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RE: Pile/Fdn check check for ortho load comb in SDC D
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- Subject: RE: Pile/Fdn check check for ortho load comb in SDC D
- From: Harold Sprague <spraguehope(--nospam--at)hotmail.com>
- Date: Fri, 29 Jul 2011 12:48:41 -0500
This is another case of ...oops (more or less).
The intent can be found in the NEHRP Provisions and Commentary. The Provisions essentially are the same in the ASCE 7. The Commentary is included below for your reference. The ASCE 7 defines minimums only. And sometimes we have problems getting the intent into the code. The intent is that orthogonal load characterization should be required where we have higher seismic demand. Seismic ground motions do not occur in just one horizontal direction. They occur in both horizontal orthogonal directions and arrive at different times. In order to account for the different arrival times, you should design for the ground motion effects in 2 horizontal orthogonal directions. And the load should be carried through to the foundations. The design of foundations to resist the applied loads is implicit in the charging language in Section 12.1.5. The acceptable approach is to apply 100 percent in one direction (x) simultaneously with 30 percent in the y direction. I say acceptable, because the nuclear world uses the 100/40/40 rule. The nuclear world requires a higher degree of conservatism and they require consideration of vertical ground motion. The ground shaking occurs in the vertical axis for buildings or nuclear structures, but I am fighting that battle in the next code cycle. The future code will require consideration for vertical ground motion effects. ....stay tuned. We have known about the vertical effects for years and design accordingly in the nuclear world. But the non-nuclear world needs to be convinced before adding a layer of complexity to design work. Regards, Harold Sprague NEHRP 03 Commentary 4.4.2 Application of loading. Earthquake forces act in both principal directions of the building simultaneously, but the earthquake effects in the two principal directions are unlikely to reach their maxima simultaneously. This section provides a reasonable and adequate method for combining them. It requires that structural elements be designed for 100 percent of the effects of seismic forces in one principal direction combined with 30 percent of the effects of seismic forces in the orthogonal direction. The following combinations of effects of gravity loads, effects of seismic forces in the x-direction, and effects of seismic forces in the y-direction (orthogonal to x-direction) thus pertain: gravity " 100% of x-direction " 30% of y-direction gravity " 30% of x-direction " 100% of y-direction The combination and signs (plus or minus) requiring the greater member strength are used for each member. Orthogonal effects are slight on beams, girders, slabs, and other horizontal elements that are essentially one-directional in their behavior, but they may be significant in columns or other vertical members that participate in resisting earthquake forces in both principal directions of the building. For two-way slabs, orthogonal effects at slab-to-column connections can be neglected provided the moment transferred in the minor direction does not exceed 30 percent of that transferred in the orthogonal direction and there is adequate reinforcement within lines one and one-half times the slab thickness either side of the column to transfer all the minor direction moment. Subject: FW: Pile/Fdn check check for ortho load comb in SDC D Date: Thu, 28 Jul 2011 14:14:24 -0500 From: Ravi.Vasu(--nospam--at)jacobs.com To: seaint(--nospam--at)seaint.org Harold, Thanks for your response. I have one more question on section 12.5. Pl. refer to ASCE 7-05 section 12.5. 1. Per 12.5.2, we do not need to take ortho load comb. for Category B structures. 2. Per 12.5.3 a(Category C), we need to consider ortho load comb. only for Structural integrity type 5 structures. Pipe rack and other refinery structures are straight forward & do not fall in type 5 irregularity. 3. Per 12.5.4(Category D,E & F), we need to consider critical loads(including ortho) only for columns or walls that forms part of two or more Intersecting seismic force resisting systems & subjected to axial load > 20% of strength….. This section does not include fdn. & piles of regular structures like pipe rack. ASCE does not address this issue of considering ortho load comb. for fdn. & piles unless they fall in the above section 12.5.3. Unless it says in the code/ASCE do we need to consider the orthogonal effect on fdns/piles? Or is there std. way of doing this like many engineers take this into consideration(conservative approach?) Can you pl. comment on this? Thanks Ravi Vasu Ravi, RE: ASCE 7-05 A pipe rack is part of a category of "nonbuilding structures similar to buildings". This means that they have structural systems and responses that are similar to buildings, but we needed a little latitude regarding the systems that we could use that we could not get if they were defined as a "building". We wanted to use as much of the information as if the systems were part of buildings as we could. In answer to your quesitons: When we design the foundation, is there any exception that the fdn & piles need not be checked for orthogonal load combinations? No Can anyone point to ASCE/IBC 2009 section about taking or not considering ortho load Comb for fdn and Pile design? Section 15.1.1 We did not provide any changes or modifications to the design of foundations, nor did we develop any exceptions to Section 12.5. Looking at Section 15.1.1 "....Design shall conform to the applicable requirements of other sections as modified by this section. ...." Thus Section 12.5 is not modified and is applicable. Chapter 15 was not intended to stand alone. It provides modifications to other sections of the ASCE 7. The ASCE 7-10 is written similarly. Regards, Harold Sprague Subject: Pile force check for orthogonal load combinations Date: Wed, 27 Jul 2011 14:38:17 -0500 From: Ravi.Vasu(--nospam--at)jacobs.com To: seaint(--nospam--at)seaint.org List, In the design of non building structures like pipe rack in SDC D, we design the steel pipe rack for orthogonal load comb. also. When we design the foundation, is there any exception that the fdn & piles need not be checked for orthogonal load combinations? The ASCE 7-05, section 12.5.4 is not clear about foundation check. It says about column or wall design in any direction. (I am not sure about orthogonal condition check for column/wall) Can anyone point to ASCE/IBC 2009 section about taking or not considering ortho load Comb for fdn and Pile design? Thanks Ravi Vasu information that is for the sole use of the intended recipient. 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