moment of a Z shaped beam

[Superchief]

For a simple clear span how does the strength of a Z shaped steel beam (2 Ibeams connected via plates) compare to the strength of a straight steel beam and more specifically, how do you calculate the moment and the connection required?

 

[hokie66]

You would have to explain further to get a reasonable response. Perhaps a sketch. Z in section, plan, or elevation?

 

[271828]

The main difference between a zee and an I-shape is that the zee will deflect downward and laterally, and twist, when subjected to regular downward loads.

Like hokie66 types, you'll need to further describe the situation.

 

[Superchief]

The issues is a 2 tier stair landing whose design was engineered with a simple span W10-30 but that does not work because it was missed that both beams must be contained within their respective floor systems.

So I need to go back to the engineer (not really their error, there was no section drawing in this area and it was not obvious the lower beam could not just go straight from the plan view but I just found it creates a headroom issue).

So, my steel fabricator suggested knife plates could tie two beams together if the engineer approves, but I am curious how the bending moment gets calculated in this situation. I assume deflection will be the major issue as the floor system resists rotation and there are no point loads.

 

[hokie66]

Can be done, but you are on the right track to go back to the design engineer for details of the moment connection required.

 

[dik]

A couple of end plates and a couple of stiffeners, and 'done like a dinner'. A local engineer should be able to handle it easily... I was thinking 'Z' in cross-section...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Superchief]

great ... but I like understanding - How is moment calculated for this Z situation?

if it is sufficiently stiffened, would it then be calculated like a straight beam except being stiffer in the section where the plates are?

 

[dik]

The moment doesn't change whether the beam is uniform, or varied x-section. It's just a matter of transferring the moment from one beam to the next.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Superchief]

Thanks!


In this case a 10" beam could go the 18' distance required, but I am curious how you solve if you had a similar condition where a 10" beam could not normally make the span.

I actually have an area like this in the building which was solved with multiple intermediate transfer beams which for various reasons is a complicated and expensive solution, but I wonder if it could have been solved with 2 beams of different sizes and a moment connection.

Actual example: it is a 29'-2" clear span with a 20'-7" section that is in the wall and can be as deep as it wants and a 7'-7" section that can only be 10" deep to fit into the floor system - there is a 12" overlap to make the moment connection.

A 10" beam would have trouble spanning that distance but if I understand correctly then the moment at 7'-7" is a little more than 1/2 the moment in the middle of the clear span?

 

[dik]

When you put a 'crank' in a beam. The whole length is subjected to the same moments across the span. The big difference is that the cranked part is subjected to the moment at that location for the full length of the 'cranked part'. This increases the deflection.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[hokie66]

Superchief,

You are trying to simplify a problem which requires a structural engineer for resolution. Don't expect us to design your structure, solving geometrical problems which are caused by architectural considerations. You can get both good and bad advice on this site.

 

[Superchief]

no, I'm trying top understand because I like understanding