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Concrete Shear Wall |
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Concrete Shear Wall |
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This program provides complete vertical and lateral load design and analysis for concrete shear walls up to five levels high. You can also design a rectangular footing supporting the wall, considering all moments, shears, and axial loads.
The program allows input of five different wall sections, each with its own base height, section height, length, offset, thickness, and material strengths. Up to ten lateral short-term loads and up to ten vertical dead, live, and short term loads can be applied to the entire wall system. The vertical and horizontal position of each load can be specified.
Concrete density and lateral weight factors can be entered to define the vertical weight and portion of vertical weight used as a generated lateral short-term load. This frees you of constantly recalculating vertical and lateral wall weights as you refine your wall and footing designs.
The program calculates shears and overturning forces for the base of each of the five wall sections, and determines required shear and bending reinforcement. Lateral loads are applied in each direction to determine the maximum moments (due to centerline offsets of portions of the wall above).
Each vertical load represents a concentrated load acting on the wall. This load is included in the shear design of the wall by providing an axial load which combines with the allowable shear equations. This load is included in the calculations for overturning moment on a wall section when it is eccentric to the centerline of the wall at that particular section.
The program also provides analysis and design capability for rectangular footings located under the wall. Total overturning forces are calculated about the bottom of the footing (with force due to wall weight included), and resultant service and factored load soil pressures are determined. The program automatically checks cases when the vertical resultant is outside the kern distance for soil pressure calculations. Also, overall overturning stability ratios are found.
A final calculation for one way shear stresses and moments at each end of the wall is performed by applying vertical loads only, vertical+lateral load to the left, and then to the right. Minimum reinforcement due to maximum moments is determined.
Basic Usage
| • | It is critical that the user understands the concept of the various ANALYSIS HEIGHTS which are used to define the bottom of each wall section. This provides the ability to have an analysis performed at any height above a datum. Also, when walls support more than five floors, the top of the wall lateral and vertical loads can be entered to apply the axial, shear, and moment loads from the elements above. |
| • | General Data allows you to enter dimension and load combination data for the entire wall. Total Height specifies the top most height of the wall. Concrete Weight and Lat. Weight Factor define the vertical and lateral loads to use for automatic application of forces due to the wall's weight. You can specify whether to Include Live Load with short-term loads (typically not for seismic design), and set a minimum reinforcing area to be used for wall bending calculations. |
| • | Lateral Loads are applied to all wall sections by specifying the load and height above the base level. This provides entry of simple point lateral loads to the wall, simulating a lumped diaphragm force. This load must not include ACI load factors, but any seismic factors should already have been applied to loads.. |
| • | Vertical Loads are applied to the wall by specifying the dead, live, and short-term components of each load. Each load has its own vertical and horizontal location which is used to calculate overturning moments and axial stress at each wall level. |
| • | Analysis Height and Wall Thickness, Offset, and Length are entered to define the vertical and horizontal position of each of the five wall sections. You can also enter the concrete and reinforcing strength of each section separately. Please see illustration above. Calculate the calcsheet and review the Story Shears, Moments, and Uplift at the ends of the base section. These values consider all vertical and lateral loads, including vertical and horizontal wall weights generated by the program. |
| • | You can now review the Shear Analysis and Bending Analysis to determine if any changes need to be made in wall thicknesses or lengths to satisfy stress and reinforcing requirements. |
| • | Continue to enter the Footing Design Values, which include material strengths, width, thickness, and projection beyond each end of the base wall section. Using these projections and the base wall length entered previously, factored load soil pressures, reinforcing requirements, and shear stresses will be calculated. |
| • | To refine your Soil Pressures and Footing Design, modify the footing dimensions as necessary. The program will automatically calculate factored soil pressures, shears, and moments for ACI combination equations 9-1, 9-2, and 9-3 for lateral loads applied in both directions. |
Unique Features
| • | This program provides a convenient method of designing a complete shear wall and footing system at once. The tedious task of estimating wall lengths, thicknesses, and footing size is performed is one step. |
| • | This program can be used in instances where the designer must consider rigidities for multi-story structures with multiple shear walls that are all connected with a rigid diaphragm. This program can quickly provide excellent information for preliminary and final wall sizes. |
| • | Vertical and lateral loads can be applied freely, and the lateral loads are applied in both directions to determine maximum moments, shears, uplift, and soil pressures. |
Assumptions & Limitations
| • | Limited loadings are available, so loads should be combined and positions adjusted when more individual loads are acting than the program input permits. |
| • | Wall weight is automatically calculated and added to the applied vertical loads and lateral loads. |
Example
The data entry for this example is shown in the screen captures that accompany the Data Entry Tabs and Results & Graphics Tabs sections to follow.
Data Entry Tabs
This set of tabs provides entries for all input in this calculation. While you are entering data and switching between these tabs you can view the desired resulting information on the tabs on the right-hand side of the screen (calculated values, sketches, diagrams, etc.). A recalculation is performed after any entry data is changed. After each data entry you can view the results on the right-hand set of tabs.
General Tab
Total Wall Height
Enter the total height of the wall. This height is used by the program to determine the total height that the topmost Analysis Height wall section should extend to for determining the weight of that uppermost section.
Concrete Weight
Enter the unit weight of concrete that should be used for wall and footing weight calculations.
Include LL & ST
This entry instructs the program when to include live load with short-term loads. Typically, wind load analysis will include live loads, while short-term loads due to seismic forces are not combined with live loads.
Seismic Factor
This factor is applied to the weight of each wall section and the resulting force is applied laterally at the center of gravity of each wall. This should be used for seismic analysis only, and is commonly called the Seismic Factor which applies only to portions of structures.
Seismic Zone
This entry is used to control overall ACI load factoring. When wind loads generate the short-term forces, enter a 0" here. When seismic loads generate the short-term loads, enter 1" through 4" here to indicate the UBC seismic zone. When zone 3 or 4 is used, the special load factoring provisions of the UBC are applied. When 1 through 4 is used, this factor will be applied to the weight of each wall section and applied as a lateral load at the wall section centroid to account for the lateral seismic force created by the wall's own weight.
Min. Wall As % for Bending
This indicates the minimum wall reinforcement percentage to be used for determining bending reinforcement. Typically this will be set to 0.0025 (per ACI for shear walls).
Analysis Height
This is the user-defined height at which the analysis of a particular wall section will be performed. All moments, shears, and vertical loads at this height will be calculated using all applied lateral and vertical loads and the wall self weight above this point. The other wall data items specified in the calcsheet column will be used between this analysis height and the next higher level in the column to the right.
ALWAYS WORK WITH THE HIGHER ANALYSIS HEIGHT IN THE COLUMN TO THE RIGHT OF A LOWER HEIGHT. This is needed due to the manner in which the program calculates the heights by comparing heights of adjacent sections.
Wall Offset
Please refer to the diagram to further understand this item. Because this program can be used with a walls that have their length changes with height, you must enter the offset from the bottom wall section to the LEFT EDGE of the wall section. This enables the program to calculate the actual X-Distance to the center of gravity of the wall.
Wall Length
Enter the length to be used in the analysis of the particular wall section. Please note that if the Wall length + Offset is greater than the Wall Length + Offset for the level below, this indicates that the section OVERHANGS the section below it. This is not allowed.
Wall Thickness
Enter the thickness to be used in the analysis of a particular wall section. This thickness will be used only between the Analysis Height for that section up to the analysis of the next higher section (or Total Wall Height if it is the highest section).
f'c
Enter the concrete compressive strength to be used in the wall calculation for a particular wall section. This does not apply to the footing, which has it's own input area for material strengths.
Fy
Enter the steel yield stress for the reinforcing to be used in the wall analysis. Please note that this value applies to BOTH shear and moment reinforcing.
d :Effective Depth
The program considers the effective shear length of the wall to be 0.8 * wall length. If you leave this entry blank, it will be filled in for you by the program.
Lateral Loads Tab
Enter the unfactored lateral loads applied to the wall. Do not apply load factors to this number; it should represent the actual lateral load for design loads (apply seismic factor to gravity loads BEFORE entering). The Height entry will specify the height at which the load is applied to the overall wall system above the top of footing, and the program will automatically determine which wall section it is applied to. This load would typically represent the total diaphragm load going into the wall at a particular level.
All lateral loads will be applied in both left and right directions; assigning a sign to them will be meaningless. Applying the loads in each direction enables this program to provide a complete analysis of the wall system for all possible combinations of loads.
Vertical Loads Tab
Loads
Enter the unfactored vertical point loads applied to the wall. These loads represent gravity loads being supported by the wall at various heights. The load may have dead, live, and short term components.
Load Location
This location is in reference to the lower left edge of the bottom wall section. By entering the coordinates of the load from this position, the program will automatically determine which wall section the load is applied to. These vertical loads are used in all calculations.
ACI Factors Tab
This tab specifies the load factors to be used by the program when calculation the factored dead, live, and short term loads to be used in the internal load combinations for determining Mu and Vu.
Footing Tab
Allowable Soil Pressure
Enter the maximum allowable soil bearing pressure that can be sustained by the soil.
Distance @ Left
Enter the distance the footing extends past the left end of the bottom most shear wall section.
Wall Length
This value is repeated from the "General" tab so that you know the proportions of footing length.
Distance @ Right
Enter the distance the footing extends past the left end of the bottom most shear wall section.
Footing Length
This value is calculated as the two "distance" projections plus wall length.
Footing Width
Enter the footing width to be used in the analysis. This width may be modified at any time for optimizing design.
Footing Thickness
Enter the footing thickness to be used in the analysis. This thickness may be modified at any time for optimizing design.
f'c
Enter the concrete compressive strength to be used in the footing calculation.
Fy
Enter the steel yield stress for the reinforcing to be used in the footing analysis.
Rebar Cover
Enter the cover over tension reinforcing which will be placed in the bottom of the footing. This distance will be subtracted from footing thickness to obtain the d to be used in calculating RU for the footing and for one-way shear calculations.
Kern Width
The kern distance is equal to footing length divided by 6. The "kern" region is the area in the center of the footing where the resultant of upward soil pressure can be and still maintain full pressure under the entire footing. This area extend Length/6 on either side of the center of the footing.
If the resultant lies outside this area the footing will be under partial compression and the soil pressure diagram will be triangular.
Results & Graphics Tabs
This set of tabs provides the calculated values resulting from your input on the "Data Entry Tabs". Because a recalculation is performed with each data entry, the information on these tabs always reflects the accurate and current results, problem sketch, or stress/deflection diagram.
Wall Results Tab
Vs : Story Shear
This value is the summation of the lateral forces applied at and above the Analysis Height for a particular section.
Ms : Story OTM
This value is the summation of the overturning moments induced from loads applied above the Analysis Height. Included are applied lateral loads, Wall Section Weight * ZIKCS, and applied vertical loads that are not at the center of the wall.
Uplift
This value first calculates the moments created by the applied lateral loads, vertical loads, wall weight, and lateral wall weight overturning the wall about the base. If overturning stability is not satisfied, the net moment is divided by ( Wall Length -1'-0" ) to obtain this number. Depending on the actual placement of tension reinforcing, multiply this force by Actual d/Wall Length to arrive at an adjusted force.
The lateral loads you have entered are combined with the calculated lateral wall weight loads and applied in both directions to get the maximum uplift at either end. Since the program allows general placement of vertical loads and offset walls, an analysis of both directions must be performed.
Vu = Vs * Factor(s)
This is equal to the Vs:Story Shear times the Load Factor specified in the loading table plus the lateral wall weight times 1.05 or 1.275 for seismic or wind cases.
Shear Analysis
Vu :
Applied To determine the actual factored unit shear at the wall section, Vu is divided by wall thickness and d.
Vc * .85
Calculated using UBC equations 11-32 and 11-33 (if applicable). These formulas consider the effect of axial load on shear stress.
Nu : Axial
This value is used in determining the allowable unit shear stress. Vertical loads and wall weight above the Analysis Height are combined with load factors to arrive at this axial load.
Mu:Moment
All lateral and vertical loads above the Analysis Height are combined with load factors to calculate the actual moment acting at the base of each wall section.
Code Min. Area %
If Phi * VU < VC/2, then Horiz % = 0.002 and Vert % = 0.0012
If Phi * VU >= VC/2, then Horiz % = MIN (0.0025,(VU - VC)/area
Vert % per ACI 11-35, 0.0025
Shear Area Req'
This is the shear reinforcing required per foot of wall to resist the shear forces.
Bending Analysis
Ru
The resisting modulus of the section is calculated using the wall thickness, steel depth equal to 0.8 * Wall length, and Phi = 0.9.
As Req'd Each End
Area of tension reinforcement required at each end of the wall to resist bending forces.
Footing Results Tab
Overturning Stability
Overall stability is calculated by including the footing weight with moments due to vertical and lateral loads about each end of the base of the footing. For load applied to the left, moments are taken about the left edge of the footing and vice versa.
One-Way Shear Check
For both of the following shear checks, Vc:Allow is calculated using : Vc = 2 * f'c½ * .85
Bending Design
Min. As
Enter the minimum allowable steel percentage you wish the program to use when finding the area of steel required.
Mu : Moment
The maximum moment acting on the footing due to factored soil pressures is calculated at the edge of the wall. The moment due to footing weight is subtracted from the soil pressure moment to determine this actual maximum moment.
Ru
Resisting modulus due to applied moment.
% As Required
The required steel percentage is calculated by first finding the required steel area due to bending moments:
% Req'd = (1/m) * (1-[1-(2*m*Ru)/Fy)½ ]½
If this area is greater than 200/Fy then it is compared to your Min AS % value and the larger used. If it is less than 200/Fy, it is multiplied by 1.33 and again compared to 200/Fy. The minimum of those two values is then compared to your Min AS % value and the larger used
As:Required
This is equal to the As% value calculated previously multiplied by the width and d distances of the footing.
Soil & Stability Tab
Wall/Footing Stability
This section provides a table of information on the overall stability and pressure resultants for the wall/footing system. To determine the maximum conditions, three lateral load analyses are performed; no lateral load at all, lateral load applied to the left, and lateral load applied to the right. This satisfies all load combinations as far as direction of forces is concerned.
For each of these force directions, ACI load factor combinations 9-1, 9-2, and 9-3 are evaluated. This results in a set of eccentricities for all possible conditions which will be used to determine factored soil pressures for footing design.
Sketch Tab
This tab provides a sketch of the beam with loads and resulting values shown. Using the [Print Sketch] button will print the sketch in large scale on a single sheet of paper.
Printing Tab
This tab allows you to control which areas of the calculation to print. Checking a box will signal that the information described by the item will be printed. However, if there is no information in for a particular selection it will not be printed. So these checkboxes are best described as "If this particular area of the calculations contains data then print it".
Sample Printout
