Composite Beams and rigid diaphragms 1

[OUe]

This is my first post on this website, so please be kind.

I am starting a new job on Monday, May 14th, and I'm being told that I will be working with my supervisor on a 3-story, composite beam, concentrically-braced frame of steel system for an office structure.

My experience level is doing primarily low-rise steel buildings, but not doing many (if any) multi-story structures. Therefore, I'm a little intimidated. Is there anyone out there that can recommend a good book on handling composite beam design for rigid diaphragms?

Oh, by the way: This project will be located in Washington where the SCD is category 'D'. Thanks.

 

[OUe]

Oops, I meant SDC (seismic design category). Thanks.

 

[Taro]

The AISC steel manual and most any steel textbook cover how to design composite steel beams. I would recommend designing one beam by hand just so you understand the theory. But after that, your new employer will probably have software that will do the number crunching for you. Your employer will need to instruct you on the details of how they like to do things (metal deck profile and concrete thickness, beam spacing, percent composite action, amount of camber, etc.).

 

[OUe]

Taro,

I appreciate your post, however I think I need to be more specific. I do, indeed, know how to design a composite beam (whether the percent composite is full or partial). How do I design composite beams/deck with studs for various drag struts, compression/tension chords, etc. for a rigid diaphragm? I know how to come up with the loads, I suppose I'm wondering what the major structural detailing concerns will be and if there are specific design guides or practical design practices to go by that can be mentioned within this thread.

BTW, the seismic design category is D in western Washington.

Thanks.

 

[Qshake]

For practical design references the COE Engineering Manuals are often handy and are free to download. Other refernces that may be of help are the PE/SE review texts on Building and Bridge design by Alan Williams.

Farzad Naiem offers a comprehensive look at seismic design with most any building material in his Seismic Design Handbook.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[Taro]

OUe, if you know how to determine the loads, design the composite beam and how to apply the AISC compression+flexure interaction equations, then you are 90% of the way there.

One case that isn't covered well in the AISC manual is a spandrel beam with axial compression. Lateral bracing is provided by the slab at the top flange, but the bottom flange is unrestrained. In this case, axial-torsional buckling will be the controlling limit state for the compression capacity. Good references for these members are:
- Stability Design Criteria for Metal Structures, edited by Galambos
- AISC stability bracing seminar notes by Yura and Helwig

If you don't want to go to the trouble of calculating the axial-torsional capacity, you can just provide bottom flange bracing and conservatively design for weak axis buckling with the unbraced length equal to the brace spacing. This might be easier, but is more expensive.

Another good reference for drag strut design is "Design of Shear Tab Connections for Gravity and Seismic Loads", Steel Tips article by Astaneh-Asl.

 

[OUe]

Taro and Qshake,

Thanks for the responses. I'll check the steel tips (is it in PDF off of the web?).

 

[whyun]

Composite beam design for dead and live load is independent of the seismic design typically.

Assuming that you are designing to 2006 IBC, there is a series of design manuals (three volumes of "2006 IBC Structural/Seismic Design Manual" available thru

http://www.iccsafe.org

) that may have the design examples you are looking for. Volume 3 covers steel and concrete structures.

Steel Tips can be found in

http://www.steeltips.org/steeltips/index.php

but I believe you need to sign up in order to download.

 

[OUe]

Hello all,
I have now realized that my question regarding the diaphragm requirements for the composite deck is limited by the amount of studs, the thickness of concrete above the flute, and the weight of the concrete (NW or LW). After looking at a deck manufacturer's technical guide, I have answered my own question.

For example: In verco floor decks, a concrete thickness of 2 1/2" above the flute with NW concrete and with studs at 12" o/c, the allowable diaphragm shear is 4730plf. There are also limitations on stud height verses flute height as well as minimum diameters from their product tables. Since I'm assuming that the seismic loading in Washington State controls, I will look into whether there are additional factors that affect the capacities due to seismic. It seems everyone is baulking at approving their products for seimic loading capacities these days.

 

[ttommyt]

I have a problem with a structural load in a wood frame building, a 40' clear span with truss roof system that requires a truss girder cluster with 37153 lbs load in the center of the span. I can not put columns inside this span,I've dealt with crushing loads at the ends but, I need some advice on the size steel beam that will carry this load. I thought a 6X18W60 would work but my numbers keep coming up different. Anybody?

 

[OUe]

ttommyt,

If the 37 kip load is an unfactored load at mid-span, then take PL/4 as your moment with an unbraced length equal to your truss spacing. Just make sure that there's an adequate connection to the steel beam to satisfy the lateral/torsional buckling restraint. Then look it up the ASD (green book) beam tables. Otherwise, convert your load to a factored load and use the LRFD steel tables. The beam will definitely be an ASTM 992, which is most common (GR 50). A36 would be more conservative.

Good luck!

If the 37k load is service level, then your beam will be something like W21x73. This is ignoring service-level loads, therefore it's the lightest section (with lots of assumptions).