# 14- A guide to solved problem 9-7-Lb is bigger than Lr.

## Solved problem 9-7-When Lb is bigger than Lr, What Is Flexure Strength?

From Prof. Mccormac’s book Structural Steel Design Example 9.7. Using AISC Equation F2-4, determine the values of and for a w18x 97 with fy =50 ksi and unbraced length lb =38 ft, assuming that CB =1. As a reminder, this is the graph for the relation between bracing length and Mn.

### Solved problem 9-7 Analysis is based on LRFD.

This an analysis problem, since the W section is given, for which, the distance between bracing for a beam is Lb is bigger  Lr, and also Lb is bigger than LP.
To solve the problem, we will follow the following steps:

Step-1- From Table 1-1 we can get the necessary data, such as Sx, J, h0, rts.

This is the equation used to get the value of Fcr for a beam when we have the relevant values of Sx, Jc, CB, and h0.

3-Since the given Lb is bigger than Lp and also Lb is bigger than Lr, in our solved problem 4-7, the section is in the elastic slender zone, we need to estimate the stress Fcr from the equation F2-4 as shown in the next slide.

We need to find the data expressed in that equation, we can get these data from table 1-1 for the W- section our given section is W18x97, our data can be obtained from table 3-7.

The next slide shows how to get the other values and how we find out that our given bracing distance is >lr. the table includes the value of Lr which is similar to the value estimated by the equation.

5-Estimate the strength of the section for the LRFD by using the formula ΦbMn.= Φb Fcr Sx=0.9026.156*(188)=369 ft.kips
This is the detailed calculation for The LRFD design shown in the next slide image for the solved problem 9-7.

### Solved problem 9-7 Analysis is based on ASD.

For the ASD design:  the same procedures that have been done in the LRFd, except step no.5 will be modified.
1-From table 1-1 get both Lp, Lr values for the given section W18x97.
2- since the given Lb>Lp and >Lr, in our example, the section is in the elastic slender zone.

3-We need to get Sx, J, ho =d-2tf, rts from table 1-1 for section W18x97, and also we need Lr value for the same section but from table 3-2, then Estimate Fcr from equation F2-4.

4-After evaluating Fcr, we get the value of Mn of the section, where Mn= Fcr *Sx.  cr=25.156 ksi.

Mn=fcr*Sx=25.16*188/12=410.0 Ft.kips.

5-Estimate  the strength of the section for ASD by using the formula Φb*Mn.= (1/ Ωb)* Fcr*Sx=.(1/1.67)*26.156*(188)=246 Ft.kips
This is the detailed calculation for The LRFD design shown in the next slide image for the solved problem 9-7.

This is the pdf file used for the illustration of this post.