Structural Behavior of Lateral Load Resisting Systems with Structural Membranes

When designing a structure, there are two major loads to consider; the gravity load and the lateral load. While gravity load generally acts downward, a lateral load seems to act in the horizontal (or sideways) direction. Lateral load manifests through wind and earthquakes. It is of utmost importance that a suitable path exists through which its force is transferred to the ground.

This article aims to investigate how lateral loads are being resisted by a few selected structural membranes.

The lateral force-resisting system.

Pile Foundations

Pile foundations do resist lateral loads via 3 major means;

1. Passive soil resistance on the face of the cap
2. Shear on the base of the cap
3. Passive soil resistance against pile shaft

It is noteworthy that the third means is the most reliable.

When examining the issue that comes up in the structural design of pile foundations, factors such as soil reaction, rotations, deflection, and moment, shear, and soil reaction are important for such analysis. Experts often do this examination using the Beam-on-elastic Foundation approach. This theory works because of the ability of most piles to change. This allows us to say that they have an extremely large length, for the sake of analysis only. But, when analyzing the short rigid piles, the lower boundary condition should receive consideration as well.

K Modulus

Now, steady modulus-depth and linearly increasing relationships have non-dimensional solutions. However, some solutions have steeped variations of the modulus, k. The very large amount of experimental value helps us to choose the right variation of k with depth. These values of k oftentimes come as a function of observable soil features. It is possible to determine the size of k through an experiment. This test includes adding the related non-dimensional solution with plain lateral load tests.

Important factors

There are some important factors that influence effective modulus. such as:

• Group action which brings about a 25 percent reduction in the effective modulus of a separated pile
• A cyclic load that is capable of increasing deflection, and thus further reduces the effective modulus.

The presence of these effects can make the effective modulus reduce to 10 percent of that for the first loading of an isolated pile. This makes the quality of not changing hard to reach the top of the pile. A fixity of 50 percent is good most times as it provides almost the same positive and negative moments. With this in place, the uniform flexural members are applied in an efficient manner from a structural angle. So when fixity increases, the deflection will drop through increased stiffness.

The existing technique

A technique exists through which it is possible to do the analysis for partially embedded piles. This technique uses the non-dimensional solutions readily available for fully embedded piles. Here, the designer introduces depth to fixity usually based on soil characteristics and pile stiffness. This helps in eliminating the problem of taking arbitrary depths to fixity. By conservatively assuming values for “k”, you can obtain economical designs. However, there are cases where using more accurate design data can cause significant savings. In such cases, you can conduct a simple field test program to generate the required data.

Result

Piles must resist lateral loads and moment in addition to their primary loaded members. To achieve this, the designer must determine the deflections and stresses in the prevailing soil-pile system and then find innovative ways of controlling them until they lie within a tolerable range.

The technique for Analyzing soil-pile interaction

In addition to the usual gravity load, W, the cap is often subject to moment and shear loads. The capacity of the pile axially is instrumental in resisting the axial load. However, we will not consider it here. The moment and shear loads are resisted by;

1. A passive soil resistance on the face of the cap
2. Shear along the base of the cap
3. Moment and shear resistance of the piles at the junction to the cap.

It is then easy to see that the resistance to the moment and shear of the piles are dependent on the stiffness and strength of the pile and soil respectively.

Some Details

Temporary forces like passive soil resistance can also resist lateral loads. They are temporary because basic alteration or repair jobs may remove their lateral load resisting abilities from the soil; as a consequence, researchers usually neglect them during the analysis. Another temporary resistance to lateral loads is observed in the shear along the base of the cap; in a similar fashion, a slight settlement of the soil beneath the cap is enough to destabilize its resisting property and as a result, it is also neglected.

The moment and shear resistances are much more permanent and are the only factors considered when analyzing designs.

Slab-Column Connections

Structural systems for offices and industrial buildings are usually made up of reinforced concrete (RC) sported on columns. In fact, adding reinforced concrete walls results in a drastic increase in lateral stiffness and strength of the structure. These RC walls carry the bulk of the horizontal loads in the event of an earthquake.

The lateral stiffness and strength of a structure are quite independent of the slab-column system. Manufacturers design the slab-column connection to follow the displacements (caused by the vibration of the earth)s of the structure while ensuring that the vertical load-carrying capacity from slab to column isn’t compromised. If compromised, brittle punching failure occurs in the slab. The deformation capacity of the slab-column connection is a measure of the deformation capacity of the entire building. Therefore, excess compromise can lead to collapse.

Analytical Approach

In this section, we wish to present an analytical approach that helps in predicting the moment resistance of a mechanism that adds up to the strength of the slab-column connection under earthquake-induced drifts. This approach is dependant on the Critical Shear Crack Theory (CSCT). In addition, we will also consider the effect of gravity-induced loads on the behavior of slab-column connections under loading caused by the vibration of the earth and also mechanisms that help resist increasing drifts.

This system of RC supported on column offers many benefits for offices and industrial buildings such as wide open spaces and they are relatively faster to construct. In earthquake-prone areas, it is important to include vertical spines (shear and/or core walls) to increase the horizontal stiffness to enable it to withstand a great number of horizontal loads imposed by seismic activities. The slab-column system is excluded from the systems that resist lateral force.

If compromised, brittle punching failure can occur in the slab. the deformation capacity of the building is dependent on the deformation capacity of the connection. Therefore, a large compromise can eventually lead to collapse.

When moment and shear force interacts with a slab-column connection, three mechanisms help to resist the moment. They include;

1. Shear force eccentric to the column axis
2. Flexure
3. Torsion

It can be extremely complex to calculate the contributions of the resistant moments if the slab-column connection is subject to a non-linear eccentric moment (NB such non-linearity can even be caused by a crack). This is magnified by the fact that the resistant moments withstand differing failure modes.

Mechanism of resistance to lateral load

Several researchers have investigated the mechanism of resistance to lateral load in slab-column connect. In 1968, Hanson and Hanson developed the common experimental method for experimentally determining the contribution of flexure and torsion mechanism. Their method involves cutting slots in the slab in the neighborhood of the column faces. Other researchers (Kanoh and Yoshizaki, 1979) connected a column monolithically to a slab specimen. Thereafter, a force couple was then applied in such a way that torsion alone is transferred from the column to the slab. It is important to note that these experimental methods have limits in the application.

Researchers have also used design equations in codes of practice (ACI 318, 2011; Euro code 2, 2004) in estimating these resistant moments. They rely on an empirical analysis that takes into consideration the contribution of the eccentric shear and the bending moment while neglecting the contribution from torsion.

Another important assumption made to simplify the equation is that the linear (ACI 318, 2011) or uniform (Euro code 2, (2001) distribution of shear stress on the critical perimeter. As a consequence, for a non-linear or nonuniform distribution of shear stresses, the equation breaks down by either under predicting or over predicting the moment capacity of slab-column. Furthermore, there exists a more widely accepted method of finding the moment transferred to the connection. However, the model fails to define the relationship between moment and deformation capacity.

It is essential to estimate the moment-rotation relationship and rotation capacity of the slab-column connection when designing and accessing buildings with flat slabs and columns to withstand drifts induced by seismic actions. The model used in the analysis and prediction of the moment-rotation relationship must incorporate the size effect, column size, gravity load effect, and reinforcement ratio influence.

Wall Panel

Under the action of wind and seismic loading, it is very important to understand the performance of walls panels in residential buildings. Dead and live gravitational loads affect the wall panels. In addition, under these forces, in-plane shear and/or out-of-plane lateral forces also influence wall panels.

There exist several studies to understand the behavior of wall panels under out-of-plane loading. Also, there have been various design and prescriptive methods for constructing wall panels in other to withstand this kind of duress. On the other hand, understanding the performance of in-plane forces requires further investigations. This investigation involved comparing the in-plane shear resistance of various types of wall panels commonly used in residential buildings.

There were tests on five plane specimens with an aspect ratio (height/length) of 2. Flexural From the experiment, using a standard test condition, the tests showed that ICF wall panels are much stronger and stiffer than similar wood- or steel-frame walls panels. In fact, ICF walls resisted between 6 to 8.5 times of the lateral loads resisted by framed wall panels.

Also, the initial stiffness of the ICF panel was between 18 to 38 times that of wood- or steel-frame wall panels. When ICF wall panels were subject to two times the maximum lateral load resisted by framed walls, the ICF wall was able to withstand it and even exhibit a linear behavior with no sign of physical damage of any sort. The deformation under this load level was in a range of 0.05 to 0.07 in.

Other Results

However, the maximum deflection provided by the ICF wall panels equaled or exceeded by just 2% of the height of the building. This strength advantage of ICF wall panels gave it the ability to even withstand rind and earthquakes of much stronger magnitudes. It is important to note that this study takes no account of the effect of:

• dynamic and out-of-plane loading,
• opening in the panels,
• and various wall configurations.

Therefore, there should be special care not to extend the result to panel analysis during actual earth motion. Furthermore, Lateral deformation has limits in ICF wall panels because of their stiffness. Also, their stiffness prevents potential damage to the non-structural element of the building.

In the event of a moderate earthquake or wind, most of the damage is prevalent on the non-structural components. Therefore, only these non-structural parts usually need repair. The connection between the wall panels and the footings and between the panels and the floor/roof can influence two major things which include;

1. The behavior of the wall panels
2. The development of an effective load transfer mechanism in the structure.

The lateral load distribution can also be subject to the position of the wall panels in the building plan and roof/ floor diaphragm action. From our results, we have been able to identify some design aspects that can be improved to enhance the in-plane shear behavior of the wall panel even at a reduced cost.

When using a wood frame, the use of screws (instead of nails that have a lower pull-out resistance) instead of nails for connecting the OSBs to the frame members would significantly increase the maximum lateral resistance. This is possible through increased integrity that prevents premature separation of the boards from the frame.

For steel-frame wall panels, making use of stiffening plates, small screw spacing, eliminating web holes in the lower part of the vertical studs, and making use of heavier gauge bottom tracks would ensure that premature buckling and bending of the frame member is avoided.

Finally, when using ICF wall panels, implementing a dowel with longer development lengths between the footing and the wall would ensure that early pull-out of the dowel is avoided. Also, it strengthens the base of the wall panel. It is possible to improve effectiveness by accompanying the longer dowels with vertical reinforcing bars in the wall that possess.