Masonry Structural Design
Masonry is one of the oldest building materials, with a history spanning thousands of years from the pyramids of Egypt to modern reinforced masonry buildings. The structural design of masonry follows the strength design method in the Masonry Standards Joint Committee code, which uses load and resistance factor design principles similar to reinforced concrete design. The nominal strength of masonry members is calculated from the material properties and the geometry of the cross-section, and the design strength is the nominal strength multiplied by the strength reduction factor. The strength reduction factors for masonry are 0.90 for flexure, 0.80 for shear, and 0.65 for axial compression with slender walls. The specified compressive strength of masonry ranges from 1,500 to 4,000 psi depending on the unit strength and the mortar type. The modulus of rupture for masonry used in flexural design depends on the mortar type and the direction of stress relative to the bed joints.
The design of masonry walls for axial compression and flexure uses the interaction diagram approach that defines the combinations of axial load and moment that the wall can resist. The interaction diagram for masonry walls is similar in concept to reinforced concrete columns but accounts for the orthotropic properties of masonry where the strength perpendicular to the bed joints is different from the strength parallel to the bed joints. The interaction diagram for masonry walls with out-of-plane bending considers the axial load, the wall thickness, the reinforcement, and the masonry compressive strength. Walls subjected to combined axial compression and out-of-plane bending are common in building design where wind or seismic loads push against the wall face while the wall supports vertical loads from the structure above. The slenderness of masonry walls must be considered in design, with the effective height determined by the end support conditions and the stiffness of the horizontal supports.
Reinforced masonry walls use vertical and horizontal reinforcement placed in grouted cells to provide tensile strength and ductility. The minimum reinforcement requirements for masonry walls depend on the seismic design category. Walls in moderate and high seismic zones require minimum vertical reinforcement of 0.0007 times the gross cross-sectional area and minimum horizontal reinforcement of 0.0007 times the gross area. The maximum spacing of vertical reinforcement is 48 inches, and the maximum spacing of horizontal reinforcement is 24 inches for walls in high seismic zones. The reinforcement must be fully grouted to develop the design strength, with grout that flows freely into the cells and around the reinforcement. The bond beam is a horizontally reinforced course at the top of the wall and at intermediate levels that ties the wall together and distributes lateral loads to the shear walls.
Formwork Engineering
Formwork engineering is the discipline of designing and building the temporary structures that shape and support concrete until it gains sufficient strength to support itself. The formwork must be strong enough to support the weight of the wet concrete, the reinforcement, the construction loads, and the environmental loads without collapse or excessive deflection. The formwork design is governed by ACI 347, which provides the standard requirements for formwork design and construction. The lateral pressure of fresh concrete on vertical formwork depends on the pour rate, the concrete temperature, the concrete weight, and the method of consolidation. The maximum lateral pressure typically ranges from 600 to 1,500 pounds per square foot for normal concrete placement conditions. masonry standards joint committee strength design method. aci 347 formwork lateral pressure design requirements. shoring and reshoring sequence for multistory concrete buildings. The formwork must be designed for the worst combination of pour rate and temperature that could occur during construction, with lower temperatures resulting in higher lateral pressure because the concrete takes longer to set and remains fluid longer.
The design of formwork components includes sheathing, joists, wales, ties, and shores that distribute the concrete pressure and construction loads to the ground or to the previously constructed structure. The sheathing is the surface that contacts the concrete and determines the finish quality. Plywood is the most common sheathing material, with the facing grade selected based on the required surface finish. The joists supporting the sheathing are sized based on the spacing of the wales and the maximum bending moment and shear from the concrete pressure. The wales are horizontal members that distribute the loads from the joists to the form ties. The form ties are tension members that hold the formwork together against the lateral concrete pressure. The tie spacing and capacity must be adequate for the design pressure, with typical tie capacities ranging from 3,000 to 15,000 pounds. The formwork must be braced to resist lateral wind loads and to maintain the plumbness of the formed surfaces.
The shoring system for elevated slabs supports the formwork for the slab and the wet concrete until the slab has gained sufficient strength to support itself and any construction loads applied during construction of upper floors. The shoring design must consider the load from the wet concrete, the formwork weight, the construction live load, and the loads from upper floors that may be shored on top of the freshly placed slab. The reshoring sequence for multi-story buildings involves removing the original shores after the slab gains partial strength and replacing them with reshores that distribute the construction loads from upper floors through multiple slabs below. The number of levels of shores and reshores required depends on the construction schedule, the concrete strength gain rate, and the slab design loads. The shoring removal sequence must be specified by the engineer of record to ensure that the structure is not overloaded during construction.
Construction Scheduling and Resource Management
Resource management in construction scheduling involves allocating the labor, equipment, materials, and money needed to complete the project activities according to the schedule. The resource requirements for each activity are identified during the planning phase, and the total resource demand at each point in time is calculated by aggregating the requirements of all activities that are scheduled to be in progress. The resource histogram shows the resource demand over time and identifies periods where the demand exceeds the available supply. Resource leveling adjusts the schedule to eliminate resource over-allocations by delaying non-critical activities within their available float. Resource smoothing adjusts the schedule to reduce peak resource demands without extending the project duration. The cost-loaded schedule links the project cost estimate to the schedule activities, providing a cost forecast that shows the planned expenditure over time. The S-curve of cumulative planned cost is compared with the actual cost curve to track cost performance.
Material management ensures that the right materials are available at the right time and place in the right quantities without excessive inventory carrying costs. The material procurement cycle includes identifying the material requirements, soliciting quotations from suppliers, placing purchase orders, tracking fabrication and delivery, receiving and inspecting the materials, and storing them until they are needed for installation. The procurement lead time for long-lead items such as structural steel, elevators, and electrical switchgear must be considered in the project schedule, with purchase orders placed early enough to ensure delivery when the materials are needed for installation. The just-in-time delivery approach minimizes on-site inventory by scheduling material deliveries to arrive when they are needed for installation, reducing storage requirements and the risk of theft or damage. The material management system tracks the status of each material order and provides early warning of potential delays that could affect the construction schedule.