June 7, 2016 request for formal interpretation of Section 2.3 (Combining Factored Loads Using Strength Design) of ASCE/SEI 7-10.

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1 June 7, 2016 request for formal interpretation of Section 2.3 (Combining Factored Loads Using Strength Design) of ASCE/SEI Contents p.1 Request for Interpretation p.3 Proposed response (August 5, 2016) balloted by Load Combinations and Flood Task Committees p. 4 Load Combinations Task Committee Vote p. 6 Flood Task Committee Vote p.8 Figure illustrating how ASCE 7-10 load factors for Fa and H change across flood zones Question from ASCE member number Request for Formal Interpretation, ASCE 7-10, load factors related to flood and groundwater uplift The request arises from ongoing discussions between structural engineers in the New York City design community over the appropriate load factor to apply (1.6 versus 2.0) to groundwater pressure on below-grade foundation elements in coastal flood hazard areas. The request is not a result of any specific project, but arises from discussions since Hurricane Sandy related to many projects and involving many designers. The discussions have centered on sections (Basic Combinations) and (Load Combinations Including Flood Load), but also involve sections 3.2 (Soil Loads and Hydrostatic Pressure) and (Hydrostatic Loads). Pertinent extracts from those sections are provided after the question below. Question: Per ASCE/SEI 7-10, is it correct to apply a load factor of 1.6 to below-grade hydrostatic uplift loads acting on foundations in Coastal A Zones? Relevant excerpts from ASCE/SEI 7-10 (emphasis added to highlight text) included with Request for Formal Interpretation: Chapter 2 COMBINATIONS OF LOADS 2.2 SYMBOLS Fa = flood load H = load due to lateral earth pressure, ground water pressure, or pressure of bulk materials 2.3 COMBINING FACTORED LOADS USING STRENGTH DESIGN 1

2 2.3.2 Basic Combinations Structures, components, and foundations shall be designed so that their design strength equals or exceeds the effects of the factored loads in the following combinations: (combinations 1 through 7 listed here) Where load H are present, they shall be included as follows: 1. where the effect of H adds to the primary variable load effect, include H with a load factor of 1.6; Load Combinations Including Flood Load When a structure is located in a flood zone (Section 5.3.1), the following load combinations shall be considered in addition to the basic combinations in Section 2.3.2: 1. In V-Zones or Coastal A-Zones, 1.0W in combinations 4 and 6 shall be replaced by 1.0W + 2.0Fa. Chapter 3 DEAD LOADS, SOIL LOADS, AND HYDROSTATIC PRESSURE 3.2 SOIL LOADS AND HYDROSTATIC PRESSURE Lateral Pressures In the design of structures below grade, provision shall be made for the lateral pressure of adjacent soil. If soil loads are not given in a soil investigation report approved by the authority having jurisdiction, then the soil loads specified in Table shall be used as the minimum design lateral loads. Due allowance shall be made for possible surcharge from fixed or moving loads. When a portion or the whole of the adjacent soil is below a free-water surface, computations shall be based upon the weight of the soil diminished by buoyancy, plus full hydrostatic pressure Uplift on Floors and Foundations In the design of basement floors and similar approximately horizontal elements below grade, the upward pressure of water, where applicable, shall be taken as the full hydrostatic pressure applied over the entire area. The hydrostatic load shall be measured from the underside of the construction. Any other upward loads shall be included in the design. Chapter 5 FLOOD LOADS 5.3 DESIGN REQUIREMENTS Design Loads Structural systems of buildings or other structures shall be designed, constructed, connected, and anchored to resist flotation, collapse, and permanent lateral displacement due to action of flood loads associated with the design flood (see Section 5.3.3) and other loads in accordance with the load combinations of Chapter Hydrostatic Loads Hydrostatic loads caused by a depth of water to the level of the DFE shall be applied over all surfaces involved, both above and below ground level, except that for surfaces exposed to free water, the design depth shall be increased by 1 ft (0.30 m). 2

3 Proposed Response (August 5, 2016), Balloted by Load Combinations and Flood Task Committees: Yes, based on the provisions contained in ASCE 7-10 Sections 2, 3, 5 and commentary, 1.6 is the correct load factor to be applied to below grade hydrostatic uplift on foundations in the Coastal A Zone. Although Section calls for hydrostatic loads during flooding to be calculated using the Design Flood Elevation (DFE) and applied to surfaces above and below ground, the 2.0 load factor specified by section for flood loads F a does not apply below ground: Section C2.3.3 describes the origin of the 2.0 load factor for V zones and Coastal A zones, which is based on work by Mehta, et al. (1998). That work only considers above ground lateral flood forces on vertical walls and columns. The 2.0 load factor was not derived for uplift or lateral hydrostatic loads below grade. C2.3.3 states, The recommended flood load factor of 2.0 in V Zones and Coastal A Zones is based on a statistical analysis of flood loads associated with hydrostatic pressures, pressures due to steady overland flow, and hydrodynamic pressures due to waves, as specified in Section 5.4. The relatively high flood load factor stems from the high variability in floods relative to other environmental loads. The variability in hydrostatic loads under flood conditions is small when compared with the variability in wave loads and hydrodynamic loads from overland flooding, so it does not make sense to apply 2.0 to below-grade hydrostatic loads. Section C2.3.2 states, Accordingly, load factors for fluid load (F) and lateral pressure due to soil and water in soil (H) have been chosen to yield designs that would be similar to those obtained with existing specifications, if appropriate adjustments consistent with the load combinations in Section were made to the resistance factors. Section contains the only specific mention of hydrostatic uplift on foundations in ASCE Given the uncertainty in knowledge of the groundwater elevation over the life of the structure, the 1.6 load factor for soil and hydrostatic loads H is the most appropriate choice for uplift on foundations in the Coastal A zone. Hydrostatic uplift forces, whether from groundwater or flood saturated soils, are applied in a manner consistent with live loads in load combination 2 of Section 2.3: 1.2D + 1.6(L + H) + 0.5(Lr or S or R). The attached figure (see last page) illustrates the committee s opinion as to appropriate load factors for various flood zones. Reference: Mehta, K. C., et al. (1998). An investigation of load factors for flood and combined wind and flood, Report prepared for Federal Emergency Management Agency, Washington, D.C. 3

4 Loads Combinations Task Committee Vote: 17 members (10 Affirm, 3 Affirm with Comment, 2 Negative, 2 No Vote) (Dan Howell is on both Load Combinations and Flood Committee) 76% (13) Affirm (3 with comments) Phil Line: Affirm with comment. To avoid introducing some ambiguity in the response, suggest deleting the three paragraphs that follow: Section C2.3.3 describes the origin of the 2.0 load factor for V zones and Coastal A zones, which was based on work by Mehta, et al. (1998). That work only considered above ground lateral flood forces on vertical walls and columns. The 2.0 load factor was not derived for uplift or lateral hydrostatic loads below grade. Yes, I agree with your interpretation. Unless we feel that the flood elevations have as high a variability as the wave action (seems unlikely), it appears reasonable to use the 1.6 for below-grade hydro-static loads. Rose Rodriguez: Affirm with comment. Having designed several low- and mid-rise building on the coast of Florida, I feel it might be helpful to consider what types of structures they are concerned about: * In mid-rise and high rise buildings, it would be reasonable to assume that the main lateral (shear wall) systems and foundation systems are not controlled by flood loading, but the basement walls potentially are controlled by flood loads. * In low-rise buildings and large industrial facilities, it would be reasonable to assume that flooding controls all the following: lateral systems, basement walls and basement slabs. * In transportation structures, it probably varies. If we were to enforce the 2.0, we are really penalizing the design of low rise buildings and basement walls mainly in low rise buildings. Just my interpretation. Hopefully some chart can make it into the next cycle. They would be quite helpful. Dan Howell: Affirm with comment. I believe your response makes sense and do not disagree with respect to strength design and ASD loads. However, with respect to stability checks (buoyant uplift, or flotation, which the inquiry mentioned), there does not appear to be any distinction made regarding load factors. I mentioned this in the second part of my previous comments, and I believe this should be addressed, whether in this cycle or the next. 4

5 Therefore, my vote is "affirm with comment" with the comment being the need to address stability/flotation checks and the applicable load factors. 12% (2) Negative Greg Soules: Negative I believe that the load factors, as written, are 1.6 for non-flood combinations and 2.0 for flood combinations and our response should reflect this. Yue Li: Negative My understanding is the current code for load factor related to flood load has already included that. 12% (2) No response The Load Combinations Task Committee Chair believes the negative responses should be found nonresponsive based on the rationale presented in the proposed response. The research that provided the basis for the load factor of 2.0 only considered above ground lateral flood forces on vertical walls and columns. The 2.0 load factor was not derived for uplift or lateral hydrostatic loads below grade. Given the uncertainty in knowledge of the groundwater elevation over the life of the structure, the 1.6 load factor for soil and hydrostatic loads H is the most appropriate choice for uplift on foundations in the Coastal A zone. 5

6 Flood Task Committee Vote: 9 members (6 Affirm, 1 Affirm with Comment, 2 Negative, 0 No Vote) 78% (7) Affirm (1 with comment) Dan Howell: Affirm with comment. I believe your response makes sense and do not disagree with respect to strength design and ASD loads. However, with respect to stability checks (buoyant uplift, or flotation, which the inquiry mentioned), there does not appear to be any distinction made regarding load factors. I mentioned this in the second part of my previous comments, and I believe this should be addressed, whether in this cycle or the next. Therefore, my vote is "affirm with comment" with the comment being the need to address stability/flotation checks and the applicable load factors. 22% (2) Negative Marc Levitan: Negative Comments: 1 thanks for providing all that discussion trail in the ballot it was helpful in explaining the issue 2 - The discussion points out a lot of issues that could use clarification in ASCE 7-22 chapters 2, 3, and 5 should work together on this for the next cycle 3 I think the way the standard is written, the 2.0Fa would govern computation of hydrostatic loads here, not 1.6H for the part of the hydrostatic load that comes from below ground and 2.0 for the part above ground, because section identifies that hydrostatic loads during floods include contributions from above and below ground. Therefore, the Fa in this case is all hydrostatic loads, and there would be no separate H in the load combination (double counting would make no sense) 4 The proposed rationale for explaining this seems to be problematic as well. It basically relies on commentary and on discussion of how the flood load factors were developed. Background and intent are one thing, but what s written in the standard has to govern. I think its pretty clear from sections and in the body of the standard that if you are in a coastal A flood zone, your load combination will include 2.0Fa, and Fa includes above and below ground effects. So in flood zones I think there is no separate computation of H loads needed for load combinations 4 and 6 that include flood. In strength load combinations other than 4 and 6, the normal rules for H would apply. Long Phan: Negative 6

7 My vote is Negative for this proposed interpretation. It s not clear to me why the 1.6 load factor for soil and hydrostatic loads is most appropriate for uplift forces on foundations in the Coastal A zone because of the uncertainty in knowledge of the groundwater elevation over the life of the structure. 0% (0) No response The Flood Task Committee Chair believes the negative responses should be found nonresponsive based on the rationale presented in the proposed response. The research that provided the basis for the load factor of 2.0 only considered above ground lateral flood forces on vertical walls and columns. The 2.0 load factor was not derived for uplift or lateral hydrostatic loads below grade. Given the uncertainty in knowledge of the groundwater elevation over the life of the structure, the 1.6 load factor for soil and hydrostatic loads H is the most appropriate choice for uplift on foundations in the Coastal A zone. 7

8 Load Factors per 7-10 outside 100-yr Flood Zone A zone outside 100-yr A Zone Coastal A Zone V Zone Fa H Load Factor to Apply to groundwater Option 1 (flood zone controls LF below ground) Option 2 (H controls LF below Ground; Fa controls LF above ground) Flood Zone out A zone out A Zone Coastal A Zone V Zone Figure illustrating how flood zones and ASCE 7-10 load factors for Fa and H change (light blue shaded rows) as one moves (right to left) from coastal flood zones (V, Coastal A and A, to outside 100-yr floodplain [zone X], to riverine flood zone A, to outside 100-yr floodplain). Green shaded rows show load factors that could be applied to groundwater, depending on whether flood or hydrostatic LF controls below ground. Request for formal interpretation was specific to Coastal A Zone circled in red. 8