Such combinations of axial load and moment are referred to as large moment cases. The design of plates with large moments is outlined in Section 3. Equation 3. Therefore, bearing at the interface will govern the design of the base plate thickness. Determine the axial load and moment. To determine the plate thickness, equate the right-hand sides of Equations 3. Determine the bearing length, Y. Determine the required minimum base plate thickness tp req.
Determine the anchor rod size. To determine the required thickness, substitute n for m in Equations 3. This is a common situation for rigid frames designed to resist lateral earthquake or wind loadings and is schematically presented in Figure 3. To calculate the total concrete bearing force and the anchor rod forces, consider the force diagram shown in Figure 3. Also, the summation of moments taken about the point B must equal zero.
If the expression in Equation 3. Substitution of the critical value of e from Equation 3. The concrete bearing force is given by the product qmaxY.
The anchor rod tensile force, T, is obtained by solving Equation 3. For certain force, moment, and geometry combinations, a real solution of Equation 3. In that case, an increase in plate dimensions is required. In particular, only if the following holds. Base plate with large moment. Note: When n is larger than m, the thickness will be governed by n. Alternately the bending lines could be assumed as shown in Figure 3. There are three principal ways of transferring shear from column base plates into concrete: 1.
Friction between the base plate and the grout or concrete surface. The available strength per unit length for the plate is given in Equation 3. Setting that strength equal to the applied moment given by Equation 3.
Determine the required minimum base plate thickness tp req at bearing and tension interfaces. Choose the larger value. The contribution of the shear should be based on the most unfavorable arrangement of factored compressive loads, Pu, that is consistent with the lateral force being evaluated, Vu.
Shear forces can be transferred in bearing by the use of shear lugs or by embedding the column in the foundation. These methods are illustrated in Figure 3. The commentary to ACI suggests this mechanism is developed as follows: 1.
Shear then progresses into a shear-friction mode. Pa is positive for tension and negative for compression. When Pa is negative, one must verify that Pa will actually be present while the shear force is occurring. Additional comments related to the use of shear lugs are provided here: 1.
This criterion may control or limit the shear capacity of the shear lug or column embedment details in concrete piers. Consideration should be given to bending in the base plate resulting from forces in the shear lug. This can be of special concern when the base shears most likely due to bracing forces are large and bending from the force on the shear lug is about the weak axis of the column.
As a rule of thumb, the authors generally require the base plate to be of equal or greater thickness than the shear lug. Multiple shear lugs may be used to resist large shear forces.
Appendix B of ACI provides criteria for the design and spacing of multiple shear lugs. The design of a shear lug is illustrated in Example 4. Particular attention must be paid to the manner in which the force is transferred from the base plate to the anchor rods. The effects of this slip must be evaluated by the engineer. The reader is also cautioned that, due to placement tolerances, not all of the anchor rods will receive the same force. The authors recommend a cautious approach, such as using only two of the anchor rods to transfer the shear, unless special provisions are made to equalize the load to all anchor rods Fisher, Lateral forces can be transferred equally to all anchor rods, or to selective anchor rods, by using a plate washer welded to the base plate between the anchor rod nut and the top of the base plate.
The plate washers should have holes z in. It cannot be emphasized enough that the use of shear in the anchor rods requires attention in the design process to the construction issues associated with column bases.
Once the shear is delivered to the anchor rods, the shear must be transferred into the concrete. If plate washers are used to transfer shear to the rods, some bending of the anchor rods can be expected within the thickness of the base plate.
If only two anchor rods are used for shear transfer, as suggested earlier, the shear is transferred within the base plate, and bending of the rods can be neglected. Based on shear friction theory, no bending of the anchor rod within the grout need be considered. The moment in the anchor rods can be determined by assuming reverse curvature bending.
The lever arm can be taken as the half distance between the center of bearing of the plate washer to the top of the grout surface. Where anchors are used with a built-up grout pad, ACI requires that the anchor capacity be multiplied by 0.
Limitations on grout pad thicknesses are not provided. For the typical cast-in-place anchor group used in building construction, the shear capacity determined by concrete breakout as illustrated in Figure 3. If the edge distance c1 is large enough, then the anchor rod shear strength will govern. The nominal shear strength of a single anchor rod equals 0. In evaluating the concrete breakout strength, the breakout either from the most deeply embedded anchors or breakout on the anchors closer to the edge should be checked.
When breakout is being determined on the inner two anchors those farthest from the concrete edge the outer two anchors those closest to the concrete edge should be considered to carry the same load. When the concrete breakout is considered from the outer two anchors, all of shear is to be taken by the outer anchors. Shown in Figure 3. In many cases it is necessary to use reinforcement to anchor the breakout cone to achieve the shear strength as well as the ductility desired.
Ties placed atop piers as required in Section 7. In addition to the concrete breakout strength, ACI also contains provisions for a limit state called pryout strength. The authors have checked several common situations and have not found pryout strength to control for typical anchor rod designs.
The reader is referred to ACI for further explanation. The column base shears are transferred from the anchor rods to the hairpin as shown in Figure 3. Problems have occurred with the eccentricity between the base plate and the hairpin due to bending in the anchor rods after the friction capacity is exceeded.
This problem can be avoided as shown in Figure 3. In addition, a vapor barrier should not be used under the slab. Figure 3. Concrete breakout surfaces for group anchors. Tie rods continuous rods that run through the slab to the opposite column line are typically used to counteract large shear forces associated with gravity loads on rigid frame structures. When using tie rods with large clear span rigid frames, consideration should be given to elongation of the tie rods and to the impact of these elongations on the frame analysis and design.
Tie rods and hair pin bars should be placed as close to the top surface of the concrete slab as concrete cover requirements allow. The required strength due to axial loads. Since no anchor rod forces exist, the anchor rod size can be determined based on the OSHA requirements, and practical considerations. Calculate the required axial compressive strength.
Calculate A2 geometrically similar to A1. Based on the in. Use trial and error solution. Anchor Rod Calculate the available tensile strength of a w-in. Note that the break out strength is theoretically independent of the size of the anchor rod. This embedment at only 6 in. As discussed in Section 3. In this case, the pullout strength shown in Table 3.
Procedure: 1. Determine the required strength due to uplift on the column. Select the type and number of anchor rods. Determine the appropriate base plate thickness and welding to transfer the uplift forces from the column to the anchor rods. Determine the method for developing the anchor rods in the concrete in the spread footing. Reevaluate the anchorage if the column is on a in.
The required moment strength of the base plate equals the rod force times the lever arm to the column web face. Use four anchor rods minimum per OSHA requirements. Prying forces are negligible. To simplify the analysis, conservatively assume the tensile loads in the anchor rods generate one-way bending in the base plate about the web of the column. This assumption is illustrated by the assumed bending lines shown in Figure 4. Minimum weld for 0. Table J2. Nominal weld strength per inch for a x-in.
Thus, a in. Therefore, use a heavy hex nut to develop the anchor rod. The pullout strength of a d-in. As noted earlier, this column is anchored in the middle of a large spread footing. Therefore, there are no edge constraints on the concrete tensile cones and there is no concern regarding edge distance to prevent lateral breakout of the concrete. Try using a in. As mentioned earlier in this Guide, the use of hooked anchor rods is generally not recommended.
Their use here is to demonstrate their limited pull out strength. Note that no equivalent ASD solution exists for concrete bearing capacity. With the d-in. If the anchors were installed in a in. With an 8-in. Thus, it is necessary to transfer the anchor load to the vertical reinforcing steel in the pier.
Using a Class B splice factor with a 1. Select in. Determine bearing length, Y. Verify bearing pressure: LRFD. Determine minimum plate thickness. Determine e and ecrit; check inequality in Equation 3.
ASD 0. Therefore, this is the case of base plate with large moment. Check the inequality of Equation 3. Since no anchor rod forces exist, the anchor rod size can be determined based on the OSHA requirements and practical considerations. Use four w-in. Assume that the anchor rod edge distance is 1.
Compute the required strength. Hence, a larger plate dimension Hence, a larger plate dimension is required. The new values become. The eccentricity, e, still exceeds The eccentricity, e, still exceeds ecrit; ecrit; therefore, the load combination therefore, the load combination is for is for large moments. Also: large moments. If three anchor rods are used on each face of the column, the force per rod equals 52 kips. From Table 3. The recommended hole size for the in.
AISC, Using an edge distance to the center of the hole of 24 in. The pullout strength of each anchor rod with a heavy hex nut is selected from Table 3. For completeness determine the embedment length for the anchor rods. Try 18 in. For moderate or high seismic risk, in ACI indicates that the strength of anchors is to be multiplied by 0. In this case, the steel strength would be 0. Larger anchor rods would be required.
See Figure 4. The anchor rods in this example are designed only to transfer the net uplift from the column to the pier. Determine the required embedment for the lug into the concrete pier.
Determine the appropriate thickness for the lug. Size the welds between the lug and the base plate. Solution: 1. Two criteria are used to determine the appropriate embedment for the lug. These criteria are the bearing strength of the concrete and the shear strength of the concrete in front of the lug.
The shear strength of the concrete in front of the lug is evaluated in ultimate strength terms as a. The base plate is 15 in. The design shear strength in bearing on the base plate edge per ACI is 0. The bearing area of the lug is to be excluded from the projected area.
Since this criterion is expressed in ultimate strength terms, the bearing strength of the concrete is also evaluated with an ultimate strength approach. The ultimate bearing strength of the concrete in contact with the lug is evaluated as 0. Since the anchor rods are sized for only the required uplift tension, the 1. Using this embedment, the shear strength of the concrete in front of the lug is checked. The projected area of the failure plane at the face of the pier is shown in Figure 4.
Assuming the lug is positioned in the middle of the pier and the lug is 1 in. Based on the discussion in Section 3. Consider c-in. For a c-in. Use a-in.
A 4-in. The concrete strength is 4, psi. Wind shear force is 23 kips, therefore, the required shear strength is LRFD. A total of four anchor rods is used. Use plate washers welded to the top of the base plate to transfer the shear to all four anchor rods.
Try four in. For combined shear and tension the anchor rods must meet the AISC provision. Tensile stress: The tensile stress in the rods comes from two sources: 1. The bending moment in each rod equals the shear per rod times the half distance from the center of the plate washer to the top of the grout. Determine the plate washer thickness: The bearing force per rod is.
Thus, the lever arm can be taken as one-half of the sum of the base plate thickness and 0. As a matter of interest, assume that welded washers are not provided. It should be noted that a slip of w in. Check the in. Rather than using the 0. DeWolf, J. Drake, R. Fisher, J. Frank, K. ST, June. Kaczinski, M. Koenigs, M. Thornton, W. Till, R. Wald, F. Design Requirements Anchor rods are sometimes used in special applications that require special design details, such as anchor rods designed without a grout base double-nut anchor rods , anchor rods in sleeves, pretensioned applications, and special moment bases or stools.
Double-nut anchor rods, are different from building column anchor rods that may use a setting nut but are not designed for compression in the completed structure. Doublenut joints are very stiff and reliable for transmitting moment to the foundation. Studies have shown that pretension in the rod between the two nuts improves fatigue strength and ensures good load distribution among the anchor rods Frank, ; Kaczinski et al. The base plates of light and sign standards are not grouted after erection, and the rod carries the all of the structural load.
The anchor rods must be designed for tension, compression, and shear, and the foundation must be designed to receive these loads from the anchor rods. Machinery bases and certain columns may require very close alignment of the anchor rods. The anchorage at the bottom of the rod must be designed to span the sleeve and develop the required bearing on the concrete. Often machinery, process equipment, and certain building columns may be subject to vibration or cyclical loads, which may in turn subject the anchor rod to fatigue.
Pretensioning the rod can improve its fatigue life, but anchor rods can effectively be pretensioned only against steel. Even when tensioning a Grade 55 rod 24 in. Thus, it is recommended, when it is necessary to pretension anchor rods, that a steel sleeve be used that is adequate to transfer the anchor rod pretension from the anchor plate to the base plate.
See Figure A1. Large mill building columns that have to be set accurately and have large moments at the base can be designed using a stool-type detail as shown in Figure A1. The advantage of this type of detail is that the base plate can be set in advance using large oversized holes.
If the column and base plate are over 2 in. The use of the stool has the added advantage that the extended anchor rod length will allow easier adjustment to meet the holes in the stool cap plate.
Yielding could initiate at lower load levels on the reduced area of the threads, but it is assumed that the consequences of this yielding would be relatively minor. Headed anchor rods transfer the compressive force to the concrete by bearing of the head, and deformed bars transfer the compressive force to the concrete along their length. The compressive strength of the anchor rod due to concrete failure should be calculated using the American Concrete Institute ACI criteria.
When the maximum fatigue stress range is less than the threshold fatigue stress range, 7 ksi, anchor rods need not to be further checked for fatigue. Four anchor rod joints are of low cost and suitable for small sign, signal, and light supports and other miscellaneous structures.
In other cases, although only four anchor rods may be required for strength, there should ideally be at least six and preferably eight anchor rods in a joint in a nonredundant structure subject to fatigue. There is a trend toward using fewer very large anchor rods in high-demand dynamically loaded structures. This gives the column base plate connection some measure of redundancy, even if the structure is nonredundant.
For circular patterns of six or more double-nut anchor rods, testing has shown that the thickness of the base plate must at least equal or exceed the diameter of the anchor rods, and also that the bending in the anchor rod is negligible when the distance between the bottom of the leveling nut and the top of the concrete is less than the anchor rod diameter Kaczinski et al. However, tests on four anchor rod patterns. In column-base-plate connections subject to fatigue, the anchor rod will fail before the concrete fatigue strength is reached.
Therefore, it is not necessary to consider the fatigue strength of the concrete. Corrosion protection is particularly important for fatiguecritical anchor rods, since corrosion pitting can degrade the fatigue resistance. Stresses in anchor rods for fatigue analysis should be based on elastic distribution of service loads.
The tensile stress area should be used in the computation of stresses in threaded anchors. The stress range should be calculated including the external load range due to repeated live loads and any prying action due to those loads.
The bending stress range should be added to the axial stress range to determine the total stress range to check for fatigue. In the case of anchor rods, 7 ksi is the threshold associated with Category D. This design would be tolerant of limited misalignment up to Since the fatigue resistance of various grades of anchor rod is the same, it is not advantageous to use grades higher than Grade 55 in fatigue applications.
The fracture toughness of the higher grades is generally somewhat less. For all types of joints, the entire force range is assumed to be applied to the anchor rods, even if they are pretensioned. Bending of the anchor rods need not be considered, with the exception of double-nut joints when there are only four anchor rods or when the clear distance between the bottom of the leveling nut and the concrete exceeds the diameter of the anchor rods.
In cases where the bending stress range must be calculated, the minimum bending moment is the shear force in the anchor rod times the distance between the bottom of the base plate and the top of concrete. Shear forces may be ignored for purposes of calculating the fatigue effect, even if they act in combination with the axial forces. The entire range in stress must be included, even if during part of the cycle the stress is in compression.
In the case of a load reversal, the stress range in an individual anchor rod is computed as the algebraic difference between the peak stress due to the live load applied in one direction and the peak stress due to the live load applied in the other direction.
If the base plate thickness is less than the diameter of the anchor rods, the applied stress ranges should include any additional tension resulting from prying action produced by the unfactored live load. The applied stress range is computed by dividing the axial force ranges by the tensile stress area.
If bending of the anchor rods is included in the analysis, the bending stress range must be added to the stress range from the axial forces from a consistent load case.
No further evaluation of fatigue resistance is required if the stress in the anchor rod remains in compression during the entire cycle including the minimum dead load , or if the stress range is less than the threshold stress range, FTH.
For the 1-in. This observation is consistent with crack initiation locations observed in cracked towers. In these tests, a socket joint detail with a 2-in. To assess the relative effect of base plate thickness, longitudinal stresses on the outer surface from the model are compared in Figure A1.
It can be seen that, for a base plate 24 in. For a base plate 3 in. Proper installation is usually the responsibility of the Contractor.
However, the Engineer of Record, or their representative, may witness the inspection and testing. As shown below, 3 in.
Finite-element analyses illustrate the effect of base plate thickness. In the model generated by the authors, the base plate thickness was varied from 1 to 6 in. Obviously, a 6-in. In any anchor rod installation there will be some amount of misalignment. Provisions should be made to minimize misalignments and to meet required tolerances. The best way to maintain alignment is the use of a template.
Templates comprising rings with nuts on both sides at two locations along the length of the anchor rods are recommended. Vibratory machine joints and double-nut joints designed for Seismic Design Category D or greater, according to ASCE 7, or designed for fatigue as described herein, require pretensioning.
Failure to follow the nut tightening procedure can lead to inadequately pretensioned anchor rods and associated uneven distribution of loads among the contributing anchor rods. Inadequately tightened bolts can also lead to fatigue failures and further loosening of the nuts under cyclic loading. A less likely outcome of failure to follow the tightening procedure is tightening to the point of damage—plastic deformation and stripping of the threads— which may require removal and replacement.
Other researchers have suggested a value of 0. If an anchor rod has a nut head or the head is fastened with nuts, the nut should be prevented from rotation while the anchor rod is tightened. This can be achieved with a jam nut or another type of locking device. The jam nut will affect the ultimate or fatigue strength of the rod.
Very large torques may be required to properly tighten anchor rods greater than 1 in. A slugging wrench or a hydraulic torque wrench is required. For the leveling nuts, an open-end slugging wrench may be used. This test attempts to recreate the conditions to which the anchor rod will be subjected during installation. After the test and before placing the concrete, anchor rods should be secured to a template or other device to avoid movement during placing and curing of the concrete that may lead to misalignments larger than what may be tolerated.
Beveled washers should be used: 1. Under the leveling nut if the slope of the bottom face of the base plate has a slope greater than If a beveled washer is required, the contractor should disassemble the joint, replace nuts, add the beveled washer s , and retighten in a star pattern to the initial condition.
Beveled washers can typically accommodate a slope up to Top nuts should be pretensioned. The procedure for pretensioning is a turn-of-nut procedure, although they are inspected using torque. Pretensioning the nuts should be accomplished in two full tightening cycles following a star pattern. If a rod assembly cannot achieve the required torque, is very likely that the threads have stripped.
When it is required that the nuts be prevented from loosening, a jam nut or other suitable device can be used. Any other method for preventing nut loosening should be approved by the Engineer of Record. Tack welding the top side of the top nut has been used, although this is not consistent with the AWS Structural Welding Code. While tack welding to the unstressed top of the anchor rod is relatively harmless, under no circumstance should any nut be tack welded to the washer or the base plate. A torque multiplier may be used.
Prior to placing the anchor rods in the concrete, an anchor rod rotation capacity test should be conducted with at least one anchor rod from every lot. The plate must be restrained against movement from the torque that will be applied.
The test consists of Steps 11 through 19 below, with the exception of Step 13 since there is only one anchor rod. The nut should be rotated to at least the required rotation given in Table A2. After the test, the nuts should be removed and inspected for damage to their threads.
Then, the anchor rod is removed from the test plate and restrained while the nuts should be turned onto the bolts at least one rod diameter past the location of the leveling nut and top nut in the test, then backed off by one worker using an ordinary wrench without a cheater bar. The threads are considered damaged if an unusual effort is required to turn the nut. If there is no damage to the anchor rod or nut during this test, they may be used in the joint.
A template is required for leveling the leveling nuts. Any deviation between the hole positions outside of the tolerances must be reported to the Engineer of Record. The template set or other device with anchor rods should be secured in its correct position in accordance with the contract documents.
If a top template is above the concrete surface, it may be removed 24 hours after placing the concrete. The exposed part of the anchor rods should be cleaned with a wire brush or equivalent and lubricated if galvanized. In the absence of required tolerances, the position, elevation, and projected length from the concrete should be according to the AISC Code of Standard Practice for Steel Buildings and Bridges. If the joint is required to be designed for fatigue, the misalignment from vertical should be no more than Nuts should be turned onto the bolts well past the elevation of the bottom of the leveling nut and backed off by a worker using an ordinary wrench without a cheater bar.
Thread damage requiring unusually large effort should be reported to the Engineer of Record. If threads of galvanized anchor rods were lubricated more than 24 hours before placing the leveling nut, or have been wet since they were lubricated, the exposed threads of the anchor rod should be relubricated.
Leveling nuts should be cleaned and threads and bearing surfaces lubricated if galvanized and placed on the anchor rods. Leveling nut washers should be placed on the anchor rods. The template should be placed on top of the leveling nuts to check the level of the nuts.
In some cases, if indicated in the contract documents, it is permitted to set the base plate at some other angle other than level. If this angle exceeds , beveled washers should be used. The base plate and structural element to which it is attached should be placed.
Top nut washers should be placed. Table A2. Nut rotation is relative to anchor rod. An inability to achieve this torque means it is likely that the threads have stripped and this must be reported to the Engineer of Record. The assembly of sleeve and anchor rod should be secured in its correct position in accordance with the contract documents.
If a top template is above the concrete surface, it may be removed no sooner than 24 hours after placing the concrete. The exposed part of the anchor rods should be cleaned with a wire brush or equivalent and lubricated. The nuts should be turned onto the bolts at least one rod diameter past the elevation of the bottom of the base plate and backed off by a worker using an ordinary wrench without a cheater bar.
Any damage resulting in unusual effort to turn the nut should be reported to the Engineer of Record. The base plate and attached structural element, or piece of equipment or machinery, should be placed. The sleeve must be cleaned and sealed off to prevent inclusion of debris. Anchor rods are typically tensioned using a center-hole ram with access to the nut for retightening. The nut is tightened down while the tension is maintained on the anchor rod, and the anchor rod tension is released.
It is recognized that part of the tension will be lost to relaxation after the tension is released. If threads of anchor rods were lubricated more than 24 hours before placing the nut or have been wet since they were lubricated, the exposed threads of the anchor rod should be relubricated. Nuts should be cleaned and the threads and bearing surfaces lubricated.
If more than one nut in a joint is loose, the entire joint should be disassembled, all the anchor rods visually inspected, and the joint reassembled with new nuts. If more than one nut is loose, the joint may have been poorly installed or fatigue problems may exist.
A close following of the performance of the joint should be made. Regular inspection and maintenance should be conducted for joints that are designed for the fatigue. Anchor rod appearance—Draw a diagram of the anchor rod pattern and number in a clockwise pattern. Check each anchor rod for corrosion, gouges, or cracks. Suspected cracks may be more closely examined using the dye-penetrant technique.
If there is heavy corrosion near the interface with the concrete, there may be more severe corrosion hidden below the concrete where the pocket around the anchor rod stays wet. Verify that all the anchor rods have top nuts with washers. Lock washers should not be used. Galvanized nuts or washers should not be used with unpainted weathering steel. Check for inadequately sized washers for oversize holes.
If there is no grout pad, verify that all the anchor rods have leveling nuts with washers. Check for loose nuts, gouges, thread damage, or corrosion. If the anchor rod is not projecting past the nut, measure the distance from the top of the nut to the top of the anchor rod.
Sounding the anchor rods—Anchor rods may be struck by a hammer a large ball peen hammer is suggested to detect broken bolts. Strike the side of the top nut and the top of the rod. Good tight anchor rods will all have a similar ring. Broken or loose anchor rods will have a distinctly different and duller sound.
Tack welds to the washer or the base plate are undesirable and should be reported. If similar structures subject to similar loading have had anchor rod fatigue cracking problems, an ultrasonic test of anchor rods should be performed.
Assuming that the supported column and base plate have coincident centroids, if. Throughout this Design Guide, design procedures and examples have been presented using an assumption of a uniform pressure distribution on the base plate, which is consistent with procedures adopted by ACI.
Alternatively, it is permissible to assume a triangular pressure distribution on the base plate. This alternative does not in and of itself represent an elastic design or an ASD approach to design.
Rather, both triangular and uniform distributions represent simplifying approximations that are equally applicable for LRFD and ASD applications. The use of a triangular pressure distribution, as shown in Figure B. The designer may wish to solve directly for the plate thickness based on the applied loads and the geometry of the base conditions. However, an assumption of pressure distribution must be made to determine the moment used in the above equations. This process is illustrated in the following sections.
Figure B. Elastic analysis for axial load plus moment, triangular distribution. This is the theoretical condition where no tension exists on the interface between the base plate and foundation, and any applied additional moment at the same axial compressive load will result in tension. Therefore, setting. Choose trial base plate sizes B and N based on geometry of column and four-anchor requirements. Determine plate cantilever dimension, m or n, in direction of applied moment.
See Section B4. Effect of eccentricity on bearing. When the effective eccentricity is large greater than ekern , there is a tensile force in the anchor rods due to the moment, see Figure B. To calculate this force, the anchor rod force, T, and the length of bearing, A, must be determined, as shown in Figure B. By static equilibrium, the following equations can be derived. Determine Mpl at m. By summing the moments about the resulting bolt force and solving as a quadratic function, the following expression can be determined for calculating the bearing distance, A:.
Design a base plate for axial dead and live loads equal to and kips, respectively, and moments from the dead and live loads equal to and kip-in. The design procedure is as follows: 1. Choose trial base plate sizes B and N based on geometry of column and the four-anchor requirement.
Determine the length of bearing, A, equal to the smallest positive value from the equation in Section B. If the value is reasonable, go on to the next step. If this were so, the anchor rod could not develop its full tensile strength. It is then necessary to return to Step 2 and choose another, larger plate size. Determine the resultant anchor bolt force, T, from the above equation.
If it is reasonable, go to the next step. Otherwise return to Step 2. Determine the required moment strength per inch of plate as the greater of the moment due to the pressure and the moment due to tension in the anchor rods. Each is to be determined at the appropriate critical section. Determine base pressures for a 1-in. Bending of a 1-in. Bending about a plane at n, perpendicular to applied moment.
For the case of axial loads plus small moments, the procedure shown below can be used using the axial load only. Determine required plate thickness: Note: Since the Mpl is expressed in units of kip-in.
The schedule demands along with the prob- lems that can occur at the interface of structural steel and reinforced concrete make it essential that the design details take into account not only structural requirements, but also include consideration of constructability issues, especially anchor rod setting procedures and tolerances. The impor- tance of the accurate placement of anchor rods cannot be over-emphasized.
This is the one of the key components to safely erecting and accurately plumbing the building. The material in this Guide is intended to provide guidelines for engineers and fabricators to design, detail, and specify column-base-plate and anchor rod connections in a manner that avoids common fabrication and erection problems.
Column bases and portions of the anchorage design generally can be designed in a direct approach based on either LRFD or ASD load combinations. The one area of anchorage design that is not easily designed by ASD is the embedment of anchor rods into concrete. The generic approach in development of foundation de- sign parameters taken in this Guide permits the user a choice to develop the loads based on either the LRFD or ASD ap- proach.
Many of the equations shown herein are independent of the load ap - proach and thus are applicable to either design methodology. These are shown in singular format. Other derived equations are based on the particular load approach and are presented in a side-by-side format of comparable equations for LRFD or ASD application.
The typical components of a column base are shown in Figure 1. Not only is it important to design the column-base-plate connection for strength requirements, it is also important to recognize that these connections affect the behavior of the structure. Assumptions are made in structural analysis about the boundary conditions represented by the connections. If more accurate analyses are desired, it may be necessary to input the stiffness of the column-base-plate connection in the elastic and plastic ranges, and for seismic loading, possibly even the cyclic force-deformation relations.
The forces and deformations from the structural analyses used to design the column-base- plate connection are dependent on the choice of the column- base-plate connection details. Figure 1. Column base connection components. For such col- umns, the simple column-base-plate connection detail shown in Figure 1. The design of column-base-plate connections for axial compression only is presented in Sec- tion 3. The design is simple and need not be encumbered with many of the more complex issues discussed in Appen- dix A, which pertains to special structures.
Anchor rods for gravity columns are often not required for the permanent structure and need only be sized to provide for column sta- bility during erection. Column base plate connections are also capable of trans- mitting uplift forces and can transmit shear through the an- chor rods if required. If the base plate remains in compres- sion, shear can be transmitted through friction against the grout pad or concrete; thus, the anchor rods are not required to be designed for shear.
Large shear forces can be resisted by bearing against concrete, either by embedding the col- umn base or by adding a shear lug under the base plate. Column base plate moment connections can be used to resist wind and seismic loads on the building frame. Moment at the column base can be resisted by development of a force couple between bearing on the concrete and tension in some or all of the anchor rods.
The objective of the design pro- cess in this Guide is that under service loading and under ex- treme loading in excess of the design loads, the behavior of column base plates should be close to that predicted by the approximate mathematical equations in this Design Guide.
Recent regulations of the U. This regulation has essentially eliminated the typical detail with two anchor rods except for small post- type structures that weigh less than lb e. Design guidance for anchor rods based on the ACI recommendations is included, along with practical suggestions for detailing and installing anchor rod assemblies.
These guidelines deal principally with cast-in- place anchors and with their design, installation, inspection, and repair in column-base-plate connections.
Based on cost and availability, the materials shown in Tables 2. F Gr 36 [d] 58 There is seldom a reason to use high-strength material, since in- creasing the thickness will provide increased strength where needed. When designing base plate connections, it is important to consider that material is generally less expensive than labor and, where possible, economy may be gained by using thick- er plates rather than detailing stiffeners or other reinforce- ment to achieve the same strength with a thinner base plate.
A possible exception to this rule is the case of moment-type bases that resist large moments. Most column base plates are designed as square to match the foundation shape and more readily accommodate square anchor rod patterns.
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