Double Beam Flexural Strength 1

[slickdeals]

Folks,
I am trying to analyze a situation on an existing building where we are trying to add an additional WF beam on top of an existing W beam to boost up the strength. (essentially one W beam stacked up on top of another)

How does one calculate the flexural strength of such a member? Are there prescribed code procedures for this? Can sectional properties (Z, S) be calculated for this section and applied to AISC formulae to calculate the various limit states? In addition welding between the top flange of the existing beam and bottom flange of the new beam will be made for shear flow.

 

[csd72]

slickdeals,

First principles such as parallel axis theorem, same code procedures as elsewhere, yes but use the singly symetric rules,not really shear flow but longitudinal shear yes.

Here is a spreadsheet in ASD to get you started but I cannot verify its accuracy.

 

[slickdeals]

not really shear flow but longitudinal shear yes.
Please explain.

 

[rowingengineer]

slickdeals,
This is just like a haunch for a portal frame, This member can be treated like a beam in most codes, but it will normally be a slender instead of compact classification.
I attach a spreadsheet by Alex Tomanovich, he has published these free on the web and they are bloody good. Should help you design to AISC 9th Ed. (ASD)

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[csd72]

slickdeals,

Shear flow is a different thing entirely.

It is the flow of shear stress around an unsymetric cross section such as a channel. Look up unsymetric bending and you will see what I mean.

This may seem like pedantics but every engineering tern has a specific meaning and it is best to ensure we use them properly to avoid misunderstandings.

 

[mmillerpe]

There are several ways that you can approach the problem, One way is to design the new beam to support the full load, and provide connections to transfer the loads from the existing to the new. A second would be to design the two beams to act as a single unit. If the second approach is used you would first need to think about the construction sequence. Is the load on the existing beam going to be relieved prior to adding the new member? Or would the exising beam provide temporary support? Is there going to be support added to the compression flange of the new beam or will the unbraced length of the new section be the full length of the beam? Is the load from the existing beam is not relieved, there may be some stresses "locked in" that must be accounted for. The connection between the two sections should be designed for the shear flow based on the built up properties, there are good referenced in many mechanicas of materials texts.

 

[msquared48]

If the beams do not need to be composite, then, discounting interstitial friction, there is no shear flow.

If they are separate, then they should be designed to deflect the same, sharing load based on the ratio of their respective I values. For gravity loads, if the top beam sees the load first, then there is no problem. If the bottom beam sees the load, then a connection between the two beams will be necessary to transfer some of the load to the top beam.

It all depends how you design the system as to what you end up doing.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask