A multistory office building in Los Angeles, California 3. All structures are analyzed using three-dimensional static or dynamic methods. Specification for Structural Steel Buildings. Manual of Steel Construction, 13th Edition. Seismic Design Manual. Second Edition.
Extended End- Plate Moment Connections, Steel Deck Institute. The symbols used in this chapter are from Chapter 11 of the Standard, the above referenced documents, or are as defined in the text. Customary units are used. It includes a foot-high, foot-wide mezzanine area at the east end of the building.
The structure consists of 10 gable frames spanning 90 feet in the transverse north-south direction. Spaced at 20 feet on center, these frames are braced in the longitudinal east-west direction in two bays at the east end. The building is enclosed by nonstructural insulated concrete wall panels and is roofed with steel decking covered with insulation and roofing. Columns are supported on spread footings. The elevation and transverse sections of the structure are shown in Figure 6.
Longitudinal struts at the eaves and at the mezzanine level run the full length of the building and therefore act as collectors for the distribution of forces resisted by the diagonally braced bays and as weak-axis stability bracing for the moment frame columns. The roof and mezzanine framing plans are shown in Figure 6. The framing consists of a steel roof deck supported by joists between transverse gable frames.
The mezzanine represents both an additional load and additional strength and stiffness. Because all the frames resist lateral loading, the steel deck functions as a diaphragm for distribution of the effects of eccentric loading caused by the mezzanine floor when the building is subjected to loads acting in the transverse direction. The mezzanine floor at the east end of the building is designed to accommodate a live load of psf. Its structural system is composed of a concrete slab over steel decking supported by floor beams spaced at 10 feet on center. The floor beams are supported on girders continuous over two intermediate columns spaced approximately 30 feet apart and are attached to the gable frames at each end.
Vertical deflections due to snow were limited to 3. The panels are attached with long pins perpendicular to the concrete surface. These slender, flexible pins isolate the panels from acting as shear walls.
The building is supported on spread footings based on moderately deep alluvial deposits i. The foundation plan is shown in Figure 6. Transverse ties are placed between the footings of the two columns of each moment frame to provide restraint against horizontal thrust from the moment frames. Grade beams carrying the enclosing panels serve as ties in the longitudinal direction as well as across the end walls. The design of footings and columns in the braced bays requires consideration of combined seismic loadings. The design of foundations is not included here.
Embed in symmetrical about center thickened slab 2 line Mezzanine 90'-0" 30'-0" 6'-6"x6'-8"x 6" concrete slab 1'-4" footings with 6x6-W1. See Section 3. For this example the parameters are as follows. Intermediate steel moment frames with stiffened bolted end plates and ordinary steel concentrically braced frames are used in this example. For determination of the seismic weights, the weight of the mezzanine will include the dead load plus 25 percent of the storage load psf in accordance with Standard Section Because there is a mezzanine at one end, vertical weight irregularities might be considered to apply Standard Sec.
There also are no plan irregularities in this building Standard Sec. In the N-S direction, the moment frames do not meet the requirements of Standard Section Thus, Standard Section A copy of the three-dimensional model is made, with the moment frame beam at Gridline A pinned. The structure is checked to make sure that an extreme torsional irregularity Standard Table Thus, the structure does not have an extreme torsional irregularity when a frame loses moment resistance.
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Additionally, the structure must be checked in the N-S direction to ensure that the loss of moment resistance at Beam A has not resulted in more than a 33 percent reduction in story strength. This can be checked using elastic methods based on first yield as shown below, or using strength methods.
The original model is run with the N-S load combinations to determine the member with the highest demand- capacity ratio. A combination of percent seismic forces in one direction plus 30 percent seismic forces in the orthogonal direction must be applied to the columns of this structure in Seismic Design Category D Standard Sec. The effect of seismic load Standard Sec. The seismic load is combined with the gravity loads as shown in Standard Sec.
Footnote c in Standard Table See Section 6. The main frame of the building can be considered to be a one-story building for this purpose, given that there are no interior partitions except below the mezzanine. The definition of a story in building codes generally does not require that a mezzanine be considered a story unless its area exceeds one-third the area of the room or space in which it is placed; this mezzanine is less than one-third of the footprint of the building.
The computed seismic weight is based on the assumption that the wall panels offer no shear resistance for the structure but are self-supporting when the load is parallel to the wall of which the panels are a part. Additionally, snow load does not need to be included in the seismic weight per Standard Section In the longitudinal direction where stiffness is provided only by the diagonal bracing, the approximate period is computed using Standard Equation In accordance with Standard Section For purposes of determining the required base shear strength, Tmax will be used in accordance with the Standard; drift will be calculated using the period from the model.
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In the transverse direction where stiffness is provided by moment-resisting frames Standard Eq. As in the longitudinal direction, Tmax will be used for determining the required base shear strength. The seismic response coefficient Cs is computed in accordance with Standard Section In the transverse direction: S DS 1. In both directions the value of Cs exceeds the minimum value Standard Eq.
Even though the building is considered to be one story for some purposes, it is clearly a two-level structure. Using the data in Section 6. Table 6. For this example, a three-dimensional model was created in ETABS including frame and diaphragm stiffness.
The tapered members are approximated as short, discretized prismatic segments. The collector at the knee level is included, as are those at the mezzanine level in the two east bays. The mezzanine diaphragm is modeled using planar shell elements with their in-plane rigidity being based on actual properties and dimensions of the slab.
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The roof diaphragm also is modeled using planar shell elements, but their in-plane rigidity is based on a reduced thickness that accounts for compression buckling phenomena and for the fact that the edges of the roof diaphragm panels are not connected to the wall panels. The analytical model includes elements with one-tenth the stiffness of a plane plate of 22 gauge steel.
The ELF analysis of the three-dimensional model in the transverse direction yields an important result: the roof diaphragm behaves as a rigid diaphragm. Accidental torsion is applied at the center of the roof as a moment whose magnitude is the roof lateral force multiplied by 5 percent of feet 9 feet.
A moment is also applied to the mezzanine level in a similar fashion. The resulting displacements are shown in Table 6. The displacement at the centroid of the roof is 4. Thus, the deviation of the diaphragm from a straight line is 0. Clearly, then, the diaphragm flexibility is negligible and the deck behaves as a rigid diaphragm. The ratio of maximum to average displacement is 1.
The same process needs to be repeated for the E-W direction. The demands from the three-dimensional ELF analysis are combined to meet the orthogonal combination requirement of Standard Section However, there is no story drift limit for single-story structures with interior wall, partitions, ceilings and exterior wall systems that have been designed to accommodate the story drifts. Detailing for this type of design may be problematic.
In the longitudinal direction, the lateral deflection is much smaller and is within the limits of Standard Section The deflection computations do not include the redundancy factor. The P-delta effects on the structure may be neglected in analysis if the provisions of Standard Section First, the stability coefficient maximum should be determined using Standard Equation The stability coefficient is calculated at both the roof and mezzanine levels in both orthogonal directions.
The maximum moments and axial forces caused by dead, live and earthquake loads on the gable frames are listed in Tables 6. The moments are given in Table 6. The moment diagram for the combined load condition is shown in Figure 6. The load combination is 1. The size of the members is controlled by gravity loads, not seismic loads.
The design of connections will be controlled by the seismic loads. Forces in the design of the braces are discussed in Section 6. Individual maxima are not necessarily on the same frame; combined load values are maximum for any frame.
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Using the load combinations presented in Section 6. The mezzanine framing, also shown in Figure 6. The diagonal bracing, shown in Figure 6. AISC Table 6. As Designed in. Maximum in. According to Standard Section All P-M ratios combined compression and flexure are less than 1. This is based on proper spacing of lateral bracing. Lateral bracing is provided by the roof joists.
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The maximum spacing of lateral bracing is determined using beam properties at the ends and AISC , Section F , whichever is greater, in. Adjacent to the plastic hinge regions, lateral bracing must have additional strength as defined in AISC Instead, tube brace members will be used, but they are not analyzed in this example. At the negative moment regions near the knee, lateral bracing is necessary on the bottom flange of the beams and inside the flanges of the columns Figure 6. The knee detail is shown in Figures 6.
The vertical plate shown near the upper left corner in Figure 6. The beam-to-column connection requires special consideration. The method of AISC for bolted, stiffened end plate connections is used. Refer to Figure 6. Highlights from this method are shown for this portion of the example. Refer to AISC for a discussion of the entire procedure.
Determine the maximum moment at the plastic hinge location. Find bolt size for end plates. Try A bolts. See Figure 6. Use 1 in. Step 3. Use 1. Step 4. Step 5. OK Step 6. Column flange of 2 inches is OK. Step 9. Check local column web yielding strength of the unstiffened column web at the beam flanges by AISC Equations 6. Column stiffeners need to be provided. Step Check the unstiffened column web buckling strength at the beam compression flange by AISC Equations 6.
Check the unstiffened column web crippling strength at the beam compression flange by AISC Equation In compression, the continuity plate will be designed to take the full force delivered by the beam flange, Fsu. In tension, however, the compressive limit states web buckling and web yielding are not applicable and column web yielding will control the design instead. As it will be shown later, net section rupture not gross yielding will control the design of this plate. Strength in the other direction does not need to be checked because the cruciform section will not buckle in the plane of the column web.
The continuity plate had been previously sized for adequacy to tensile yielding of the gross section. The critical section will be analyzed where the continuity plates are clipped adjacent to the k-region of the column. Although doubler plates can be added to the panel zone to increase strength, this may be an expensive solution. For simplicity, these changes are not undertaken in this example. The ridge joint detail is shown in Figure 6. An unstiffened bolted connection plate is selected. Lateral seismic forces produce no moment at the ridge until yielding takes place at one of the knees.
Vertical accelerations on the dead load do produce a moment at this point; however, the value is small compared to all other moments and does not appear to be a concern. Once seismic loads produce a plastic hinge at one knee, further lateral displacement produces positive moment at the ridge. Under the condition on which the AISC design is based a full plastic moment is produced at each knee , the moment at the ridge will simply be the static moment from the gravity loads less the horizontal thrust times the rise from knee to ridge.
Analyzing this frame under the gravity load case 1. The design of the framing for the mezzanine floor at the east end of the building is controlled by gravity loads. The concrete-filled 3-inch, gauge steel deck of the mezzanine floor is supported on steel beams spaced at 10 feet and spanning 20 feet Figure 6.
The steel beams rest on three-span girders connected at each end to the portal frames and supported on two intermediate columns Figure 6. The girder spans are approximately 30 feet each. Those lateral forces that are received by the mezzanine are distributed to the frames and diagonal bracing via the floor diaphragm. A typical beam-column connection at the mezzanine level is provided in Figure 6. The design of the end plate connection is similar to that at the knee, but simpler because the beam is horizontal and not tapered. Also note that demands on the end-plate connection will be less because this connection is not at the end of the column.
The strength of the members and connections, including the columns in this area but excluding the brace connections, must be based on Standard Section For simplicity, we can assume that the lateral force is equally divided among the roof level braces and is slightly amplified to account for torsional effects. Demand will be taken as either the expected yield strength of the brace or the amplified seismic load. In ordered to calculate U, the weld length along the double angles needs to be determined.
The eave strut, part of the braced frame, also acts as a collector element and must be designed using the overstrength factor per Standard Section Figure 6. There are deviations from simple approximations in both directions. In the E-W direction, the base shear is kips Section 6. The plot shows that the shear in the edge of the diaphragm is significantly higher in the two braced bays.
In the N-S direction, the shear is generally highest in the bay between the mezzanine frame and the first frame without the mezzanine. This is expected given the significant change in stiffness. There is no simple approximation to estimate the shear here without a three-dimensional model. The shear is also high at the longitudinal braced bays because they tend to resist the horizontal torsion.
However, the shear at the braced bays is lower than observed for the E-W motion. This seven-story office building of rectangular plan configuration is feet, 4 inches long in the E-W direction and feet, 4 inches wide in the N-S direction Figure 6. The building has a penthouse. It is framed in structural steel with foot bays in each direction. The typical story height is 13 feet, 4 inches; the first story is 22 feet, 4 inches high. The penthouse extends 16 feet above the roof level of the building and covers the area bounded by Gridlines C, F, 2 5 in Figure 6.
The elevators and stairs are located in the central three bays. There are five bays of moment frames on each line. The braced frames are in a two-story X configuration. The frames are identical in brace size and configuration, but there are some minor differences in beam and column sizes. Braced frame elevations are shown in Figures 6.
The building has no vertical irregularities despite the relatively tall height of the first story. The exception of Standard Section In the three-dimensional analysis, the first story drift ratio is less than percent of that for the story above. Because the building is symmetrical in plan, plan irregularities would not be expected. Analysis reveals that Alternative B is torsionally irregular, which is not uncommon for core-braced buildings. A combination of percent of the seismic forces in one direction with 30 percent of the seismic forces in the orthogonal direction is required for structures in Seismic Design Category D for certain elements—namely, the shared columns in the SCBF Standard Sec.
In using modal response spectrum analysis MRSA , the bidirectional case is handled by using the square root of the sum of the squares SRSS of the orthogonal spectra. Steel frames are used in many commercial high-rise buildings, as well as industrial structures, such as ore mines and oilrigs. Enabling construction of ever lighter and safer structures, steel frames have become an important topic for engineers.
Important features of the this book include: fundamental equations governing the elastic and elasto-plastic equilibrium of beam, sheer-beam, column, joint-panel, and brace elements for steel frames; analysis of elastic buckling, elasto-plastic capacity and earthquake-excited behaviour of steel frames; background knowledge of more precise analysis and safer design of steel frames against gravity and wind, as well as key discussions on seismic analysis.
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That same year he started working at the University as a lecturer in Structural Engineering, and over the next six years he worked his way up to Associate Professor, and then Professor in His research interests lie mainly in the behavior and design of multi-storey steel buildings, the fire-resistance of steel structures and the dynamic identification of structures. He is an active member on the Editorial board of five International journals covering areas of research in steel and composite structures, structural engineering and materials, computational structural engineering, and advanced steel construction.
He is the author of four books in Chinese, and over eighty research papers. The main parameters are consider deflection, shear force, bending moment, axial force. The investigation should include, but not be limited to the following: Read More. LRFD Design example for steel girder superstructure design example includes detailed design computations for the following bridge featuresconc rete deck, steel plate girder, bolted field splice, shear connectors, bearing stiffeners, welded connections, elastomeric bearing, cantilever abutment and Read More.
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