How to reduce Unbraced length of Steel Beam? 1

[Ahmad Uzair Arshad]

Dear All,

Hope you are doing well,
I am working on a rehabilitation of fire damaged structure, New steel beam (W section) is provided between two existing concrete beams to reduce span of existing concrete slab, I cant add Lateral bracing, can I reduce the unbraced length by bolting Top flange of steel beam in existing concrete slab,

Kindly suggest,

Thank you

 

[Tomfh]

Yes

 

[JoshPlumSE]

Yes. That would be top flange bracing.

 

[StrucPEng]

Yes, but I would avoid just directly bolting the beam flange to the u/s of slab. Ideally you want the slab to be supported by the beam and not the beam to hang up on the slab. In the past I have used a detail like the attached with intermittent angles bolted to the u/s slab and bearing tight against the beam but left unconnected. This will restrain the top flange from LTB but not hang the beam up on the slab.

 

[hotmailbox]

I personally prefer bolting the top flange to the existing slab. With proper support at each end of the beam & adequate beam design, the beam is not hung from the slab. On top of that we may benefit some composite action.

 

[canwesteng]

I like that detail, not because I'm concerned in the slightest about the beam hanging from the slab, but it's much easier to erect. How are you drilling holes in the concrete with the tool between the flanges otherwise?

 

[bones206]

I recently used a detail with a continuous wide plate stitch welded to the top flange, with a hole pattern for the anchor bolts. Pretty sure I got that detail from a previous thread about supporting hollowcore planks (which is what I used it for). I think I like the clip angle detail better though. (

 

[Ingenuity]

How are you drilling holes in the concrete with the tool between the flanges otherwise?

SDS-Plus 90[sup]o[/sup] drill attachment, with a shallow-bit, if the beam depth permits:

 

[Tomfh]

I would have some side tabs and bolt straight up into the concrete.

Rotary hammers deliver axial hammer blows. How does that 90 degree mount achieve that?

 

[Ingenuity]

Rotary hammers deliver axial hammer blows. How does that 90 degree mount achieve that?

Operator's hand - or plywood/wood lever etc - at the base, in alignment with C/L of bit. Works well for shallow depths (less than 6"), and small bit diameters (say less than 3/4").

You still have to drill the steel flange too - I use a compact low-profile mag drill for such that fits between flanges.

But, I agree weld tabs or bolt-on angle clips are easier.

 

[canwesteng]

Yeah I can see complaints arising from making the contractor hammer drill holes at a 90 like that. Good to know it's possible for the cases I'll need it though.

 

[Agent666]

If you can afford it strengthwise for your member, you can put a matching hole in the bottom flange to drill through. Done it before when things are tight. Just get a longer drill bit and feed it through both flanges.

https://engineervsheep.com

 

[hotmailbox]

Normally the steel beam in this case is deep so the anchors can be tilted a bit for easy hole drilling. Need bevel washers.

 

[HTURKAK]

Dear Ahmad Uzair,

Will you provide some sktches showing the size of the subject slab and beams? is this slab two way slab ? With your proposed set up, are the beam reinforcements sufficient ? Will the new steel beam be connected to slab to obtain composite action?

If only to reduce span and provide simple support, I will suggest ,
-remove the plastering to match W beam and RC slab
- install the steel beam to supporting RC beams,
- uplift the RC slab along the steel beam with using props,
- shimming the gap with steel plates and plastering..

IMO, the friction and new plaster will provide required bracing and no need to bolting.

Good luck..

 

[Liam Nesson]

Yes, make sure you have a load path for the lateral force, i believe its 0.5% of the compression force in the flange.