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This post was last edited by hesonchang214 on November 2, 2019, at 08:12. Summary of 20 issues in building structure design. This article discusses the basic methods of building structure design as well as the issues that need to be paid attention to during such design processes. It is hoped that this will be helpful to structural designers. I. Regarding the issue of the external corners of the slabs of box and raft foundations: (1) The area of these external corners constitutes a very small proportion of the total area of the foundation slab. It can be changed to a right angle or an oblique angle. (2). If the bottom plate rebar is arranged in two rows in both directions and remains unchanged in the overhanging section, there is no need to add radial rebar at the external corners. (3). The distribution rebar of the one-way slab in the cantilever slab can be changed to a diameter of 12. Don’t underestimate this change – it can save 30,000 to 200,000 yuan on a project. II. Regarding the overhanging slabs of box and raft foundation slabs: (1) From a structural perspective, if overhanging slabs can be designed, it is possible to distribute the reinforcement in the edge-span slabs more evenly. Particularly when the reinforcement in the slab is arranged continuously along its length, this approach prevents the need to increase the amount of continuous reinforcement throughout the entire slab due to the reinforcement in the edge spans, thus resulting in cost savings. (2) After adding a cantilever slab, the additional stress on the foundation can be reduced. When the foundation type lies at the threshold between a natural foundation and other types of artificial foundations, the use of a natural foundation becomes feasible by incorporating such a cantilever slab. Circumferential window wells with larger spans can be added if necessary. (3). It can reduce overall settlement; when the load is eccentric, the use of cantilever slabs at specific locations can also help adjust the differences in settlement and overall inclination. (4). The window well area can be considered as a wall built on a cantilevered slab; it is not advisable to have further long cantilevered slabs. Although in calculations, this slab should not be treated as a cantilever slab. Of course, this rule is not absolute; when there are multiple basement levels, the cross-walls between window wells are relatively numerous, and these cross-walls can be connected to interior walls, then flexible considerations may be applied. (5). When the groundwater level is high, extending the foundation slab helps to address the issue of buoyancy resistance. (6) From an architectural perspective, removing the overhangs facilitates the use of flexible waterproofing methods. When it is a multi-story building, the structure can also yield to the building. III. Regarding the proportion of stirrups in beam reinforcement (approximately 10~20%): For example, in a beam with a span of 8 meters and a cross-section of 400X600, the reinforcement consists of 6 bars of 25 mm diameter at the top, with 1/3 of them truncated, and 5 bars of 25 mm diameter at the bottom; the stirrups are placed more densely within that area. Total amount of longitudinal bars: 3.85*9*8=281kg. Reinforcing bars: 0.395*3.5*50=69; ratio of reinforcing bars to longitudinal bars = 1/4. If the rebar for the two limbs is only 1/8, the proportion of reinforcing bars relative to the longitudinal bars is small, so there is no need to be stingy when using them. Not to mention the need to strongly cut and weakly bend. It already excludes the construction stirrups. IV. Regarding the calculated span of beams and slabs: The calculated spans mentioned in general manuals or textbooks—such as 1.1 times the clear span—are rules and concepts applicable only to conventional structural designs. They are not suitable for wide and flat beams, which are being used increasingly nowadays. In simple terms, a beam-slab structure can be considered as having a rigid support on the center line of the beam; by eliminating the concept of a separate beam, the beam and the slab are treated together as a single member with a variable cross-section. In flat-beam structures, when the beam height is only slightly greater than the slab thickness, the calculated length should be taken as the center of the beam. The bending moment at the beam center together with the beam thickness, as well as the bending moment at the beam edges together with the slab thickness, should be considered; reinforcement should be selected based on the larger of these values. (Borrowing the concept of the variable cross-section at the stepped independent foundation,) columns can also be regarded as beams with extremely large cross-sections; therefore, when designing the reinforcement for such beams, the bending moment at the edge of the column should be considered. Peak shaving is normal; it’s a problem only if peak shaving isn’t done. V. The lap length of the longitudinal bars is several times the diameter d of the steel bars; generally, d is taken as the smaller value of the bar diameters. This is on the premise that the strength of the bars with larger diameters is not fully utilized. Otherwise, the larger value of the rebar diameter should be used. If the longitudinal reinforcement in the columns at the top of a frame structure is sometimes larger than that in the lower layers, a larger diameter for the reinforcement should be used; in some cases, the longitudinal reinforcement should even extend down one more layer. In reality, putting two steel bars together and tying them with wire doesn’t serve much purpose at all; it even reduces the bond strength between the steel bars and the concrete. Therefore, whenever possible, mechanical connections or welding should be used for rebar. VI. The rebar anchorage length is a multiple of the rebar diameter d; this is a requirement under the premise that the strength of the rebar is fully utilized. When the strength of the rebar is not fully utilized—such as in small projections on beams where longitudinal rebars are used, or at the ends of horizontal rebars in shear walls—the anchorage length may be reduced. For example, for the ends of horizontal reinforcement bars in shear walls, it is sufficient to have a straight hook length of 10d. 7. The cost of columns is very low in frame structures; however, they play a decisive role in earthquake resistance. Experimental results show that, when spatial effects are taken into account, the longitudinal reinforcement in columns must be increased to about 2.5 times the calculated value in order to ensure that plastic hinges do not form in the columns. It is not necessary to follow the calculated reinforcement requirements; the longitudinal reinforcement can be increased significantly, along with the stirrups. VIII. Seismic joints should be enlarged; statistics show that 40% of the seismic joints installed in accordance with regulatory requirements collided during earthquakes. Therefore, the spacing of seismic joints should be increased. IX. Anchorage lap: For example, at the middle column joints, the longitudinal reinforcement beneath the frame beams is anchored into the column at the LAE length; the lap length is 2*LAE – column width, such as when the reinforcement diameter is 25. LAE=40D, column width 500; 2*25*40-500=1500, which is the lap length. This value of 1500 is much greater than 1.2*LAE=1200. For columns with varying cross-sections, if the cross-sectional areas of the upper and lower columns differ by 50, and the upper column is anchored into the lower column by 40D, lap is considered in this case as part of the anchoring mechanism. X. Regarding rebound and re-compression: During the excavation of the foundation pit, the soil at the edge of the pit within the friction angle range is constrained and does not rebound; whereas the soil at the center of the pit rebounds. The rebound is primarily elastic in nature, and the rebounded material is removed manually. When the foundation is small and the bottom of the pit is under significant restraint, such as in the case of isolated foundations, the rebound effect can be ignored; when calculating settlement, the additional stress at the base should be taken into account. When the foundation pit is large, it is relatively less constrained. In the case of a box foundation, settlement should be calculated based on the base pressure, with the portion constrained by the soil at the edges of the pit considered as a safety margin; this is one of the reasons why the calculated settlement is higher than the actual settlement. 11. It is generally believed that for column footings, when the stiffness is high and there is not much difference between the axial force on the columns and their spans, they can be calculated as a slabs-on-grade structure. In practice, most of it can be calculated based on the floor slab. That is, a modified inverted roof slab is used. First, calculate the continuous beam based on the average reaction forces. Then, balance the resulting support reactions with the axial forces of the columns; add the positive values of this difference to the 1/3 portion of the beam on each side of the column, and add the negative values to the 1/3 portion of the beam at the mid-span. Relatively speaking, the compressive stress in the 1/3 portion at the mid-span is lower. Multiple revisions may be required until the support reactions are in equilibrium with the column axial forces. 12. Where the main beam is accompanied by secondary beams, additional reinforcement should generally be added in the form of stirrups. These additional stirrups can be considered as a way to compensate for the inability to place stirrups within the cross-section of the secondary beam or when there is a shortage of stirrups; they are added on both sides of the secondary beam, similar to the additional reinforcement used for openings in slabs. Additional reinforcement is generally required, but it is not absolute. The specifications state clearly that concentrated loads located below the beam or within the height range of the beam cross-section should be entirely carried by additional transverse reinforcement. In other words, concentrated forces located on the beam, such as columns on the beam, or beams behind the beam, such as the bearing beam under the water tank, do not require additional reinforcement. Additional reinforcement should be added to the concentrated force at the bottom of the beam. However, the concentrated load within the height range of the beam section can be determined based on specific circumstances. When the cross-sectional areas of the main and secondary beams are not significantly different and the load on the secondary beams is high, additional reinforcement should be added. When the main beam is very tall, the secondary beam has a small cross-section and the loads are low, such as when it is close to an additional hidden beam on the slab, the main beam may not require additional reinforcement. Also, when both the primary and secondary beam sections are very large, such as those formed due to process requirements, and the loads are relatively low, the primary beam may not require additional reinforcement. As a general principle, when the main beam cracks due to the secondary beam, if the concrete and stirrups in the section from the top of the compressed zone of the secondary beam to the bottom of the main beam are capable of withstanding the shear force generated by the secondary beam, no additional reinforcement is required for the main beam. For a concentrated force on the beam, the shear force generated is the same throughout the entire length of the beam; thus, the shear resistance requirement is satisfied, and it is naturally met at the location of the concentrated force. For primary and secondary deep beams as well as secondary beams, when the cross-sections and loads are small relative to those of the primary beams, this requirement can also be met. 13. Under normal circumstances, cantilever beams should have a constant cross-section, especially when the overhang length is short. Unlike cantilever slabs, the self-weight of cantilever beams accounts for only a small proportion of the total load, and making them of variable cross-section does not effectively reduce their self-weight. The stirrups of the variable-section cantilever beams are all different, which increases the difficulty of construction. The deflection of beams with variable cross-sections is also greater than that of beams with constant cross-sections. Of course, except for those with exposed main beams. Exposed large beams have a better visual effect when their cross-section is varied appropriately. 14. Cast-in-place slabs should generally be designed as two-way slabs. Firstly, double-sided panels have multiple support edges, resulting in good seismic stability; even if two sides collapse, there are still the other two sides. When a one-way slab collapses, one of its panels falls down. Secondly, from a computational standpoint, for two-way slabs—for example, those with simply supported edges on all four sides—the mid-span bending moment coefficient is approximately 1/27. For one-way slabs with simply supported edges on two opposite sides, this coefficient is 1/8. The ratio between the two is 2 * (1/27) / (1/8), which is roughly 60%. Structurally, the thickness of double-directional slabs ranges from 1/40 to 50, while that of single-directional slabs ranges from 1/3 to 40. Double-directional slabs are thin; furthermore, even in the case of single-directional slabs, structural reinforcement must be provided on their non-load-bearing edges. 15. Beam pads: To reduce the additional bending moment exerted on brick walls due to the eccentricity of the support reactions, beam pads with internal notches can be used. 16. Specifications for the diameter of rebar in slabs: It is generally believed that when the diameter of the upper rebar in a slab is 8 mm or more, it can prevent bending due to stepping on it during construction; however, based on on-site experience, only rebar with a diameter of 12 mm or more can ensure this. 17. For cast-in-place balcony balustrades, in terms of construction requirements, the slab thickness should be greater than 80 mm when using a single row of reinforcement, and greater than 120 mm when using two rows of reinforcement. Since the minimum length of the vibration rod is 30, when arranging the rebar in a single row and the slab thickness is 60, with rebar diameters of 8+6 on both sides, only 23 remains on each side of the rebar, which is not sufficient for vibration. 18. When a room uses double-direction cross-beams, the slab should have its overall bending moment taken into account. That is, for the small panels in the 4 corners separated by the cross-beams of the grid structure, the negative reinforcement should be considered to be cut according to the depth of the room’s bay, rather than merely based on the dimensions of that small panel. That is, the secondary beam is considered merely as a stiffening rib for the large slab. 19. When most of the rooms in a building are small with only one or two larger rooms, determining the thickness of the foundation slab based on the larger rooms would result in waste, while doing so based on the smaller rooms would make it difficult to arrange the reinforcement properly. When the load-bearing capacity meets the requirements, polypropylene insulation can be placed in the center of the larger rooms to reduce the load, and the thickness of the foundation slab can then be determined based on the smaller rooms. 20. The deflection at the end of a cantilever beam does not depend solely on its own deformation; the effect of inward collapse of its supports is likely to exceed the beam’s own deformation.