1. CEE 4606 - Capstone II Structural Engineering
CEE 7402 Forensic Engineering
CEE Capstone II Structural Engineering
Lecture 6 – Seismic Detailing

2. CEE 7402 Forensic Engineering
Outline
1. IBC Seismic Detailing Requirements
2. Shear Design
3. Torsion Design
3. Progress Reports
4. Work Tasks

3. IBC Seismic Detailing Requirements
CEE 7402 Forensic Engineering
IBC Seismic Detailing Requirements
How does having a structure in Seismic Design Category D affect the design of our building frame for gravity load?
What did we find out?

4. IBC Seismic Detailing Requirements
CEE 7402 Forensic Engineering
IBC Seismic Detailing Requirements
General concept:
In a moderate to severe EQ, inelastic behavior is expected
Yielding is allowed to take place
Fairly large displacements occur
We must have ductility for this to occur!
Ductility is provided by good detailing

5. IBC Seismic Detailing Requirements
CEE 7402 Forensic Engineering
IBC Seismic Detailing Requirements
Level of detailing required depends on the level of seismic risk:
Low seismic risk: SDC A, B
Medium seismic risk: SDC C
High seismic risk: SDC D, E, F

6. IBC Seismic Detailing – SDC D
CEE 7402 Forensic Engineering
IBC Seismic Detailing – SDC D
IBC
For SDC D, all requirements of SDC C (and thus all requirements of SDC A & B) must be met
Seismic-force-resisting systems
Frame members not proportioned to resist EQ forces

7. IBC Seismic Detailing – 1910.5.2
CEE 7402 Forensic Engineering
IBC Seismic Detailing –
Since our frame is not designed to resist EQ forces (our masonry shear walls handle this load), we essentially are required to meet the provisions of Section 21.9 of ACI
Note that we will have special detailing requirements for our masonry shear walls when we design them!

8. IBC Seismic Detailing – 1910.5.2
CEE 7402 Forensic Engineering
IBC Seismic Detailing –
Since our frame is not designed to resist EQ forces (our masonry shear walls handle this load), we essentially are required to meet the provisions of Section 21.9 of ACI
Note that we will have special detailing requirements for our masonry shear walls when we design them!

9. CEE 7402 Forensic Engineering
ACI Section 21.9
Principle:
Our frame is designed only for gravity loads
However, EQ forces will cause the structure to displace some laterally
This lateral displacement will cause some forces to develop in the frame elements

10. CEE 7402 Forensic Engineering
ACI Section 21.9
Principle:
Design the frame system such that the gravity load system maintains its vertical load capacity when subjected to the maximum lateral displacement expected for the design-basis earthquake

11. CEE 7402 Forensic Engineering
ACI Section 21.9
Example:
Two parking garages collapsed in the 1994 Northridge earthquake when columns that were designed only for gravity load failed
We are forced to detail the elements of our gravity load resisting system to behave in a ductile manner, even if we do not design them to resist EQ loads!

12. CEE 7402 Forensic Engineering
ACI Section 21.9
Two options:
1) Perform analysis to determine moments and shears in frame system resulting from the design EQ (apply ACI )
You may do this analysis if you so desire (not easy!; use STAAD, consider cracking, etc.)
2) Conservative, simple approach in which no analysis is required but more stringent detailing is necessary (apply ACI )

13. CEE 7402 Forensic Engineering
ACI Section
Requirements if no lateral analysis is conducted based on design displacement:
ACI : General requirements
ACI : Beam requirements
ACI : Column requirements

14. CEE 7402 Forensic Engineering
ACI Section
States four requirements by reference:
21.2.4: Min. conc. strength = 3000 psi
21.2.5: ASTM reinforcement
21.2.6: Mechanical splices
: Welded splices

15. CEE 7402 Forensic Engineering
ACI Section
Three requirements for beams:
: Special minimum & maximum reinforcement limits
21.3.4: Specifies a “capacity” design for shear
Max. stirrup spacing = d/2 for entire member

16. CEE 7402 Forensic Engineering
ACI Section
Three requirements for columns:
21.4.4: Special requirements for transverse reinforcement (ties & spirals)
21.4.5: Specifies a “capacity” design for shear
: Requires confining reinforcement in beam-column joints

17. Shear Design
Review your CEE 3422 notes and McCormac text Ch. 7
Shear covered in ACI Chapter 11
Seismic detailing requires that we use a “capacity design” approach
Reference: Ch. 15, Nawy (handed out)

18. Shear Design – “Capacity Design”
Which is more brittle: flexural failure or shear failure???
Remember the pictures of columns that failed in shear?
Avoid brittle failures in general, but we really need to avoid them in seismic design!

19. Shear – “Capacity Design” Philosphy
Design for shear based on the highest possible shear that could occur in the structure, not the shear computed by loads
Assume that when flexural failure occurs, we still have enough shear capacity
Design assuming that plastic hinges (flexural failure) occur at each end of the beam

20. Shear Design – “Capacity Design”
Reverse cyclic loading that occurs during earthquakes will cause plastic hinges to form at beam ends – in positive moment on one end and negative on the other
This is a worst case scenario for shear!
Mpr = Probable flexural moment strength
Mpr1
Mpr2

21. Shear Design – “Capacity Design”
Probable moment strength uses f = 1.0 and assumes steel is at 125% of fy to consider effect of strain hardening
We still must consider load along the beam!
Design for shear based on worst of two cases:

22. Shear Design – Final Comments
Conservative assumption: Vc = 0
Considers that considers effects of repeated cracking and loss of concrete contribution near plastic hinges
Apply in accordance with ACI
Note: You will probably wind up with a fairly tight stirrup/hoop spacing

23. Does torsion scare anybody? We design for torsion together with shear
ACI Chapter 11
See McCormac text, Chapter 14
Text provides a good simple discussion
Remember, we will design for minimal torsion

24. Two Types of Torsion Equilibrium torsion Compatibility torsion
Actually a torsional load acting directly on the structural member
Compatibility torsion
Torsion occurs in the member only because it is connected to another member that frames in
Indirect torsion, dependent on compatibility of deformations
Which type do we have?

25. ACI Torsion Design
ACI uses an approximation that all members are thin walled tubes to develop equations
If torsion is equilibrium torsion, we must design for the total torsional load
If torsion is compatibility torsion, we are only required to design for the torsional load that causes cracking
Torsion could never exceed this value!

26. Cracking Torque Design for this value of torsion
See code and text for definitions of variables
After cracking, ACI assumes no strength contribution is supplied by concrete, so the entire torque must be resisted by closed hoops

27. ACI Torsion Design
Above equations are very similar to shear equations – they give the required torsion reinforcement for a given hoop spacing or vice versa.

28. An additional small amount of longitudinal steel will also be required
ACI Torsion Design
Simply add the torsion reinforcement requirement to the shear reinforcement requirement
They are the same steel, not different bars!
An additional small amount of longitudinal steel will also be required
This steel must be distributed around the cross-section

29. Torsion Design – Additional Guidance
Text – see specifically Sections 14.7 and 14.9 (example problem)
This should not add a significant amount of steel to your beams, but will add some
Don’t be afraid to ask questions

30. Comments on Progress Report #1
CEE 7402 Forensic Engineering
Comments on Progress Report #1
Agendas lack important details
Use the passive voice!!!
Good background information, some took a minimalist approach
Good wind and earthquake research, but 1 or 2 sources is not enough
Tables and Figures
Consistent format, sig figs, color, format, location in document, references

31. Comments on Progress Report #1
CEE 7402 Forensic Engineering
Comments on Progress Report #1
References!!!
Appendices
Originator, checker, and dates for all drawings and calculations
Good meeting minutes
Good outcomes, but some of you need a little more support for your conclusions
Proof read everything after you put all of the parts together

32. CEE 7402 Forensic Engineering
Work Tasks
Determine final loads on the structure
Gravity loads (dead, live)
Lateral loads (seismic, wind)
Should probably be done by now!
Truss analysis on roof & design of roof members
Should be nearing completion
Detailing of roof-to-structure connection
Hint: See IBC

33. CEE 7402 Forensic Engineering
Work Tasks
Review IBC Seismic Detailing Requirements in detail
Work on design of one-way slab, beams, and girders
Framing plan should be finalized
Design for flexure, shear, and torsion
Determine column loadings
Column design discussed in CASE