Need a book? Engineering books recommendations...
Return to index: [Subject] [Thread] [Date] [Author]
Concrete shear wall shear equa 11-32 & 21-7
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "SEAINT" <seaint(--nospam--at)seaint.org>
- Subject: Concrete shear wall shear equa 11-32 & 21-7
- From: "David Merrick" <mrkgp(--nospam--at)winfirst.com>
- Date: Wed, 9 Apr 2003 16:29:28 -0700
This is a summary of a previous discussion of shear design for concrete shear walls. I fear there are shear wall programs not checking both equations 11-32 and 21-7. Equation 21-7 is for the special case of high seismic zones. Some design cases equation 11-32 will require more shear strength. There are three discussed affects of a higher shear demand in equation 11-32, comparing to 21-7. I. Varying phi factors. UBC requires 0.6 in zone 3&4. It was intended that phi is 0.85 for equation 11-32 and 0.6 for equation 21-7. ACI 318-99 requires a phi of 0.85 for shear in all shear wall applications. ACI 318-02 requires a phi of 0.75 for shear in all shear wall applications. II. Varying shear wall length. Equation 11-32 is limited by the tension steel position, leaving about 80% of the wall for shear resistance. III. Varying concrete strength. For a 10ft tall, 5.5ft long wall with an OTM only due to the shear at the top. The factor of sqrt(fc') is 89% of the equivalent factor in equation 21-7. The equation 11-32 is reduced further when an OTM is increased, but limited by code section 1911.10.5 with a value of 2*sqrt(fc'). The largest factor for equation 21-7 is 3*sqrt(fc'). This is independent of any OTM. Eq 11-32 is dependent on OTM. For a large OTM the smallest factor is 2*sqrt(fc '). Equation 11-32 then has a strength 66% of equation 21-7. In summary, the ratio of strength for equation 11-32 divide by equation 21-7 is as follows. ***UBC: 83% If wind is not less than 83% of zone 3&4 EQ, then wind will control shear. Literal interpretation of UBC concludes that equation 11-32 may also control EQ shear design. Zone 2 seismic forces (67% to 74% of zone 3) are required to have more shear strength than zone 3 (11% to 19%). I hope and suspect that equation 21-7 is based on better research. The later ACI codes exclude equation 11-32 for seismic shear of zones 3&4. ***ACI 318-99 59% If wind is not less than 59% of zone 3&4 EQ, then wind will control shear. ***ACI 318-02 59% If wind is not less than 59% of zone 3&4 EQ, then wind will control shear. Because design examples may only design for a seismic force in zone 3&4, they leave out equation 11-32. The omission of equation 11-32 may have worked its way into design programs. If so, one will need to check for wind shear by setting the seismic zone to 1 or 2. This will not work if the program does not do equation 11-32 for any zone or wind check. That goes to another issue of protected computer code and whose is responsible. One could also find 41 and 26 year-old research, and prove that equation 11-32 might be too conservative and hope the opposite is not concluded. (ACI 318-02 ref. 21.3 & 21.38) As always, this is for discussion only. I do hope readers will review what is said and comment. David Merrick, SE MRKGP(--nospam--at)winfirst.com Sacramento, Ca ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
- Prev by Subject: RE: Concrete Reinforcing Choices
- Next by Subject: concrete shear walls
- Previous by thread: BOCA 1999 Table 1610.6.3
- Next by thread: Question about shear stress in steel plates, tees and angles
- About this archive
- Messages sorted by: [Subject][Thread][Author][Date]