete slab segments. The top surface of the slab within the cantilevered portion of the slab will be in tension whereas the bottom portion of the slab will be in compression. If there is any sort of "Fixity" of the slab at it's supports, then even the portion of the slab spanning between the girders will have negative bending moments (tension in the upper portion of the slab, necessitating rebar installation in the upper region of the slab) occurring there, located between the girder centerlines to the Design Inflection Points (IP) that'd be so located somewhere around both 1/4 span locations, give or take. The remaining portion of the slab span between the IP's will fall within the Positive Bending Moment realm. And since the overall span of this slab may be somewhere between 12-to-15 feet (the OP hasn't given us this information. or little else for that matter), it may be best to provide upper and lower steel reinforcement (ie, Doubly-Reinforced Slab) throughout the slab span transversely (one-way slab action). Additionally, longitudinal reinforcement must be provided in the slab as well for concrete Shrinkage and Temperature Steel requirements.

I hope this helps clear up some the questions, as well as providing design alternatives.

Have a great sunny day people!

===Signed,

CaptMoosie LPE/PhD