Steel Continuous Beam Structural Diagram

Question

Here's an image of my structural plan that my group and I are designing for a university assignment. It is a 7-story steel structure with bracing at the two ends and at the verticle core near the center.

Our professors have suggested that we use continuous primary and secondary beams however we are unsure what they are. All of the structural calculations (bending moment, shear, deflection check) will be done with a computer program however we must first understand the structural diagram of a continuous beam in order to enter the proper information into the program.

What would the continuous beam structural diagram look like for the primary and secondary beams of this structure? Is the diagram below correct?

Any explanation of how a steel continuous beam works in a multi-level structure is greatly appreciated. Thank you for your help.

Answer 1

For multiple stories, you usually have continuous columns going up and simple span beams in between. Having continuous beams would require putting the top level column on top of the beam, so you would have an extra beam bearing calculation which would become more difficult as you have more stories. There is also the question of stability of the beam with a top flange load which would have to be addressed.

Your diagram is about right, but catches my eye as maybe a bit long for a beam. Usually steel rolled shapes will be max. 60 feet long, so if you want to do multi-span, you will be limited by this. Id suggest you take a look at "Gerber beam systems" which are sometimes used for economical multi span beams going over columns.

Also there is no need to use fixed ends at both ends, there are quite a few column connections that can be considered flexible enough to be designed as pins (Shear tabs, Single and double angle, beam face plate). While putting a rigid (fixed) connection in steel is possible, it is rarely done outside of framed construction because the connection is more complex and you have to design your column for the transferred moment. If I understand your drawing correctly, you have X braces here so no need for steel frames.

Answer 2

In real life you would simulate the whole building as a 3D frame. Particularly for lateral loads (such as wind or eartquakes) it is almost necessary. You could omit secondary beams from such a model though.With an assignment things can be more simplistic though.

A beam can be considered continuous if it can transfer its full end moments and forces to the next span. This, doesn't neccessarily mean that the beam is physically continuous, as a non-split member. It rather means, that an appropriate connection exists at beam end that can carry the full moments and other forces (aka full-strength, rigid connection).

Your structural model seems appropriate, except for the end supports. Making them fully fixed you assume that the columns at that points are infinitely less flexible than the beam, which is not correct. A detailed approach would be to envelope the beam response between the two extremes of fully fixed and pinned end supports. However you may skip this if the beam cross-section remains unchanged and loads are also similar for all spans. In that case, it would be ok if you considered only the pinned ended model, because this would give you simultaneously worst sagging moments (at the end spans) and almost the same hogging moments (at the interior supports). Thus, you cover both extremes simultaneously.