3 Secret Technical Tips for Optimising Steel-Concrete Composite Floor Beams

The 3 secret technical tips for optimised steel concrete composite floor beams are discussed below for the benefit of steel designers. These are particularly applicable for commercial buildings, storage spaces, departmental shops , car park etc with more column-free areas. The tips to enjoy major advantages of composite steel beam over naked steel beam for the same span and loading are as below:

Tip#1

As a flexure/bending member, the least section area for a naked steel beam happens when the web area equals half of total area of section and other half are is equally distributed in top and bottom flanges. Thus Atotal = 2Aw, and Aw=2Af where Aw is the area of web, Af is the area of each of top and bottom flange to resist design moment, ie. A =Aw+2Af. In other words, web resists 25% of BM and each of Flange resists 75% of BM either in compression or tension. Of course, serviceability criteria to be looked into separately by providing appropriate second moment of inertia (I) required for rigidity and stiffenes in order to control deflection.

Non-Composite Section and Composite Section

Let us take an example of a steel floor beam of Span L=10m and total DL+LL as udl w = 20kN/m. Assuming this is a simply supported beam having only shear connections between columns

Span Moment M = 0.125(wL^2) = 250kNm so Mweb = 62.5 kNm. Mflange = 187.5 kNm

If t is the thickness and d is the depth of web and limiting the Allowable Stress in steel(Fs) to 206MPa for Yst=350MPA steel we get  Fs.(t.d^2)/6 = Mweb which gives, d = 477mm when t is 8mm assumed.

Select 480x8 web plate. Aw = 3840 mm^2 = 2Af, Top and Bottom Flange Each 165x12 thick. A = 7800 mm^2, and I = 313418883 mm^4

Serviceability Check: Mid span deflection y = (ML^2)/(10EI) =  40.5mm almost Span/250

Hence provided least steel beam section area A = 7800 mm^2 ie. weight@61 kg/m

Tip#2

Let us design the steel concrete composite beam for same loading, span and BM. From strength point of view, the section modulas of composite beam is always more than 50% greater than the steel only beam because the neutral axis mostly lies in concrete flange which means same sterngth is achieved by reducing the web depth by 22.5 to 20%. Web depth in composite section = 0.8x480 = 385mm. Hence height saving in floor zone = 480-385 = 95mm and savings in web area = 0.2x3840 = 768 mm^2.

Composite Steel Beam with Metal Decking & Studs

Tip#3

Since the neutral axis lies within the concrete slab for floor beams, and concrete behaves as a compression flange to resist remaining 75%BM, the area of tension flange is calculated as

Af(Bottom Flange) = Mflange/(Fs.d) = 187.5x10^6/(206x385) = 2364mm^2, , say 200x12 plate

As for the top flange, sufficient width and thikness are just required for welding of stud connector only, say, 100mmwidex8mm thk plate is taken for Af(Top Flange). 

Toal areas for both flanges = Af = 800+2400 = 3200 mm^2

Total Steel Area A = (100x8)+(385x8)+(200x12)  = 6280 mm^2, weight@49kg/m

Saving in weight of steel for 10m length = 120kg per beam or straight 20% saving in weight of beams.

It is apparerent that substantial savings on steel quantities possible for large floor areaa, and multi storey buildings where beams are duplicated thousand times or more.

Conditions

Propped construction of composite beam gives more economy in steel beam size, quantity and saving in height of building because both DL and LL  moments are resisted by full composite section unlike unpropped construction where DL of green concrete is resisted by naked steel beam and only LL part is resisted by composite section after concrete hardens. This is not out of the box thinking but you can get the props at the fraction of a price and the same props may be reused in hundreds of projects. This may create disadvantageg like mobility on floor but think of the steel you save in the long run.

Recommendation

The above writing is meant to inspire steel designers and decision makers to know the essence of propped steel-concrete composite beam design that can be used to save steel as natural resources and to reduce overall building height in multistorey building over 10 storeys as a possible diversification of using props below beams at green concrete stage. It comes at a small fraction of a price for the re-usable props. Hope this information would serve as an aid to steel/PEB engineers and decision makers to explore feasibility of using propped composite beams in long span buildings frames with more zeal and confidence. The author has no conflict of interest and the opinions are of his own.

Disclaimer

The contents of this article is for informational purpose only and proper professional guidence must be sought before implementation of design and construction of propped steel-concrete composite floor beams.