strength , c psi Aggregates all lightweight, lb/yd3 Sand aggregate lightweight, lb/yd3 weights range from 90 lb / ft3 (all aggregates lightweight) to 115 lb / ft3 (sand lightweight). Typically, compressive strengths range from 2500 to 4000 psi. Highstrength lightweight concretes, however, have been produced with maximum unit weights of 125 lb / ft3 and strengths from 6000 to 9000 psi. Structural lightweight concretes are defined by the ACI as concretes with a 28-day compressive strength more than 2500 psi and air-dry unit weight of 115 lb / ft3 or less. The variable amount of water absorbed in the voids of lightweight aggregates makes use of W/C difficult in design of a lightweight-aggregate mix (Table 4.5). Air entrainment of 4 to 6% is desirable to prevent segregation. Maximum size of the coarse aggregate should not exceed half the depth of cover over the reinforcing steel. Lightweight-aggregate concrete exposed to sulfates should have a compressive strength ranging from 3750 to 4750 psi (see ACI 318). For marine structures, the W/C should not exceed 0.40 and at least seven bags of cement should be used per cubic yard of concrete. The modulus of elasticity Ec of lightweight concrete generally ranges from 1,500,000 to 3,000,000 psi. It may be estimated from E  w1.5
 (4.1) c c where w  unit weight of concrete, lb / ft3

tan . Thus, the frictional resistance force (Qsn ) in Eq. (6.27) is equal to the perimeter surface area times the shear strength of the soil at the pile or pier surface. Combined End-Bearing and Friction Pile in Granular Soil. Piles and piers subjected to vertical compressive loads and embedded in a deposit of granular soil are usually treated in the design analysis as combined end-bearing and friction piles or piers. This is because the pile or pier can develop substantial load-carrying capacity from both end-bearing and frictional resistance. To calculate the ultimate pile or pier capacity for a condition of combined end-bearing and friction, the value of Qp from Eq. (6.26) is added to the value of Qs from Eq. (6.27). Usually the ultimate capacity is divided by a factor of safety of 3 in order to calculate the allowable pile or pier load. Pile Groups in Granular Soil. The previous discussion dealt with the load capacity of a single pile in cohesionless soil. Usually pile groups are used to support the foundation elements, such as a group of piles supporting a pile cap or a mat slab. In loose sand and gravel deposits, the load-carrying capacity of each pile in the group may be greater than that of a single pile because of the densification effect due to driving the piles. Because of this densification effect, the load capacity of the group is often taken as the load capacity of a single pile times the number of piles in the group. An exception would be a situation where a weak layer underlies the cohesionless soil. In this case, group action of the piles could cause them to punch through the granular soil and into the weaker layer or cause excessive settlement of the weak layer located below the pile tips. In order to determine the settlement of the strata underlying the pile group, the 2:1 approximation (see Art. 6.4.2) can be used to determine the increase in vertical

The building team should make it standard practice to have the output of the various disciplines checked at the end of each design step and especially before incorporation in the contract documents. Checking of the work of each discipline should be performed by a competent practitioner of that discipline other than the original designer and reviewed by principals and other senior professionals. Checkers should seek to ensure that calculations, drawings, and specifications are free of errors, omissions, and conflicts between building components. For projects that are complicated, unique, or likely to have serious effects if failure should occur, the client or the building team may find it advisable to request a peer review of critical elements of the project or of the whole project. In such cases, the review should be conducted by professionals with expertise equal to or greater than that of the original designers, that is, by peers; and they should be independent of the building team, whether part of the same firm or an outside organization. The review should be paid for by the organization that requests it. The scope may include investigation of site conditions, applicable codes and governmental regulations, environmental impact, design assumptions, calculations, drawings, specifications, alternative designs, constructibility, and conformance with the building program. The peers should not be considered competitors or replacements of the original designers, and there should be a high level of respect and communication between both groups. A report of the results of the review should be submitted to the authorizing agency and the leader of the building team. (The Peer Review Manual, American Consulting Engineers Council, 1015 15th St., NW, Washington, D.C. 20005, and Peer Review, a Program Guide for Members of the Association of Soil and Foundation Engineers, ASFE, Silver